Basictronics

Rheostat

2008-07-24T04:53:00.001-07:00

A rheostat is a two-terminal variable resistor. Often these are designed to handle much higher voltage and current. Typically these are constructed as a resistive wire wrapped to form a toroid coil with the wiper moving over the upper surface of the toroid, sliding from one turn of the wire to the next. Sometimes a rheostat is made from resistance wire wound on a heat resisting cylinder with the slider made from a number of metal fingers that grip lightly onto a small portion of the turns of resistance wire. The 'fingers' can be moved along the coil of resistance wire by a sliding knob thus changing the 'tapping' point. They are usually used as variable resistors rather than variable potential dividers.

Any three-terminal potentiometer can be used as a two-terminal variable resistor, by not connecting to the 3rd terminal. It is common practice to connect the wiper terminal to the unused end of the resistance track to reduce the amount of resistance variation caused by dirt on the track.

How to read Resistor Color Codes

2008-07-15T17:32:00.000-07:00

How to read Resistor Color Codes

First the code

Black	Brown	Red	Orange	Yellow	Green	Blue	Violet	Grey	White
0	1	2	3	4	5	6	7	8	9

The mnemonic

Bad Boys Ravish Only Young Girls But Violet Gives Willingly PC_fascists

Black is also easy to remember as zero because of the nothingness common to both.

(Please don't add or change the mnemonic - it will only get reverted -admin)

How to read the code

First find the tolerance band, it will typically be gold ( 5%) and sometimes silver (10%).

Starting from the other end, identify the first band - write down the number associated with that color; in this case Blue is 6.

Now 'read' the next color, here it is red so write down a '2' next to the six. (you should have '62' so far.)

Now read the third or 'multiplier exponent' band and write down that as the number of zeros.

In this example it is two so we get '6200' or '6,200'. If the 'multiplier exponent' band is Black (for zero) don't write any zeros down.

If the 'multiplier exponent' band is Gold move the decimal point one to the left. If the 'multiplier exponent' band is Silver move the decimal point two places to the left. If the resistor has one more band past the tolerance band it is a quality band.

Read the number as the '% Failure rate per 1000 hour' This is rated assuming full wattage being applied to the resistors. (To get better failure rates, resistors are typically specified to have twice the needed wattage dissipation that the circuit produces). Some resistors use this band for temco information. 1% resistors have three bands to read digits to the left of the multiplier. They have a different temperature coefficient in order to provide the 1% tolerance.

At 1% the temperature coefficient starts to become an important factor. at +/-200 ppm a change in temperature of 25 Deg C causes a value change of up to 1%

BS 1852 Coding for resistor values

BS 1852(British Standard 1852). The letter R is used for Ohms and K for Kohms M for Megohms and placed where the decimal point would go.

At the end is a letter that represents tolerance Where M=20%, K=10%, J=5%, G=2%, and F=1% D=.5% C=.25 B=.1%

BS 1852 coding examples
R33	0.33 ohms
2R2	2.2 ohms
470R	470 Ohms
1K2	1.2K ohms
22K	22K ohms
22K2	22.2K ohms
4M7	4.7M ohms
5K6G	5.6K ohms 2%
33KK	33k Ohms 10%
47K3F	47.3 K Ohms 1%

Common surface mount coding

The third or forth digit is the multiplier

Thus 103 is a 10K resistor

475 is a 4.7M resistor

Measure resistors with Volt Ohm meter.

We now have a program that calculates the minimum error on resistor dividers of up to 4 values.

Other Components

2008-06-30T04:58:00.000-07:00

Other Components

Photograph © Rapid Electronics

circuit symbol

Light Dependent Resistor (LDR)

An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is made from cadmium sulphide (CdS) and the resistance decreases as the brightness of light falling on the LDR increases.

A multimeter can be used to find the resistance in darkness and bright light, these are the typical results for a standard LDR:

Darkness: maximum resistance, about 1M.
Very bright light: minimum resistance, about 100.

For many years the standard LDR has been the ORP12, now the NORPS12, which is about 13mm diameter. Miniature LDRs are also available and their diameter is about 5mm.

An LDR may be connected either way round and no special precautions are required when soldering.

Thermistor


Photograph © Rapid Electronics

circuit symbol

A thermistor is an input transducer (sensor) which converts temperature (heat) to resistance. Almost all thermistors have a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. It is possible to make thermistors with a positive temperature coefficient (resistance increases as temperature increases) but these are rarely used. Always assume NTC if no information is given.

A multimeter can be used to find the resistance at various temperatures, these are some typical readings for example:

Icy water 0°C: high resistance, about 12k.
Room temperature 25°C: medium resistance, about 5k.
Boiling water 100°C: low resistance, about 400.

Suppliers usually specify thermistors by their resistance at 25°C (room temperature). Thermistors take several seconds to respond to a sudden temperature change, small thermistors respond more rapidly.

A thermistor may be connected either way round and no special precautions are required when soldering. If it is going to be immersed in water the thermistor and its connections should be insulated because water is a weak conductor; for example they could be coated with polyurethane varnish.

Piezo transducer

Photograph © Rapid Electronics

circuit symbol

Piezo transducers are output transducers which convert an electrical signal to sound. They require a driver circuit (such as a 555 astable) to provide a signal and if this is near their natural (resonant) frequency of about 3kHz they will produce a particularly loud sound.

Piezo transducers require a small current, usually less than 10mA, so they can be connected directly to the outputs of most ICs. They are ideal for buzzes and beeps, but are not suitable for speech or music because they distort the sound. They are sometimes supplied with red and black leads, but they may be connected either way round. PCB-mounting versions are also available.

Piezo transducers can also be used as input transducers for detecting sudden loud noises or impacts, effectively behaving as a crude microphone.

Photograph © Rapid Electronics

capacitor in series to block DC

circuit symbol

Loudspeaker

Loudspeakers are output transducers which convert an electrical signal to sound. Usually they are called 'speakers'. They require a driver circuit, such as a 555 astable or an audio amplifier, to provide a signal. There is a wide range available, but for many electronics projects a 300mW miniature loudspeaker is ideal. This type is about 70mm diameter and it is usually available with resistances of 8 and 64. If a project specifies a 64 speaker you must use this higher resistance to prevent damage to the driving circuit.

Most circuits used to drive loudspeakers produce an audio (AC) signal which is combined with a constant DC signal. The DC will make a large current flow through the speaker due to its low resistance, possibly damaging both the speaker and the driving circuit. To prevent this happening a large value electrolytic capacitor is connected in series with the speaker, this blocks DC but passes audio (AC) signals. See capacitor coupling.

Loudspeakers may be connected either way round except in stereo circuits when the + and - markings on their terminals must be observed to ensure the two speakers are in phase.

Correct polarity must always be observed for large speakers in cabinets because the cabinet may contain a small circuit (a 'crossover network') which diverts the high frequency signals to a small speaker (a 'tweeter') because the large main speaker is poor at reproducing them.

Miniature loudspeakers can also be used as a microphone and they work surprisingly well, certainly good enough for speech in an intercom system for example.


Buzzer (about 400Hz)	Bleeper (about 3kHz)
Photographs © Rapid Electronics
circuit symbol

Buzzer and Bleeper

These devices are output transducers converting electrical energy to sound. They contain an internal oscillator to produce the sound which is set at about 400Hz for buzzers and about 3kHz for bleepers.

Buzzers have a voltage rating but it is only approximate, for example 6V and 12V buzzers can be used with a 9V supply. Their typical current is about 25mA.

Bleepers have wide voltage ranges, such as 3-30V, and they pass a low current of about 10mA.

Buzzers and bleepers must be connected the right way round, their red lead is positive (+).

Inductor (miniature)

Ferrite rod
Photographs © Rapid Electronics

circuit symbol

Inductor (coil)

An inductor is a coil of wire which may have a core of air, iron or ferrite (a brittle material made from iron). Its electrical property is called inductance and the unit for this is the henry, symbol H. 1H is very large so mH and µH are used, 1000µH = 1mH and 1000mH = 1H. Iron and ferrite cores increase the inductance. Inductors are mainly used in tuned circuits and to block high frequency AC signals (they are sometimes called chokes). They pass DC easily, but block AC signals, this is the opposite of capacitors.

Inductors are rarely found in simple projects, but one exception is the tuning coil of a radio receiver. This is an inductor which you may have to make yourself by neatly winding enamelled copper wire around a ferrite rod. Enamelled copper wire has very thin insulation, allowing the turns of the coil to be close together, but this makes it impossible to strip in the usual way - the best method is to gently pull the ends of the wire through folded emery paper.
Warning: a ferrite rod is brittle so treat it like glass, not iron!

An inductor may be connected either way round and no special precautions are required when soldering.

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Variable Resistors

2008-06-30T04:57:00.001-07:00

Variable Resistors

Construction | LIN & LOG | Rheostat | Potentiometer | Presets

Construction

Standard Variable Resistor
Photograph © Rapid Electronics

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.

Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.

The resistance and type of track are marked on the body:
4K7 LIN means 4.7 k linear track.
1M LOG means 1 M logarithmic track.

Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board.

Linear (LIN) and Logarithmic (LOG) tracks

Linear (LIN) track means that the resistance changes at a constant rate as you move the wiper. This is the standard arrangement and you should assume this type is required if a project does not specify the type of track. Presets always have linear tracks.

Logarithmic (LOG) track means that the resistance changes slowly at one end of the track and rapidly at the other end, so halfway along the track is not half the total resistance! This arrangement is used for volume (loudness) controls because the human ear has a logarithmic response to loudness so fine control (slow change) is required at low volumes and coarser control (rapid change) at high volumes. It is important to connect the ends of the track the correct way round, if you find that turning the spindle increases the volume rapidly followed by little further change you should swap the connections to the ends of the track.

Rheostat

Rheostat Symbol

This is the simplest way of using a variable resistor. Two terminals are used: one connected to an end of the track, the other to the moveable wiper. Turning the spindle changes the resistance between the two terminals from zero up to the maximum resistance.

Rheostats are often used to vary current, for example to control the brightness of a lamp or the rate at which a capacitor charges.

If the rheostat is mounted on a printed circuit board you may find that all three terminals are connected! However, one of them will be linked to the wiper terminal. This improves the mechanical strength of the mounting but it serves no function electrically.

Potentiometer

Potentiometer Symbol

Variable resistors used as potentiometers have all three terminals connected.

This arrangement is normally used to vary voltage, for example to set the switching point of a circuit with a sensor, or control the volume (loudness) in an amplifier circuit. If the terminals at the ends of the track are connected across the power supply then the wiper terminal will provide a voltage which can be varied from zero up to the maximum of the supply.

Presets

Preset Symbol

These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. For example to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit. A small screwdriver or similar tool is required to adjust presets.

Presets are much cheaper than standard variable resistors so they are sometimes used in projects where a standard variable resistor would normally be used.

Multiturn presets are used where very precise adjustments must be made. The screw must be turned many times (10+) to move the slider from one end of the track to the other, giving very fine control.


Preset (open style)	Presets (closed style)	Multiturn preset
Photographs © Rapid Electronics

Heat sinks for transistors

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Heat sinks for transistors

Heat sinks are needed for transistors passing large currents.

Why is a heat sink needed?

Heat sink

Photograph © Rapid Electronics

Waste heat is produced in transistors due to the current flowing through them. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air.

The rate of producing waste heat is called the thermal power, P. Usually the base current I_B is too small to contribute much heat, so the thermal power is determined by the collector current I_C and the voltage V_CE across the transistor:

P = I_C × V_CE (see diagram below)

Insulation kit

Heat-conducting paste

Photographs © Rapid Electronics

The heat is not a problem if I_C is small or if the transistor is used as a switch because when 'full on' V_CE is almost zero. However, power transistors used in circuits such as an audio amplifier or a motor speed controller will be partly on most of the time and V_CE may be about half the supply voltage. These power transistors will almost certainly need a heat sink to prevent them overheating.

Power transistors usually have bolt holes for attaching heat sinks, but clip-on heat sinks are also available. Make sure you use the right type for your transistor. Many transistors have metal cases which are connected to one of their leads so it may be necessary to insulate the heat sink from the transistor. Insulating kits are available with a mica sheet and a plastic sleeve for the bolt. Heat-conducting paste can be used to improve heat flow from the transistor to the heat sink, this is especially important if an insulation kit is used.

Heat sink ratings

Heat sinks are rated by their thermal resistance (Rth) in °C/W. For example 2°C/W means the heat sink (and therefore the component attached to it) will be 2°C hotter than the surrounding air for every 1W of heat it is dissipating. Note that a lower thermal resistance means a better heat sink.

This is how you work out the required heat sink rating:

Work out thermal power to be dissipated, P = I_C × V_CE
If in doubt use the largest likely value for I_C and assume that V_CE is half the supply voltage.
For example if a power transistor is passing 1A and connected to a 12V supply, the power P is about 1 × ½ × 12 = 6W.
Find the maximum operating temperature (Tmax) for the transistor if you can, otherwise assume Tmax = 100°C.
Estimate the maximum ambient (surrounding air) temperature (Tair). If the heat sink is going to be outside the case Tair = 25°C is reasonable, but inside it will be higher (perhaps 40°C) allowing for everything to warm up in operation.
Work out the maximum thermal resistance (Rth) for the heat sink using: Rth = (Tmax - Tair) / P
With the example values given above: Rth = (100-25)/6 = 12.5°C/W.
Choose a heat sink with a thermal resistance which is less than the value calculated above (remember lower value means better heat sinking!) for example 5°C/W would be a sensible choice to allow a safety margin. A 5°C/W heat sink dissipating 6W will have a temperature difference of 5 × 6 = 30°C so the transistor temperature will rise to 25 + 30 = 55°C (safely less than the 100°C maximum).
All the above assumes the transistor is at the same temperature as the heat sink. This is a reasonable assumption if they are firmly bolted or clipped together. However, you may have to put a mica sheet or similar between them to provide electrical insulation, then the transistor will be hotter than the heat sink and the calculation becomes more difficult. For typical mica sheets you should subtract 2°C/W from the thermal resistance (Rth) value calculated in step 4 above.

If this all seems too complex you can try attaching a moderately large heat sink and hope for the best. Cautiously monitor the transistor temperature with your finger, if it becomes painfully hot switch off immediately and use a larger heat sink!

Why thermal resistance?

The term 'thermal resistance' is used because it is analagous to electrical resistance:

The temperature difference across the heat sink (between the transistor and air) is like voltage (potential difference) across a resistor.
The thermal power (rate of heat) flowing through the heat sink from transistor to air is like current flowing through a resistor.
So R = V/I becomes Rth = (Tmax - Tair)/P
Just as you need a voltage difference to make current flow, you need a temperature difference to make heat flow.

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Transistors

2008-06-30T04:54:00.000-07:00

Transistors

This page covers practical matters such as precautions when soldering and identifying leads. The operation and use of transistors is covered by the Transistor Circuits page.

Also see: Heat sinks | Transistor Circuits

Function

Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

The amount of current amplification is called the current gain, symbol h_FE.
For further information please see the Transistor Circuits page.

Types of transistor


Transistor circuit symbols

There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors.

The leads are labelled base (B), collector (C) and emitter (E).
These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels!

A Darlington pair is two transistors connected together to give a very high current gain.

In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Transistor leads for some common case styles.

Connecting

Transistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on.

If you are lucky the orientation of the transistor will be clear from the PCB or stripboard layout diagram, otherwise you will need to refer to a supplier's catalogue to identify the leads.

The drawings on the right show the leads for some of the most common case styles.

Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above.

Please see below for a table showing the case styles of some common transistors.

Crocodile clip
Photograph © Rapid Electronics.

Soldering

Transistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink.

Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation.

Heat sink

Photograph © Rapid Electronics

Heat sinks

Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air.

For further information please see the Heat sinks page.

Testing a transistor

Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it:

Testing an NPN transistor

1. Testing with a multimeter

Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range.

Test each pair of leads both ways (six tests in total):

The base-emitter (BE) junction should behave like a diode and conduct one way only.
The base-collector (BC) junction should behave like a diode and conduct one way only.
The collector-emitter (CE) should not conduct either way.

The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.

A simple switching circuit
to test an NPN transistor

2. Testing in a simple switching circuit

Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboard for example. Take care to include the 10k resistor in the base connection or you will destroy the transistor as you test it!

If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released.

To test a PNP transistor use the same circuit but reverse the LED and the supply voltage.

Some multimeters have a 'transistor test' function which provides a known base current and measures the collector current so as to display the transistor's DC current gain h_FE.

Transistor codes

There are three main series of transistor codes used in the UK:

Codes beginning with B (or A), for example BC108, BC478
The first letter B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. The rest of the code identifies the particular transistor. There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type, for example a higher current gain or a different case style. If a project specifies a higher gain version (BC108C) it must be used, but if the general code is given (BC108) any transistor with that code is suitable.
Codes beginning with TIP, for example TIP31A
TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings.
Codes beginning with 2N, for example 2N3053
The initial '2N' identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system.

Choosing a transistor

Most projects will specify a particular transistor, but if necessary you can usually substitute an equivalent transistor from the wide range available. The most important properties to look for are the maximum collector current I_C and the current gain h_FE. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating.

To make a final choice you will need to consult the tables of technical data which are normally provided in catalogues. They contain a great deal of useful information but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows the most important technical data for some popular transistors, tables in catalogues and reference books will usually show additional information but this is unlikely to be useful unless you are experienced. The quantities shown in the table are explained below.

NPN transistors
Code	Structure	Case style	I_C max.	V_CE max.	h_FE min.	P_tot max.	Category (typical use)	Possible substitutes
BC107	NPN	TO18	100mA	45V	110	300mW	Audio, low power	BC182 BC547
BC108	NPN	TO18	100mA	20V	110	300mW	General purpose, low power	BC108C BC183 BC548
BC108C	NPN	TO18	100mA	20V	420	600mW	General purpose, low power
BC109	NPN	TO18	200mA	20V	200	300mW	Audio (low noise), low power	BC184 BC549
BC182	NPN	TO92C	100mA	50V	100	350mW	General purpose, low power	BC107 BC182L
BC182L	NPN	TO92A	100mA	50V	100	350mW	General purpose, low power	BC107 BC182
BC547B	NPN	TO92C	100mA	45V	200	500mW	Audio, low power	BC107B
BC548B	NPN	TO92C	100mA	30V	220	500mW	General purpose, low power	BC108B
BC549B	NPN	TO92C	100mA	30V	240	625mW	Audio (low noise), low power	BC109
2N3053	NPN	TO39	700mA	40V	50	500mW	General purpose, low power	BFY51
BFY51	NPN	TO39	1A	30V	40	800mW	General purpose, medium power	BC639
BC639	NPN	TO92A	1A	80V	40	800mW	General purpose, medium power	BFY51
TIP29A	NPN	TO220	1A	60V	40	30W	General purpose, high power
TIP31A	NPN	TO220	3A	60V	10	40W	General purpose, high power	TIP31C TIP41A
TIP31C	NPN	TO220	3A	100V	10	40W	General purpose, high power	TIP31A TIP41A
TIP41A	NPN	TO220	6A	60V	15	65W	General purpose, high power
2N3055	NPN	TO3	15A	60V	20	117W	General purpose, high power
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
PNP transistors
Code	Structure	Case style	I_C max.	V_CE max.	h_FE min.	P_tot max.	Category (typical use)	Possible substitutes
BC177	PNP	TO18	100mA	45V	125	300mW	Audio, low power	BC477
BC178	PNP	TO18	200mA	25V	120	600mW	General purpose, low power	BC478
BC179	PNP	TO18	200mA	20V	180	600mW	Audio (low noise), low power
BC477	PNP	TO18	150mA	80V	125	360mW	Audio, low power	BC177
BC478	PNP	TO18	150mA	40V	125	360mW	General purpose, low power	BC178
TIP32A	PNP	TO220	3A	60V	25	40W	General purpose, high power	TIP32C
TIP32C	PNP	TO220	3A	100V	10	40W	General purpose, high power	TIP32A
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.

Structure	This shows the type of transistor, NPN or PNP. The polarities of the two types are different, so if you are looking for a substitute it must be the same type.
Case style	There is a diagram showing the leads for some of the most common case styles in the Connecting section above. This information is also available in suppliers' catalogues.
I_C max.	Maximum collector current.
V_CE max.	Maximum voltage across the collector-emitter junction. You can ignore this rating in low voltage circuits.
h_FE	This is the current gain (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Note that current gain is just a number so it has no units. The gain is often quoted at a particular collector current I_C which is usually in the middle of the transistor's range, for example '100@20mA' means the gain is at least 100 at 20mA. Sometimes minimum and maximum values are given. Since the gain is roughly constant for various currents but it varies from transistor to transistor this detail is only really of interest to experts. Why h_FE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here.
P_tot max.	Maximum total power which can be developed in the transistor, note that a heat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifiers, the power is roughly I_C × V_CE. For transistors operating as switches the maximum collector current (I_C max.) is more important.
Category	This shows the typical use for the transistor, it is a good starting point when looking for a substitute. Catalogues may have separate tables for different categories.
Possible substitutes	These are transistors with similar electrical properties which will be suitable substitutes in most circuits. However, they may have a different case style so you will need to take care when placing them on the circuit board.

Darlington pair

This is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. Darlington pairs are sold as complete packages containing the two transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor.

You can make up your own Darlington pair from two transistors.
For example:

For TR1 use BC548B with h_FE1 = 220.
For TR2 use BC639 with h_FE2 = 40.

The overall gain of this pair is h_FE1 × h_FE2 = 220 × 40 = 8800.
The pair's maximum collector current I_C(max) is the same as TR2.

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Switches

2008-06-30T04:52:00.001-07:00

Switches

Switch Contacts - pole, throw etc.
Standard Switches - SPST, SPDT, DPST, DPDT.
Special Switches - multiway, key, tilt, reed etc.

Also see: Relays | Series and Parallel Connections - Switches

Circuit symbol for a
simple on-off switch

Selecting a Switch

There are three important features to consider when selecting a switch:

Contacts (e.g. single pole, double throw)
Ratings (maximum voltage and current)
Method of Operation (toggle, slide, key etc.)

Switch Contacts

Several terms are used to describe switch contacts:

Pole - number of switch contact sets.
Throw - number of conducting positions, single or double.
Way - number of conducting positions, three or more.
Momentary - switch returns to its normal position when released.
Open - off position, contacts not conducting.
Closed - on position, contacts conducting, there may be several on positions.

For example: the simplest on-off switch has one set of contacts (single pole) and one switching position which conducts (single throw). The switch mechanism has two positions: open (off) and closed (on), but it is called 'single throw' because only one position conducts.

Switch Contact Ratings

Switch contacts are rated with a maximum voltage and current, and there may be different ratings for AC and DC. The AC values are higher because the current falls to zero many times each second and an arc is less likely to form across the switch contacts.

For low voltage electronics projects the voltage rating will not matter, but you may need to check the current rating. The maximum current is less for inductive loads (coils and motors) because they cause more sparking at the contacts when switched off.

Standard Switches

Type of Switch	Circuit Symbol	Example
ON-OFF Single Pole, Single Throw = SPST A simple on-off switch. This type can be used to switch the power supply to a circuit. When used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral. Photograph © Rapid Electronics		SPST toggle switch
(ON)-OFF Push-to-make = SPST Momentary A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around ON. This is the standard doorbell switch. Photograph © Rapid Electronics		Push-to-make switch
ON-(OFF) Push-to-break = SPST Momentary A push-to-break switch returns to its normally closed (on) position when you release the button. Photograph © Rapid Electronics		Push-to-break switch
ON-ON Single Pole, Double Throw = SPDT This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position. A SPDT toggle switch may be used as a simple on-off switch by connecting to COM and one of the A or B terminals shown in the diagram. A and B are interchangeable so switches are usually not labelled. ON-OFF-ON SPDT Centre Off A special version of the standard SPDT switch. It has a third switching position in the centre which is off. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released. Photographs © Rapid Electronics		SPDT toggle switch SPDT slide switch (PCB mounting) SPDT rocker switch
Dual ON-OFF Double Pole, Single Throw = DPST A pair of on-off switches which operate together (shown by the dotted line in the circuit symbol). A DPST switch is often used to switch mains electricity because it can isolate both the live and neutral connections. Photograph © Rapid Electronics		DPST rocker switch
Dual ON-ON Double Pole, Double Throw = DPDT A pair of on-on switches which operate together (shown by the dotted line in the circuit symbol). A DPDT switch can be wired up as a reversing switch for a motor as shown in the diagram. ON-OFF-ON DPDT Centre Off A special version of the standard SPDT switch. It has a third switching position in the centre which is off. This can be very useful for motor control because you have forward, off and reverse positions. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released. Photograph © Rapid Electronics		DPDT slide switch Wiring for Reversing Switch
Rapid Electronics stock a wide range of switches and they have kindly allowed me to use their photographs on this page. The photographs are from their Image Gallery CD-ROM.

Special Switches

Type of Switch	Example
Push-Push Switch (e.g. SPST = ON-OFF) This looks like a momentary action push switch but it is a standard on-off switch: push once to switch on, push again to switch off. This is called a latching action. Photograph © Rapid Electronics
Microswitch (usually SPDT = ON-ON) Microswitches are designed to switch fully open or closed in response to small movements. They are available with levers and rollers attached. Photograph © Rapid Electronics
Keyswitch A key operated switch. The example shown is SPST. Photograph © Rapid Electronics
Tilt Switch (SPST) Tilt switches contain a conductive liquid and when tilted this bridges the contacts inside, closing the switch. They can be used as a sensor to detect the position of an object. Some tilt switches contain mercury which is poisonous. Photograph © Rapid Electronics
Reed Switch (usually SPST) The contacts of a reed switch are closed by bringing a small magnet near the switch. They are used in security circuits, for example to check that doors are closed. Standard reed switches are SPST (simple on-off) but SPDT (changeover) versions are also available. Warning: reed switches have a glass body which is easily broken! For advice on handling please see the Electronics in Meccano website. Photograph © Rapid Electronics
DIP Switch (DIP = Dual In-line Parallel) This is a set of miniature SPST on-off switches, the example shown has 8 switches. The package is the same size as a standard DIL (Dual In-Line) integrated circuit. This type of switch is used to set up circuits, e.g. setting the code of a remote control. Photograph © Rapid Electronics
Multi-pole Switch The picture shows a 6-pole double throw switch, also known as a 6-pole changeover switch. It can be set to have momentary or latching action. Latching action means it behaves as a push-push switch, push once for the first position, push again for the second position etc. Photograph © Rapid Electronics
Multi-way Switch Multi-way switches have 3 or more conducting positions. They may have several poles (contact sets). A popular type has a rotary action and it is available with a range of contact arrangements from 1-pole 12-way to 4-pole 3 way. The number of ways (switch positions) may be reduced by adjusting a stop under the fixing nut. For example if you need a 2-pole 5-way switch you can buy the 2-pole 6-way version and adjust the stop. Contrast this multi-way switch (many switch positions) with the multi-pole switch (many contact sets) described above. Photograph © Rapid Electronics	Multi-way rotary switch 1-pole 4-way switch symbol

Resistor Colour Code Calculator

2008-06-30T04:50:00.000-07:00

Resistor Colour Code Calculator

The Resistor Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

The Resistor Colour Code Calculator can be used to identify resistors. It consists of three card discs showing the colours and values, these are fastened together so you can simply turn the discs to select the value or colour code required. Simple but effective!

There are two versions to download and print on A4 white card (two per sheet):

Coloured (for a colour printer)
Black and White (for a black only printer)
This version must be coloured manually, it is easiest to do this before cutting out.

To make the calculator, carefully cut out the three discs and fasten them together with a small brass paper fastener.

The calculator design is copyright but it may be freely copied for educational purposes.

The Resistor Colour Code Calculator is supplied as a PDF file. To view and print PDF files you need an Acrobat Reader which may be downloaded free for Windows, Mac, RISC OS, or UNIX/Linux computers. If you are not sure which type of computer you have it is probably Windows.

Resistors

2008-06-30T04:49:00.000-07:00

Resistors

Colour Code | Tolerance | Real Values (E6 & E12 series) | Power Rating

Also see: Resistance | Ohm's Law

Example: Circuit symbol:

Function

Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.

Connecting and soldering

Resistors may be connected either way round. They are not damaged by heat when soldering.

The Resistor Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

Resistor values - the resistor colour code

Resistance is measured in ohms, the symbol for ohm is an omega .
1 is quite small so resistor values are often given in k and M.
1 k = 1000 1 M = 1000000 .

Resistor values are normally shown using coloured bands.
Each colour represents a number as shown in the table.

Most resistors have 4 bands:

The first band gives the first digit.
The second band gives the second digit.
The third band indicates the number of zeros.
The fourth band is used to shows the tolerance (precision) of the resistor, this may be ignored for almost all circuits but further details are given below.

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.
So its value is 270000 = 270 k.
On circuit diagrams the is usually omitted and the value is written 270K.

Find out how to make your own Resistor Colour Code Calculator

Small value resistors (less than 10 ohm)

The standard colour code cannot show values of less than 10. To show these small values two special colours are used for the third band: gold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.

For example:
red, violet, gold bands represent 27 × 0.1 = 2.7
green, blue, silver bands represent 56 × 0.01 = 0.56

Tolerance of resistors (fourth band of colour code)

The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 resistor with a tolerance of ±10% will have a value within 10% of 390, between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390).

A special colour code is used for the fourth band tolerance:
silver ±10%, gold ±5%, red ±2%, brown ±1%.
If no fourth band is shown the tolerance is ±20%.

Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.

Resistor shorthand

Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.

For example:

560R

2K7

39K

1M0

Real resistor values (the E6 and E12 series)

You may have noticed that resistors are not available with every possible value, for example 22k and 47k are readily available, but 25k and 50k are not!

Why is this? Imagine that you decided to make resistors every 10 giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits. In fact it would be difficult to make resistors sufficiently accurate.

To produce a sensible range of resistor values you need to increase the size of the 'step' as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value.

The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance)
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc.
Notice how this is the E6 series with an extra value in the gaps.

The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390 could vary by ±10% × 390 = ±39, so it could be any value between 351 and 429.

Resistors in Series and Parallel

For information on resistors connected in series and parallel please see the Resistance page,

Power Ratings of Resistors

High power resistors
(5W top, 25W bottom)

Photographs © Rapid Electronics

Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.

Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300) or high voltages (more than 15V).

The power, P, developed in a resistor is given by:

P = I² × R
or
P = V² / R

where:

P = power developed in the resistor in watts (W)
I = current through the resistor in amps (A)
R = resistance of the resistor in ohms ()
V = voltage across the resistor in volts (V)

Examples:

A 470 resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W.
In this case a standard 0.25W resistor would be suitable.
A 27 resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W.
A high power resistor with a rating of 5W would be suitable.

Relays

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Relays

Choosing a relay | Protection diodes | Reed relays | Advantages & disadvantages

Also see: Switches | Diodes

Circuit symbol for a relay

Relays

Photographs © Rapid Electronics

Relay showing coil and switch contacts

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.

Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches.

Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay.

The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.

The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.

The relay's switch connections are usually labelled COM, NC and NO:

COM = Common, always connect to this, it is the moving part of the switch.
NC = Normally Closed, COM is connected to this when the relay coil is off.
NO = Normally Open, COM is connected to this when the relay coil is on.

Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.

Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.

Choosing a relay

You need to consider several features when choosing a relay:

Physical size and pin arrangement
If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue.
Coil voltage
The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.
Coil resistance
The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current:

Relay coil current = supply voltage

coil resistance

For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current.
Switch ratings (voltage and current)
The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".
Switch contact arrangement (SPDT, DPDT etc)
Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches.

Protection diodes for relays

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection.

Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

Reed relays

Reed Relay

Photograph © Rapid Electronics

Reed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch.

Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of supply voltages (9-20V for example). They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example).

The reed relay shown in the photograph will plug into a standard 14-pin DIL socket ('IC holder').

For further information about reed switches please see the page on switches.

Relays and transistors compared

Like relays, transistors can be used as an electrically operated switch. For switching small DC currents (< 1A) at low voltage they are usually a better choice than a relay. However transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! The main advantages and disadvantages of relays are listed below:

Advantages of relays:

Relays can switch AC and DC, transistors can only switch DC.
Relays can switch high voltages, transistors cannot.
Relays are a better choice for switching large currents (> 5A).
Relays can switch many contacts at once.

Disadvantages of relays:

Relays are bulkier than transistors for switching small currents.
Relays cannot switch rapidly (except reed relays), transistors can switch many times per second.
Relays use more power due to the current flowing through their coil.
Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil.

Further information

For further information about relays please see the Electronics in Meccano website.

http://www.kpsec.freeuk.com/

Light Emitting Diodes (LEDs)

2008-06-30T04:46:00.001-07:00

Light Emitting Diodes (LEDs)

Example: Circuit symbol:

Function

LEDs emit light when an electric current passes through them.

Connecting and soldering

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).

LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.

Testing an LED

Never connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.

LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

For an accurate value please see Calculating an LED resistor value below.

Colours of LEDs

LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours.

The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent.

Tri-colour LEDs

The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.

The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour.

Bi-colour LEDs

A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.

Sizes, Shapes and Viewing angles of LEDs

LED Clip

Photograph © Rapid Electronics

LEDs are available in a wide variety of sizes and shapes. The 'standard' LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular.

Round cross-section LEDs are frequently used and they are very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if necessary. LED clips are also available to secure LEDs in holes. Other cross-section shapes include square, rectangular and triangular.

As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less.

Rapid Electronics stock a wide selection of LEDs and their catalogue is a good guide to the range available.

Calculating an LED resistor value

An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.

The resistor value, R is given by:

R = (V_S - V_L) / I

V_S = supply voltage
V_L = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted

If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright.

For example

If the supply voltage V_S = 9V, and you have a red LED (V_L = 2V), requiring a current I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater).

Working out the LED resistor formula using Ohm's law

Ohm's law says that the resistance of the resistor, R = V/I, where:
V = voltage across the resistor (= V_S - V_L in this case)
I = the current through the resistor

So R = (V_S - V_L) / I

For more information on the calculations please see the Ohm's Law page.

Connecting LEDs in series

If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED.

All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for V_L.

Example calculations:
A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal.
V_L = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage V_S is 9V and the current I must be 15mA = 0.015A,
Resistor R = (V_S - V_L) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200,
so choose R = 220 (the nearest standard value which is greater).

Avoid connecting LEDs in parallel!

Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea.

If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

Reading a table of technical data for LEDs

Suppliers' catalogues usually include tables of technical data for components such as LEDs. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used.

The table below shows typical technical data for some 5mm diameter round LEDs with diffused packages (plastic bodies). Only three columns are important and these are shown in bold. Please see below for explanations of the quantities.

Type	Colour	I_F max.	V_F typ.	V_F max.	V_R max.	Luminous intensity	Viewing angle	Wavelength
Standard	Red	30mA	1.7V	2.1V	5V	5mcd @ 10mA	60°	660nm
Standard	Bright red	30mA	2.0V	2.5V	5V	80mcd @ 10mA	60°	625nm
Standard	Yellow	30mA	2.1V	2.5V	5V	32mcd @ 10mA	60°	590nm
Standard	Green	25mA	2.2V	2.5V	5V	32mcd @ 10mA	60°	565nm
High intensity	Blue	30mA	4.5V	5.5V	5V	60mcd @ 20mA	50°	430nm
Super bright	Red	30mA	1.85V	2.5V	5V	500mcd @ 20mA	60°	660nm
Low current	Red	30mA	1.7V	2.0V	5V	5mcd @ 2mA	60°	625nm

I_F max.	Maximum forward current, forward just means with the LED connected correctly.
V_F typ.	Typical forward voltage, V_L in the LED resistor calculation. This is about 2V, except for blue and white LEDs for which it is about 4V.
V_F max.	Maximum forward voltage.
V_R max.	Maximum reverse voltage You can ignore this for LEDs connected the correct way round.
Luminous intensity	Brightness of the LED at the given current, mcd = millicandela.
Viewing angle	Standard LEDs have a viewing angle of 60°, others emit a narrower beam of about 30°.
Wavelength	The peak wavelength of the light emitted, this determines the colour of the LED. nm = nanometre.

Flashing LEDs

Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit.

LED Displays

LED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). The pictures below illustrate some of the popular designs:


Bargraph	7-segment	Starburst	Dot matrix
Photographs © Rapid Electronics

Pin connections of LED displays

Pin connections diagram
© Rapid Electronics

There are many types of LED display and a supplier's catalogue should be consulted for the pin connections. The diagram on the right shows an example from the Rapid Electronics catalogue. Like many 7-segment displays, this example is available in two versions: Common Anode (SA) with all the LED anodes connected together and Common Cathode (SC) with all the cathodes connected together. Letters a-g refer to the 7 segments, A/C is the common anode or cathode as appropriate (on 2 pins). Note that some pins are not present (NP) but their position is still numbered. Also see: Display Drivers.

http://www.kpsec.freeuk.com/

Lamps

2008-06-30T04:44:00.002-07:00

Lamps

Function | Symbols | Selecting | Types of lamp | Connecting

Function and Construction

Lamps emit light when an electric current passes through them. All of the lamps shown on this page have a thin wire filament which becomes very hot and glows brightly when a current passes through it. The filament is made from a metal with a high melting point such as tungsten and it is usually wound into a small coil. Filament lamps have a shorter lifetime than most electronic components because eventually the filament 'blows' (melts) at a weak point.

Circuit symbols

There are two circuit symbols for a lamp, one for a lamp used to provide illumination and another for a lamp used as an indicator. Small lamps such as torch bulbs can be used for both purposes so either circuit symbol may used in simple educational circuits.


Lamp used for lighting (for example a car headlamp or torch bulb)	Lamp used as an indicator (for example a warning light on a car dashboard)

Selecting a Lamp

There are three important features to consider when selecting a lamp:

Voltage rating - the supply voltage for normal brightness.
Power or current rating - small lamps are usually rated by current.
Lamp type - please see the table below.

The voltage and power (or current) ratings are usually printed or embossed on the body of a lamp.

Voltage rating

This is the supply voltage required for normal brightness. If a slightly higher voltage is used the lamp will be brighter but its lifetime will be shorter. With a lower supply voltage the lamp will be dimmer and its lifetime will be longer. The light from dim lamps has a yellow-orange colour.

Torch lamps pass a relatively large current and this significantly reduces the output voltage of the battery. Some voltage is used up inside the battery driving the large current through the small resistance of the battery itself (its 'internal resistance'). As a result the correct voltage rating for a torch lamp is lower than the normal voltage of the battery which lights it!

For example: a lamp rated 3.5V 0.3A is correct for a 4.5V battery (three 1.5V cells) because when the lamp is connected the voltage across the battery falls to about 3.5V.

Power or current rating

This is the power or current for the lamp when connected to its rated voltage. Low power lamps are usually rated by their current and high power lamps by their power. It is easy to convert between the two ratings:

P = I × V
or
I = P / V

where:

P = power in watts (W)
I = current in amps (A)
V = voltage in volts (V)

Examples:

A lamp rated 3.5V 0.3A has a power rating P = I × V = 0.3 × 3.5 = 1.05W
A lamp rated 6V 0.06A has a power rating P = I × V = 0.06 × 6 = 0.36W
A lamp rated 12W 2.4W has a current rating I = P / V = 2.4 / 12 = 0.2A

Lamp Type

Type of Lamp	Example
MES Miniature Edison Screw These are the standard small lamps. The bulb diameter is usually about 10mm, but tubular bulbs are also available. MES lamps have one contact on the base and the body forms the other contact. They are available with a good range of voltage and power (or current) ratings. Lens ended versions are available to produce a focused beam of light. LES Lilliput Edison Screw Smaller than MES, these have a bulb diameter of about 5mm. Photograph © Rapid Electronics
MCC Miniature Centre Contact These have a bayonet style fitting, like a standard mains lamp in the UK. They have one contact on the base and the body forms the other contact. The bulb diameter is about 10mm. Photograph © Rapid Electronics
SBC Small Bayonet Cap These have a bayonet style fitting, like a standard mains lamp in the UK. They have two contacts on the base so the metal body is not connected in the circuit. SBC lamps have high power ratings (24W for example) and their bulbs are large with a diameter of up to about 40mm. Note the two filament arrangements in the lamps shown, horizontal on the left, vertical on the right. Photograph © Rapid Electronics
Pre-focus This type of lamp is used in torches and lanterns. The flange at the top of the metal body is used to hold the lamp in place. Lampholders are not readily available so this type is unsuitable for most projects. Photograph © Rapid Electronics
Wire ended These are very small lamps with a bulb about 3mm diameter and 6mm long. Take care to avoid snapping the wires where they enter the glass bulb. Photograph © Rapid Electronics
Grain of Wheat These are similar to the wire ended lamps above but they have stranded wire leads usually about 150mm long. The bulb is about 3mm diameter and 6mm long - the size of a grain of wheat! Photograph © Rapid Electronics
Rapid Electronics stock a wide range of lamps and they have kindly allowed me to use their photographs on this page. The photographs are from their Image Gallery CD-ROM.


screw terminals	solder tags
Lampholders Photographs © Rapid Electronics

Connecting and soldering

Lamps may be connected either way round in a circuit and the supply may be AC or DC.

Most lamps are designed to be used in a lampholder but the small 'wire ended' and 'grain of wheat' lamps have wires which may be soldered directly onto a circuit board.

Lampholders usually have screw terminals or solder tags to attach wires. Some small holders have contacts which may be soldered directly to a circuit board.

Lamps in Series

Several lamps can be successfully connected in series provided they all have identical voltage and power (or current) ratings. The supply voltage is divided equally between identical lamps so their voltage rating must be suitable for this. For example Christmas tree lights may have 20 lamps connected in series to a 240V supply, so each lamp will have 240V ÷ 20 = 12V across it.

A disadvantage of connecting lamps in series is that if one lamp blows all of them will go out because the circuit is broken. Christmas tree lamps have a special feature to overcome this problem; they are designed to short circuit (conduct like a wire link) when they blow, so the circuit is not broken and the other lamps remain lit, making it easier to locate the faulty lamp. Sets also include one 'fuse' lamp which blows normally.

WARNING! The Christmas tree lamps may seem safe because they use only 12V but they are connected to the mains supply which can be lethal. Always unplug from the mains before changing lamps. The voltage across the holder of a missing lamp is the full 240V of the mains supply! (Yes, it really is!)

http://www.kpsec.freeuk.com/

74 Series Logic ICs

2008-06-30T04:44:00.001-07:00

74 Series Logic ICs

HC & HCT families | LS family | Open collector outputs

Gates: 2-input | 3-input | 4-input | 8-input | NOT
Decade and 4-bit counters: 7490 | 7493 | 74390 | 74393 | 74160-3 | 74192-3 | 74HC4017
12-bit and 14-bit counters: 74HC4020 | 74HC4040 | 74HC4060
Decoders & display drivers: 7442 | 7447 | 74HC4511

Also see: 4000 Series | Logic Gates | Counting Circuits | ICs (chips) (with summary of logic ICs)

Quick links to
individual ICs

7400 7432
7402 7442
7403 7447
7404 7486
7405 7490
7408 7493
7409 74132
7410 74160
7411 74161
7412 74162
7414 74163
7420 74192
7421 74193
7427 74390
7430 74393

74HC4017
74HC4020
74HC4040
74HC4060
74HC4511

General characteristics

There are several families of logic ICs numbered from 74xx00 onwards with letters (xx) in the middle of the number to indicate the type of circuitry, eg 74LS00 and 74HC00. The original family (now obsolete) had no letters, eg 7400.

The 74LS (Low-power Schottky) family (like the original) uses TTL (Transistor-Transistor Logic) circuitry which is fast but requires more power than later families. The 74 series is often still called the 'TTL series' even though the latest ICs do not use TTL!

The 74HC family has High-speed CMOS circuitry, combining the speed of TTL with the very low power consumption of the 4000 series. They are CMOS ICs with the same pin arrangements as the older 74LS family. Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead.

The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower immunity to noise, but this is unlikely to be a problem in most situations.

The CMOS circuitry used in the 74HC and 74HCT series ICs means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them.

To compare the different logic families please see the Summary table of logic families

For most new projects the 74HC family is the best choice.
The 74LS and 74HCT families require a 5V supply so they are not convenient for battery operation.

74HC and 74HCT family characteristics:

74HC Supply: 2 to 6V, small fluctuations are tolerated.
74HCT Supply: 5V ±0.5V, a regulated supply is best.
Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit!
Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible. For reliability use 74HCT if the system includes some 74LS ICs.
Outputs can sink and source about 4mA if you wish to maintain the correct output voltage to drive logic inputs, but if there is no need to drive any inputs the maximum current is about 20mA. To switch larger currents you can connect a transistor.
Fan-out: one output can drive many inputs (50+), except 74LS inputs because these require a higher current and only 10 can be driven.
Gate propagation time: about 10ns for a signal to travel through a gate.
Frequency: up to 25MHz.
Power consumption (of the IC itself) is very low, a few µW. It is much greater at high frequencies, a few mW at 1MHz for example.

74LS family TTL characteristics:

Supply: 5V ±0.25V, it must be very smooth, a regulated supply is best. In addition to the normal supply smoothing, a 0.1µF capacitor should be connected across the supply near the IC to remove the 'spikes' generated as it switches state, one capacitor is needed for every 4 ICs.
Inputs 'float' high to logic 1 if unconnected, but do not rely on this in a permanent (soldered) circuit because the inputs may pick up electrical noise. 1mA must be drawn out to hold inputs at logic 0. In a permanent circuit it is wise to connect any unused inputs to +Vs to ensure good immunity to noise.
Outputs can sink up to 16mA (enough to light an LED), but they can source only about 2mA. To switch larger currents you can connect a transistor.
Fan-out: one output can drive up to 10 74LS inputs, but many more 74HCT inputs.
Gate propagation time: about 10ns for a signal to travel through a gate.
Frequency: up to about 35MHz (under the right conditions).
Power consumption (of the IC itself) is a few mW.

Open Collector Outputs

Some 74 series ICs have open collector outputs, this means they can sink current but they cannot source current. They behave like an NPN transistor switch.

The diagram shows how an open collector output can be connected to sink current from a supply which has a higher voltage than the logic IC supply. The maximum load supply is 15V for most open collector ICs.

Open collector outputs can be safely connected together to switch on a load when any one of them is low; unlike normal outputs which must be combined using diodes.

There are many ICs in the 74 series and this page only covers a selection, concentrating on the most useful gates, counters, decoders and display drivers. For each IC there is a diagram showing the pin arrangement and brief notes explain the function of the pins where necessary. For simplicity the family letters after the 74 are omitted in the diagrams below because the pin connections apply to all 74 series ICs with the same number. For example 7400 NAND gates are available as 74HC00, 74HCT00 and 74LS00.

If you are using another reference please be aware that there is some variation in the terms used to describe pin functions, for example reset is also called clear. Some inputs are 'active low' which means they perform their function when low. If you see a line drawn above a label it means it is active low, for example: (say 'reset-bar').

Datasheets are available from:

Gates

Quad 2-input gates

7400 quad 2-input NAND
7403 quad 2-input NAND with open collector outputs
7408 quad 2-input AND
7409 quad 2-input AND with open collector outputs
7432 quad 2-input OR
7486 quad 2-input EX-OR
74132 quad 2-input NAND with Schmitt trigger inputs

The 74132 has Schmitt trigger inputs to provide good noise immunity. They are ideal for slowly changing or noisy signals.

7402 quad 2-input NOR
Note the unusual gate layout.

Triple 3-input gates

7410 triple 3-input NAND
7411 triple 3-input AND
7412 triple 3-input NAND with open collector outputs
7427 triple 3-input NOR

Notice how gate 1 is spread across the two sides of the package.

Dual 4-input gates

7420 dual 4-input NAND
7421 dual 4-input AND

NC = No Connection (a pin that is not used).

7430 8-input NAND gate

NC = No Connection (a pin that is not used).

Hex NOT gates

7404 hex NOT
7405 hex NOT with open collector outputs
7414 hex NOT with Schmitt trigger inputs

The 7414 has Schmitt trigger inputs to provide good noise immunity. They are ideal for slowly changing or noisy signals.

Counters

7490 decade (0-9) ripple counter
7493 4-bit (0-15) ripple counter

NC = No Connection (a pin that is not used).
# on the 7490 pins 6 and 7 connect to an
internal AND gate for resetting to 9.

For normal use connect QA to clockB and
connect the external clock signal to clockA.

These are ripple counters so beware that glitches may occur in any logic gate systems connected to their outputs due to the slight delay before the later counter outputs respond to a clock pulse.

The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means a counter output can directly drive the clock input of the next counter in a chain.

The counter is in two sections: clockA-QA and clockB-QB-QC-QD. For normal use connect QA to clockB to link the two sections, and connect the external clock signal to clockA.

For normal operation at least one reset0 input should be low, making both high resets the counter to zero (0000, QA-QD low). Note that the 7490 has a pair of reset9 inputs on pins 6 and 7, these reset the counter to nine (1001) so at least one of them must be low for counting to occur.

Counting to less than the maximum (9 or 15) can be achieved by connecting the appropriate output(s) to the two reset0 inputs. If only one reset input is required the two inputs can be connected together. For example: to count 0 to 8 connect QA (1) and QD (8) to the reset inputs.

Connecting ripple counters in a chain: please see 74393 below.

74390 dual decade (0-9) ripple counter

For normal use connect QA to clockB and
connect the external clock signal to clockA.

The 74390 contains two separate decade (0 to 9) counters, one on each side of the IC. They are ripple counters so beware that glitches may occur in any logic gate systems connected to their outputs due to the slight delay before the later counter outputs respond to a clock pulse.

Each counter is in two sections: clockA-QA and clockB-QB-QC-QD. For normal use connect QA to clockB to link the two sections, and connect the external clock signal to clockA.

For normal operation the reset input should be low, making it high resets the counter to zero (0000, QA-QD low).

Counting to less than 9 can be achieved by connecting the appropriate output(s) to the reset input, using an AND gate if necessary. For example: to count 0 to 7 connect QD (8) to reset, to count 0 to 8 connect QA (1) and QD (8) to reset using an AND gate.

Connecting ripple counters in a chain: please see 74393 below.

74393 dual 4-bit (0-15) ripple counter

The 74393 contains two separate 4-bit (0 to 15) counters, one on each side of the IC. They are ripple counters so beware that glitches may occur in logic systems connected to their outputs due to the slight delay before the later outputs respond to a clock pulse.

The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means means a counter output can directly drive the clock input of the next counter in a chain.

For normal operation the reset input should be low, making it high resets the counter to zero (0000, QA-QD low).

Counting to less than 15 can be achieved by connecting the appropriate output(s) to the reset input, using an AND gate if necessary. For example to count 0 to 8 connect QA (1) and QD (8) to reset using an AND gate.

Connecting ripple counters in a chain
The diagram below shows how to link ripple counters in a chain, notice how the highest output QD of each counter drives the clock input of the next counter.

74160-3 synchronous counters

* reset and preset are both active-low
preset is also known as parallel enable (PE)

74160 synchronous decade counter (standard reset)
74161 synchronous 4-bit counter (standard reset)
74162 synchronous decade counter (synchronous reset)
74163 synchronous 4-bit counter (synchronous reset)

These are synchronous counters so their outputs change precisely together on each clock pulse. This is helpful if you need to connect their outputs to logic gates because it avoids the glitches which occur with ripple counters.

The count advances as the clock input becomes high (on the rising-edge). The decade counters count from 0 to 9 (0000 to 1001 in binary). The 4-bit counters count from 0 to 15 (0000 to 1111 in binary).

For normal operation (counting) the reset, preset, count enable and carry in inputs should all be high. When count enable is low the clock input is ignored and counting stops.

The counter may be preset by placing the desired binary number on the inputs A-D, making the preset input low, and applying a positive pulse to the clock input. The inputs A-D may be left unconnected if not required.

The reset input is active-low so it should be high (+Vs) for normal operation (counting). When low it resets the count to zero (0000, QA-QD low), this happens immediately with the 74160 and 74161 (standard reset), but with the 74162 and 74163 (synchronous reset) the reset occurs on the rising-edge of the clock input.

Counting to less than the maximum (15 or 9) can be achieved by connecting the appropriate output(s) through a NOT or NAND gate to the reset input. For the 74162 and 74163 (synchronous reset) you must use the output(s) representing one less than the reset count you require, e.g. to reset on 7 (counting 0 to 6) use QB (2) and QC (4).

Connecting synchronous counters in a chain
The diagram below shows how to link synchronous counters such as 74160-3, notice how all the clock (CK) inputs are linked. Carry out (CO) is used to feed the carry in (CI) of the next counter. Carry in (CI) of the first 74160-3 counter should be high.

74192 up/down decade (0-9) counter
74193 up/down 4-bit (0-15) counter

* preset is active-low

These counters have separate clock inputs for counting up and down. The count increases as the up clock input becomes high (on the rising-edge). The count decreases as the down clock input becomes high (on the rising-edge). In both cases the other clock input should be high.

For normal operation (counting) the preset input should be high and the reset input low. When the reset input is high it resets the count to zero (0000, QA-QD low)

The counter may be preset by placing the desired binary number on the inputs A-D and briefly making the preset input low. Note that a clock pulse is not required to preset, unlike the 74160-3 counters. The inputs A-D may be left unconnected if not required.

Connecting counters with separate up and down clock inputs in a chain
The diagram below shows how to link 74192-3 up/down counters with separate up and down clock inputs, notice how carry and borrow are connected to the up clock and down clock inputs respectively of the next counter.

74HC4017 decade counter (1-of-10)
74HC4020 14-bit ripple counter
74HC4040 12-bit ripple counter
74HC4060 14-bit ripple counter with internal oscillator

These are the 74HC equivalents of 4000 series CMOS counters. Like all 74HC ICs they need a power supply of 2 to 6V. For pin connections and functions please see: 4017 | 4020 | 4040 | 4060

Decoders

7442 BCD to decimal (1 of 10) decoder

The 7442 outputs are active-low which means they become low when selected but are high at other times. They can sink up to about 20mA.

The appropriate output becomes low in response to the BCD (binary coded decimal) input. For example an input of binary 0101 (=5) will make output Q5 low and all other outputs high.

The 7442 is a BCD (binary coded decimal) decoder intended for input values 0 to 9 (0000 to 1001 in binary). With inputs from 10 to 15 (1010 to 1111 in binary) all outputs are high.

Note that the 7442 can be used as a 1-of-8 decoder if input D is held low.

Also see: 74HC4017 and 4017 both are a decade counter and 1-of-10 decoder in a single IC.

7-segment Display Drivers

7447 BCD to 7-segment display driver

The appropriate outputs a-g become low to display the BCD (binary coded decimal) number supplied on inputs A-D. The 7447 has open collector outputs a-g which can sink up to 40mA. The 7-segment display segments must be connected between +Vs and the outputs with a resistor in series (330 with a 5V supply). A common anode display is required.

Display test and blank input are active-low so they should be high for normal operation. When display test is low all the display segments should light (showing number 8).

If the blank input is low the display will be blank when the count input is zero (0000). This can be used to blank leading zeros when there are several display digits driven by a chain of counters. To achieve this blank output should be connected to blank input of the next display down the chain (the next most significant digit).

The 7447 is intended for BCD (binary coded decimal) which is input values 0 to 9 (0000 to 1001 in binary). Inputs from 10 to 15 (1010 to 1111 in binary) will light odd display segments but will do no harm.

74HC4511 BCD to 7-segment display driver

This is the 74HC equivalent of the CMOS 4511 display driver. Like all 74HC ICs it needs a power supply of 2 to 6V. For pin connections and functions please see 4511.

4000 series CMOS Logic ICs

2008-06-30T04:43:00.001-07:00

4000 series CMOS Logic ICs

Gates: 2-input | 3-input | 4-input | 8-input | 4069 NOT | 4049 NOT | 4050 Buffer | 4000
Decade and 4-bit counters: 4017 | 4026 | 4029 | 4510 | 4516 | 4518 | 4520
7-bit, 12-bit & 14-bit counters: 4020 | 4024 | 4040 | 4060
Decoders and display drivers: 4028 | 4511

Also see: 74 Series | Logic Gates | Counting Circuits | ICs (chips) (with summary of logic ICs)

Quick links to
individual ICs

4000 4060
4001 4068
4002 4069
4011 4070
4012 4071
4017 4072
4020 4073
4023 4075
4024 4077
4025 4081
4026 4082
4028 4093
4029 4510
4030 4511
4040 4516
4049 4518
4050 4520

General characteristics

Supply: 3 to 15V, small fluctuations are tolerated.
Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit!
Outputs can sink and source only about 1mA if you wish to maintain the correct output voltage to drive CMOS inputs. If there is no need to drive any inputs the maximum current is about 5mA with a 6V supply, or 10mA with a 9V supply (just enough to light an LED). To switch larger currents you can connect a transistor.
Fan-out: one output can drive up to 50 inputs.
Gate propagation time: typically 30ns for a signal to travel through a gate with a 9V supply, it takes a longer time at lower supply voltages.
Frequency: up to 1MHz, above that the 74 series is a better choice.
Power consumption (of the IC itself) is very low, a few µW. It is much greater at high frequencies, a few mW at 1MHz for example.

There are many ICs in the 4000 series and this page only covers a selection, concentrating on the most useful gates, counters, decoders and display drivers. For each IC there is a diagram showing the pin arrangement and brief notes explain the function of the pins where necessary. The notes also explain if the IC's properties differ substantially from the standard characteristics listed above.

If you are using another reference please be aware that there is some variation in the terms used to describe input pins. I have tried to be logically consistent so the term I have used describes the pin's function when high (true). For example 'disable clock' on the 4026 is often labelled 'clock enable' but this can be confusing because it enables the clock when low (false). An input described as 'active low' is like this, it performs its function when low. If you see a line drawn above a label it means it is active low, for example: (say 'reset-bar').

Datasheets are available from:

Static precautions

The CMOS circuitry means that 4000 series ICs are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them.

Gates

Quad 2-input gates

4001 quad 2-input NOR
4011 quad 2-input NAND
4030 quad 2-input EX-OR (now obsolete)
4070 quad 2-input EX-OR
4071 quad 2-input OR
4077 quad 2-input EX-NOR
4081 quad 2-input AND
4093 quad 2-input NAND with Schmitt trigger inputs

The 4093 has Schmitt trigger inputs to provide good noise immunity. They are ideal for slowly changing or noisy signals. The hysteresis is about 0.5V with a 4.5V supply and almost 2V with a 9V supply.

Triple 3-input gates

4023 triple 3-input NAND
4025 triple 3-input NOR
4073 triple 3-input AND
4075 triple 3-input OR

Notice how gate 1 is spread across the two ends of the package.

Dual 4-input gates

4002 dual 4-input NOR
4012 dual 4-input NAND
4072 dual 4-input OR
4082 dual 4-input AND

NC = No Connection (a pin that is not used).

4068 8-input NAND/AND* gate

This gate has a propagation time which is about 10 times longer than normal so it is not suitable for high speed circuits.

NC = No Connection (a pin that is not used).

* = The AND output (pin 1) is not available on some versions of the 4068.

4069 hex NOT (inverting buffer)

4049 hex NOT and 4050 hex buffer

4049 hex NOT (inverting buffer)
4050 hex non-inverting buffer

Inputs: These ICs are unusual because their gate inputs can withstand up to +15V even if the power supply is a lower voltage.

Outputs: These ICs are unusual because they are capable of driving 74LS gate inputs directly. To do this they must have a +5V supply (74LS supply voltage). The gate output is sufficient to drive four 74LS inputs.

NC = No Connection (a pin that is not used).

Note the unusual arrangement of the power supply pins for these ICs!

4000 dual 3-input NOR gate and NOT gate

Two 3-input NOR gates and a single NOT gate in one package.

NC = No Connection (a pin that is not used).

Decade and 4-bit Counters

4017 decade counter (1-of-10)

The count advances as the clock input becomes high (on the rising-edge). Each output Q0-Q9 goes high in turn as counting advances. For some functions (such as flash sequences) outputs may be combined using diodes.

The reset input should be low (0V) for normal operation (counting 0-9). When high it resets the count to zero (Q0 high). This can be done manually with a switch between reset and +Vs and a 10k resistor between reset and 0V. Counting to less than 9 is achieved by connecting the relevant output (Q0-Q9) to reset, for example to count 0,1,2,3 connect Q4 to reset.

The disable input should be low (0V) for normal operation. When high it disables counting so that clock pulses are ignored and the count is kept constant.

The ÷10 output is high for counts 0-4 and low for 5-9, so it provides an output at ¹/₁₀ of the clock frequency. It can be used to drive the clock input of another 4017 (to count the tens).

Example projects: Heart-shaped badge | Network Lead Tester | Traffic Light | Dice | Model Lighthouse

4026 decade counter and 7-segment display driver

The count advances as the clock input becomes high (on the rising-edge). The outputs a-g go high to light the appropriate segments of a common-cathode 7-segment display as the count advances. The maximum output current is about 1mA with a 4.5V supply and 4mA with a 9V supply. This is sufficient to directly drive many 7-segment LED displays. The table below shows the segment sequence in detail.

The reset input should be low (0V) for normal operation (counting 0-9). When high it resets the count to zero.

The disable clock input should be low (0V) for normal operation. When high it disables counting so that clock pulses are ignored and the count is kept constant.

The enable display input should be high (+Vs) for normal operation. When low it makes outputs a-g low, giving a blank display. The enable out follows this input but with a brief delay.

The ÷10 output (h in table) is high for counts 0-4 and low for 5-9, so it provides an output at ¹/₁₀ of the clock frequency. It can be used to drive the clock input of another 4026 to provide multi-digit counting.

The not 2 output is high unless the count is 2 when it goes low.

Example project: 'Random' flasher for 8 LEDs
This project uses the 4026 in an unconventional way, the outputs a-g and the ÷10 output (h) are used to flash individual LEDs in a complex pattern which appears random if not studied too closely!

4029 up/down synchronous counter with preset

The 4029 is a synchronous counter so its outputs change precisely together on each clock pulse. This is helpful if you need to connect the outputs to logic gates because it avoids the glitches which occur with ripple counters.

The count occurs as the clock input becomes high (on the rising-edge). The up/down input determines the direction of counting: high for up, low for down. The state of up/down should be changed when the clock is high.

For normal operation (counting) preset, and carry in should be low.

The binary/decade input selects the type of counter: 4-bit binary (0-15) when high; decade (0-9) when low.

The counter may be preset by placing the desired binary number on the inputs A-D and briefly making the preset input high. There is no reset input, but preset can be used to reset the count to zero if inputs A-D are all low.

Connecting synchronous counters in a chain: please see 4510/16 below.

4510 up/down decade (0-9) counter with preset
4516 up/down 4-bit (0-15) counter with preset

For normal operation (counting) preset, reset and carry in should be low. When reset is high it resets the count to zero (0000, QA-QD low). The clock input should be low when resetting.

The counter may be preset by placing the desired binary number on the inputs A-D and briefly making the preset input high, the clock input should be low when this happens.

Connecting synchronous counters in a chain
The diagram below shows how to link synchronous counters, notice how all the clock (CK) inputs are linked. Carry out (CO) feeds carry in (CI) of the next counter. Carry in (CI) of the first counter should be low for 4029, 4510 and 4516 counters.

4518 dual decade (0-9) counter
4520 dual 4-bit (0-15) counter

These contain two separate synchronous counters, one on each side of the IC.

Normally a clock signal is connected to the clock input, with the enable input held high. Counting advances as the clock signal becomes high (on the rising-edge). Special arrangements are used if the 4518/20 counters are linked in a chain, as explained below.

For normal operation the reset input should be low, making it high resets the counter to zero (0000, QA-QD low).

Counting to less than the maximum (9 or 15) can be achieved by connecting the appropriate output(s) to the reset input, using an AND gate if necessary. For example to count 0 to 8 connect QA (1) and QD (8) to reset using an AND gate.

Connecting 4518 and 4520 counters in a chain
The diagram below shows how to link 4518 and 4520 counters. Notice how the normal clock inputs are held low, with the enable inputs being used instead. With this arrangement counting advances as the enable input becomes low (on the falling-edge) allowing output QD to supply a clock signal to the next counter. The complete chain is a ripple counter, although the individual counters are synchronous! If it is essential to have truly synchronous counting a system of logic gates is required, please see a 4518/20 datasheet for further details.

7-bit, 12-bit and 14-bit counters

4020 14-bit (÷16,384) ripple counter

The 4020 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse.

Output Qn is the nth stage of the counter, representing 2ⁿ, for example Q4 is 2⁴ = 16 (¹/₁₆ of clock frequency) and Q14 is 2¹⁴ = 16384 (¹/₁₆₃₈₄ of clock frequency). Note that Q2 and Q3 are not available.

The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low).

Also see: 4040 (12-bit) and 4060 (14-bit with internal oscillator).

4024 7-bit (÷128) ripple counter

The 4024 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse.

Output Qn is the nth stage of the counter, representing 2ⁿ, for example Q4 is 2⁴ = 16 (¹/₁₆ of clock frequency) and Q7 is 2⁷ = 128 (¹/₁₂₈ of clock frequency).

The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low).

4040 12-bit (÷4096) ripple counter

The 4040 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse.

Output Qn is the nth stage of the counter, representing 2ⁿ, for example Q4 is 2⁴ = 16 (¹/₁₆ of clock frequency) and Q12 is 2¹² = 4096 (¹/₄₀₉₆ of clock frequency).

The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low).

Also see these 14-bit counters: 4020 and 4060 (includes internal oscillator).

4060 14-bit (÷16,384) ripple counter with internal oscillator

The 4060 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse.

Output Qn is the nth stage of the counter, representing 2ⁿ, for example Q4 is 2⁴ = 16 (¹/₁₆ of clock frequency) and Q14 is 2¹⁴ = 16384 (¹/₁₆₃₈₄ of clock frequency). Note that Q1-3 and Q11 are not available.

The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low).

The 4060 includes an internal oscillator. The clock signal may be supplied in three ways:

From an external source to the clock input, as for a normal counter. In this case there should be no connections to external C and external R (pins 9 and 10).
RC oscillator as shown in the diagram. The oscillator drives the clock input with an approximate frequency f = ¹/_(2×R1×C) (it partly depends on the supply voltage). R1 should be at least 50k if the supply voltage is less than 7V. R2 should be between 2 and 10 times R1.
Crystal oscillator as shown in the diagram, note that there is no connection to pin 9. The 32768 Hz crystal will give a 2Hz signal at the last output, Q14.

Also see: 4020 (14-bit) and 4040 (12-bit), neither have internal oscillators.

Example projects: Christmas Decoration | Valentine Heart

Decoders

4028 BCD to decimal (1 of 10) decoder

The appropriate output Q0-9 becomes high in response to the BCD (binary coded decimal) input. For example an input of binary 0101 (=5) will make output Q5 high and all other outputs low.

The 4028 is a BCD (binary coded decimal) decoder intended for input values 0 to 9 (0000 to 1001 in binary). With inputs from 10 to 15 (1010 to 1111 in binary) all outputs are low.

Note that the 4028 can be used as a 1-of-8 decoder if input D is held low.

Also see: 4017 (a decade counter and 1-of-10 decoder in a single IC).

7-segment Display Drivers

4511 BCD to 7-segment display driver

The appropriate outputs a-g become high to display the BCD (binary coded decimal) number supplied on inputs A-D. The outputs a-g can source up to 25mA. The 7-segment display segments must be connected between the outputs and 0V with a resistor in series (330 with a 5V supply). A common cathode display is required.

Display test and blank input are active-low so they should be high for normal operation. When display test is low all the display segments should light (showing number 8). When blank input is low the display will be blank (all segments off).

The store input should be low for normal operation. When store is high the displayed number is stored internally to give a constant display regardless of any changes which may occur to the inputs A-D.

The 4511 is intended for BCD (binary coded decimal). Inputs values from 10 to 15 (1010 to 1111 in binary) will give a blank display (all segments off).

Integrated Circuits (Chips)

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Integrated Circuits (Chips)

Also see: 4000 Series ICs | 74 Series ICs | 555 and 556 Timer Circuits

Integrated Circuits are usually called ICs or chips. They are complex circuits which have been etched onto tiny chips of semiconductor (silicon). The chip is packaged in a plastic holder with pins spaced on a 0.1" (2.54mm) grid which will fit the holes on stripboard and breadboards. Very fine wires inside the package link the chip to the pins.

Pin numbers

The pins are numbered anti-clockwise around the IC (chip) starting near the notch or dot. The diagram shows the numbering for 8-pin and 14-pin ICs, but the principle is the same for all sizes.

IC holders (DIL sockets)

ICs (chips) are easily damaged by heat when soldering and their short pins cannot be protected with a heat sink. Instead we use an IC holder, strictly called a DIL socket (DIL = Dual In-Line), which can be safely soldered onto the circuit board. The IC is pushed into the holder when all soldering is complete.

IC holders are only needed when soldering so they are not used on breadboards.

Commercially produced circuit boards often have ICs soldered directly to the board without an IC holder, usually this is done by a machine which is able to work very quickly. Please don't attempt to do this yourself because you are likely to destroy the IC and it will be difficult to remove without damage by de-soldering.

Removing an IC from its holder

If you need to remove an IC it can be gently prised out of the holder with a small flat-blade screwdriver. Carefully lever up each end by inserting the screwdriver blade between the IC and its holder and gently twisting the screwdriver. Take care to start lifting at both ends before you attempt to remove the IC, otherwise you will bend and possibly break the pins.

Static precautions

Antistatic bags for ICs
Photograph © Rapid Electronics

Many ICs are static sensitive and can be damaged when you touch them because your body may have become charged with static electricity, from your clothes for example. Static sensitive ICs will be supplied in antistatic packaging with a warning label and they should be left in this packaging until you are ready to use them.

It is usually adequate to earth your hands by touching a metal water pipe or window frame before handling the IC but for the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces. You can make an earthed work surface with a sheet of aluminium kitchen foil and using a crocodile clip to connect the foil to a metal water pipe or window frame with a 10k resistor in series.

Datasheets

PDF files
To view and print PDF files you need an Acrobat Reader which may be downloaded free for Windows, Mac, RISC OS, or UNIX/Linux computers. If you are not sure which type of computer you have it is probably Windows.

Datasheets are available for most ICs giving detailed information about their ratings and functions. In some cases example circuits are shown. The large amount of information with symbols and abbreviations can make datasheets seem overwhelming to a beginner, but they are worth reading as you become more confident because they contain a great deal of useful information for more experienced users designing and testing circuits.

Datasheets are available as PDF files from:

Sinking and sourcing current

IC outputs are often said to 'sink' or 'source' current. The terms refer to the direction of the current at the IC's output.

If the IC is sinking current it is flowing into the output. This means that a device connected between the positive supply (+Vs) and the IC output will be switched on when the output is low (0V).

If the IC is sourcing current it is flowing out of the output. This means that a device connected between the IC output and the negative supply (0V) will be switched on when the output is high (+Vs).

It is possible to connect two devices to an IC output so that one is on when the output is low and the other is on when the output is high. This arrangement is used in the Level Crossing project to make the red LEDs flash alternately.

The maximum sinking and sourcing currents for an IC output are usually the same but there are some exceptions, for example 74LS TTL logic ICs can sink up to 16mA but only source 2mA.

Using diodes to combine outputs

The outputs of ICs must never be directly connected together. However, diodes can be used to combine two or more digital (high/low) outputs from an IC such as a counter. This can be a useful way of producing simple logic functions without using logic gates!

The diagram shows two ways of combining outputs using diodes. The diodes must be capable of passing the output current. 1N4148 signal diodes are suitable for low current devices such as LEDs.

For example the outputs Q0 - Q9 of a 4017 1-of-10 counter go high in turn. Using diodes to combine the 2nd (Q1) and 4th (Q3) outputs as shown in the bottom diagram will make the LED flash twice followed by a longer gap. The diodes are performing the function of an OR gate.

Example projects: Traffic Light | Dice | Model Lighthouse

The 555 and 556 Timers

The 8-pin 555 timer IC is used in many projects, a popular version is the NE555. Most circuits will just specify '555 timer IC' and the NE555 is suitable for these. The 555 output (pin 3) can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply LEDs, relay coils and low current lamps. To switch larger currents you can connect a transistor.

The 556 is a dual version of the 555 housed in a 14-pin package. The two timers (A and B) share the same power supply pins.

Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.

For further information please see the page on 555 and 556 timer circuits.

Logic ICs (chips)

Logic ICs process digital signals and there are many devices, including logic gates, flip-flops, shift registers, counters and display drivers. They can be split into two groups according to their pin arrangements: the 4000 series and the 74 series which consists of various families such as the 74HC, 74HCT and 74LS.

For most new projects the 74HC family is the best choice. The older 4000 series is the only family which works with a supply voltage of more than 6V. The 74LS and 74HCT families require a 5V supply so they are not convenient for battery operation.

The table below summarises the important properties of the most popular logic families:

Property	4000 Series	74 Series 74HC	74 Series 74HCT	74 Series 74LS
Technology	CMOS	High-speed CMOS	High-speed CMOS TTL compatible	TTL Low-power Schottky
Power Supply	3 to 15V	2 to 6V	5V ±0.5V	5V ±0.25V
Inputs	Very high impedance. Unused inputs must be connected to +Vs or 0V. Inputs cannot be reliably driven by 74LS outputs unless a 'pull-up' resistor is used (see below).		Very high impedance. Unused inputs must be connected to +Vs or 0V. Compatible with 74LS (TTL) outputs.	'Float' high to logic 1 if unconnected. 1mA must be drawn out to hold them at logic 0.
Outputs	Can sink and source about 5mA (10mA with 9V supply), enough to light an LED. To switch larger currents use a transistor.	Can sink and source about 20mA, enough to light an LED. To switch larger currents use a transistor.	Can sink and source about 20mA, enough to light an LED. To switch larger currents use a transistor.	Can sink up to 16mA (enough to light an LED), but source only about 2mA. To switch larger currents use a transistor.
Fan-out	One output can drive up to 50 CMOS, 74HC or 74HCT inputs, but only one 74LS input.	One output can drive up to 50 CMOS, 74HC or 74HCT inputs, but only 10 74LS inputs.		One output can drive up to 10 74LS inputs or 50 74HCT inputs.
Maximum Frequency	about 1MHz	about 25MHz	about 25MHz	about 35MHz
Power consumption of the IC itself	A few µW.	A few µW.	A few µW.	A few mW.

Driving 4000 or 74HC inputs from a
74LS output using a pull-up resistor.

Mixing Logic Families

It is best to build a circuit using just one logic family, but if necessary the different families may be mixed providing the power supply is suitable for all of them. For example mixing 4000 and 74HC requires the power supply to be in the range 3 to 6V. A circuit which includes 74LS or 74HCT ICs must have a 5V supply.

A 74LS output cannot reliably drive a 4000 or 74HC input unless a 'pull-up' resistor of 2.2k is connected between the +5V supply and the input to correct the slightly different logic voltage ranges used.

Note that a 4000 series output can drive only one 74LS input.

Quick links to
individual ICs

4000 4060
4001 4068
4002 4069
4011 4070
4012 4071
4017 4072
4020 4073
4023 4075
4024 4077
4025 4081
4026 4082
4028 4093
4029 4510
4030 4511
4040 4516
4049 4518
4050 4520

4000 Series CMOS

This family of logic ICs is numbered from 4000 onwards, and from 4500 onwards. They have a B at the end of the number (e.g. 4001B) which refers to an improved design introduced some years ago. Most of them are in 14-pin or 16-pin packages. They use CMOS circuitry which means they use very little power and can tolerate a wide range of power supply voltages (3 to 15V) making them ideal for battery powered projects. CMOS is pronounced 'see-moss' and stands for Complementary Metal Oxide Semiconductor.

However the CMOS circuitry also means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. For the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces.

For further information, including pin connections, please use the quick links on the right or go to 4000 Series ICs.

Quick links to
individual ICs

7400 7432
7402 7442
7403 7447
7404 7486
7405 7490
7408 7493
7409 74132
7410 74160
7411 74161
7412 74162
7414 74163
7420 74192
7421 74193
7427 74390
7430 74393

74HC4017
74HC4020
74HC4040
74HC4060
74HC4511

74 Series: 74LS, 74HC and 74HCT

The 74LS (Low-power Schottky) family (like the original) uses TTL (Transistor-Transistor Logic) circuitry which is fast but requires more power than later families.

Beware that the 74 series is often still called the 'TTL series' even though the latest ICs do not use TTL!

For further information, including pin connections, please use the quick links on the right or go to 74 series ICs.

PIC microcontrollers

A PIC is a Programmable Integrated Circuit microcontroller, a 'computer-on-a-chip'. They have a processor and memory to run a program responding to inputs and controlling outputs, so they can easily achieve complex functions which would require several conventional ICs.

www.picaxe.co.uk

Programming a PIC microcontroller may seem daunting to a beginner but there are a number of systems designed to make this easy. The PICAXE system is an excellent example because it uses a standard computer to program (and re-program) the PICs; no specialist equipment is required other than a low-cost download lead. Programs can be written in a simple version of BASIC or using a flowchart. The PICAXE programming software and extensive documentation is available to download free of charge, making the system ideal for education and users at home. For further information (including downloads) please see www.picaxe.co.uk

If you think PICs are not for you because you have never written a computer program, please look at the PICAXE system! It is very easy to get started using a few simple BASIC commands and there are a number of projects available as kits which are ideal for beginners. The system is stocked by Rapid Electronics.

Diodes

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Diodes

Signal diodes | Rectifier diodes | Bridge rectifiers | Zener diodes

Also see: LEDs | AC and DC | Power Supplies

Example: Circuit symbol:

Function

Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.

Forward Voltage Drop

Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).

Reverse Voltage

When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents. In addition there are LEDs (which have their own page) and Zener diodes (at the bottom of this page).

Connecting and soldering

Diodes must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is marked by a line painted on the body. Diodes are labelled with their code in small print, you may need a magnifying glass to read this on small signal diodes!

Small signal diodes can be damaged by heat when soldering, but the risk is small unless you are using a germanium diode (codes beginning OA...) in which case you should use a heat sink clipped to the lead between the joint and the diode body. A standard crocodile clip can be used as a heat sink.

Rectifier diodes are quite robust and no special precautions are needed for soldering them.

Testing diodes

You can use a multimeter or a simple tester (battery, resistor and LED) to check that a diode conducts in one direction but not the other. A lamp may be used to test a rectifier diode, but do NOT use a lamp to test a signal diode because the large current passed by the lamp will destroy the diode!

Signal diodes (small current)

Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.

General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.

Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.

For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.

Protection diodes for relays

Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil is switched off. The diagram shows how a protection diode is connected 'backwards' across the relay coil.

Diode	Maximum Current	Maximum Reverse Voltage
1N4001	1A	50V
1N4002	1A	100V
1N4007	1A	1000V
1N5401	3A	100V
1N5408	3A	1000V

Rectifier diodes (large current)

Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode.

All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. The table shows maximum current and maximum reverse voltage for some popular rectifier diodes. The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.

Also see: Power Supplies

Bridge rectifiers

There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled .

The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct.

Also see: Power Supplies


Various types of Bridge Rectifiers Note that some have a hole through their centre for attaching to a heat sink Photographs © Rapid Electronics

Zener diodes

Example: Circuit symbol:
a = anode, k = cathode

Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current.

Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them. Zener diode codes begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example.

Zener diodes are rated by their breakdown voltage and maximum power:

The minimum voltage available is 2.7V.
Power ratings of 400mW and 1.3W are common.

http://www.kpsec.freeuk.com/

Connectors and Cables

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Connectors and Cables

Connectors: Battery clips | Terminal blocks | Croc clips | 4mm & 2mm | DC power

Audio & communication: Jack | Phono | Coax | BNC | DIN | D | IDC & RJ45

Photographs © Rapid Electronics

Battery clips and holders

The standard battery clip fits a 9V PP3 battery and many battery holders such as the 6 × AA cell holder shown. Battery holders are also available with wires attached, with pins for PCB mounting, or as a complete box with lid, switch and wires.

Many small electronic projects use a 9V PP3 battery but if you wish to use the project for long periods a better choice is a battery holder with 6 AA cells. This has the same voltage but a much longer battery life and it will work out cheaper in the long run.

Larger battery clips fit 9V PP9 batteries but these are rarely used now.


PCB terminal block	Terminal block Photographs © Rapid Electronics

Terminal blocks and PCB terminals

Terminal blocks are usually supplied in 12-way lengths but they can be cut into smaller blocks with a sharp knife, large wire cutters or a junior hacksaw. They are sometimes called 'chocolate blocks' because of the way they can be easily cut to size.

PCB mounting terminal blocks provide an easy way of making semi-permanent connections to PCBs. Many are designed to interlock to provide more connections.

Crocodile clips


Crocodile clips Photographs © Rapid Electronics

The 'standard' crocodile clip has no cover and a screw contact. However, miniature insulated crocodile clips are more suitable for many purposes including test leads. They have a solder contact and lugs which fold down to grip the cable's insulation, increasing the strength of the joint. Remember to feed the cable through the plastic cover before soldering! Add and remove the cover by fully opening the clip, a piece of wood can be used to hold the jaws open.



4mm terminal and solder tag

Photographs © Rapid Electronics

4mm plugs, sockets and terminals

These are the standard single pole connectors used on meters and other electronic equipment. They are capable of passing high currents (typically 10A) and most designs are very robust. Shrouded plugs and sockets are available for use with high voltages where there is a risk of electric shock. A wide variety of colours is available from most suppliers.

Plugs
Plugs may have a screw or solder terminal to hold the cable. Check if you need to thread the cable through the cover before connecting it. Some plugs, such as those illustrated, are 'stackable' which means that they include a socket to accept another plug, allowing several plugs to be connected to the same point - a very useful feature for test leads.

Sockets
These are usually described as 'panel mounting' because they are designed to be fitted to a case. Most sockets have a solder contact but the picture shows other options. Fit the socket in the case before attaching the wire otherwise you will be unable to add the mounting nut.

Terminals
In addition to a socket these have provision for attaching a wire by threading it through a hole (or wrapping it around the post) and tightening the top nut by hand. They usually have a threaded stud to fit a solder tag inside the case.

Photograph © Rapid Electronics

2mm plugs and sockets

These are smaller versions of the 4mm plugs and sockets described above, but terminals are not readily available. The plugs illustrated are stackable. Despite their small size these connectors can pass large currents and some are rated at 10A.

DC power plugs and sockets


Photographs © Rapid Electronics

These 2-pole plugs and sockets ensure that the polarity of a DC supply cannot be accidentally reversed. The standard sizes are 2.1 and 2.5mm plug diameter. Standard plugs have a 10mm shaft, 'long' plugs have a 14mm shaft. Sockets are available for PCB or chassis mounting and most include a switch on the outer contact which is normally used to disconnect an internal battery when a plug is inserted.

Miniature versions with a 1.3mm diameter plug are used where small size is essential, such as for personal cassette players.


¼" (6.3mm) jack plug and socket

3.5mm jack plug and socket

3.5mm jack line socket (for fitting to a cable) Photographs © Rapid Electronics

Jack plugs and sockets

These are intended for audio signals so mono and stereo versions are available. The sizes are determined by the plug diameter: ¼" (6.3mm), 3.5mm and 2.5mm. The 2.5mm size is only available for mono.

Screened plugs have metal bodies connected to the COM contact. Most connections are soldered, remember to thread cables through plug covers before soldering! Sockets are designed for PCB or chassis mounting.

¼" plug connections are similar to those for 3.5mm plugs shown below. ¼" socket connections are COM, R and L in that order from the mounting nut, ignore R for mono use. Most ¼" sockets have switches on all contacts which open as the plug is inserted so they can be used to isolate internal speakers for example.

The connections for 3.5mm plugs and sockets are shown below. Plugs have a lug which should be folded down to grip the cable's insulation and increase the strength of the joint. 3.5mm mono sockets have a switch contact which can be used to switch off an internal speaker as the plug is inserted. Ignore this contact if you do not require the switching action.

3.5mm jack plug and socket connections
(the R connection is not present on mono plugs)

L = left channel signal
R = right channel signal
COM = common (0V, screen)

Do not use jack plugs for power supply connections because the contacts may be briefly shorted as the plug is inserted. Use DC power connectors for this.

Photographs © Rapid Electronics

Phono plugs and sockets

These are used for screened cables carrying audio and video signals. Stereo connections are made using a pair of phono plugs and sockets. The centre contact is for the signal and the outer contact for the screen (0V, common). Screened plugs have metal bodies connected to the outer contact to give the signal additional protection from electrical noise. Sockets are available for PCB or chassis mounting, singly for mono, or in pairs for stereo. Line sockets are available for making extension leads.

Construction of a screened cable


Photographs © Rapid Electronics

Coax plugs and sockets

These are similar to the phono plugs and sockets described above but they are designed for use with screened cables carrying much higher frequency signals, such as TV aerial leads. They provide better screening because at high frequencies this is essential to reduce electrical noise.

BNC plug, photograph © Rapid Electronics

BNC plugs and sockets

These are designed for screened cables carrying high frequency signals where an undistorted and noise free signal is essential, for example oscilloscope leads. BNC plugs are connected with a push and twist action, to disconnect you need to twist and pull.

Plugs and sockets are rated by their impedance (50 or 75) which must be the same as the cable's impedance. If the connector and cable impedances are not matched the signal will be distorted because it will be partly reflected at the connection, this is the electrical equivalent of the weak reflection which occurs when light passes through a glass window.

DIN plug

5 way 180° DIN socket
(chassis mounting)

Photographs © Rapid Electronics

DIN plugs and sockets

These are intended for audio signals but they can be used for other low-current purposes where a multi-way connector is required. They are available from 3 way to 8 way. 5 way is used for stereo audio connections. The contacts are numbered on the connector, but they are not in numerical order! For audio use the 'common' (0V) wire is connected to contact 2. 5 way plugs and sockets are available in two versions: 180° and 270° (the angle refers to the arc formed by the contacts).

Plastic covers of DIN plugs (and line sockets) are removed by depressing the retaining lug with a small screwdriver. You may also need small pliers to extract the body from the cover but do not pull on the pins themselves to avoid damage. Remember to thread the cable through the cover before starting to solder the connections!

Soldering DIN plugs is easier if you clamp the insert with the pins. Wires should be pushed into the hollow pins - first 'tin' the wires (coat them with a thin layer of solder) then melt a little solder into the hollow pin and insert the wire while keeping the solder molten. Take care to avoid melting the plastic base, stop and allow the pin to cool if necessary.

Mini-DIN connectors are used for computer equipment such as keyboards and mice but they are not a good choice for general use unless small size is essential.

Photographs © Rapid Electronics

D connectors

These are multi-pole connectors with provision for screw fittings to make semi-permanent connections, for example on computer equipment. The D shape prevents incorrect connection. Standard D-connectors have 2 rows of contacts (top picture); 9, 15 and 25-way versions are the most popular. High Density D-connectors have 3 rows of contacts (bottom picture); a 15-way version is used to connect computer monitors for example.

Note that covers (middle picture) are usually sold separately because both plugs and sockets can be fitted to cables by fitting a cover to a chassis mounted connector. PCB mounting versions of plugs and sockets are also available. The contacts are usually numbered on the body of the connector, although you may need a magnifying glass to see the very small markings. Soldering D-connectors requires a steady hand due to the closeness of the contacts, it is easy to accidently unsolder a contact you have just completed while attempting to solder the next one!

Photographs © Rapid Electronics

IDC communication connectors

These multi-pole insulation displacement connectors are used for computer and telecommunications equipment. They automatically cut through the insulation on wires when installed and special tools are required to fit them. They are available as 4, 6 and 8-way versions.

The 8-way RJ45 is the standard connector for modern computer networks. If you regularly use these you may be interested in our network lead tester project.

Standard UK telephone connectors are similar in style but a slightly different shape. They are called BT (British Telecom) connectors.

Cables

Cable... flex... lead... wire... what do all these terms mean?

A cable is an assembly of one or more conductors (wires) with some flexibility.
A flex is the proper name for the flexible cable fitted to mains electrical appliances.
A lead is a complete assembly of cable and connectors.
A wire is a single conductor which may have an outer layer of insulation (usually plastic).

Single core equipment wire

This is one solid wire with a plastic coating available in a wide variety of colours. It can be bent to shape but will break if repeatedly flexed. Use it for connections which will not be disturbed, for example links between points of a circuit board.

Typical specification: 1/0.6mm (1 strand of 0.6mm diameter), maximum current 1.8A.

Stranded wire

This consists of many fine strands of wire covered by an outer plastic coating. It is flexible and can withstand repeated bending without breaking. Use it for connections which may be disturbed, for example wires outside cases to sensors and switches. A very flexible version ('extra-flex') is used for test leads.

Typical specifications:
10/0.1mm (10 strands of 0.1mm diameter), maximum current 0.5A.
7/0.2mm (7 strands of 0.2mm diameter), maximum current 1.4A.
16/0.2mm (16 strands of 0.2mm diameter), maximum current 3A.
24/0.2mm (24 strands of 0.2mm diameter), maximum current 4.5A.
55/0.1mm (55 strands of 0.1mm diameter), maximum current 6A, used for test leads.

'Figure 8' (speaker) cable

Photograph © Rapid Electronics

'Figure 8' cable consists of two stranded wires arranged in a figure of 8 shape. One wire is usually marked with a line. It is suitable for low voltage, low current (maximum 1A) signals where screening from electrical interference is not required. It is a popular choice for connecting loudspeakers and is often called 'speaker cable'.

Photograph © Rapid Electronics

Signal cable

Signal cable consists of several colour-coded cores of stranded wire housed within an outer plastic sheath. With a typical maximum current of 1A per core it is suitable for low voltage, low current signals where screening from electrical interference is not required.

The picture shows 6-core cable, but 4-core and 8-core are also readily available.

Screened cable (mono)

Screened cable (stereo)

Photographs © Rapid Electronics

Screened cable

The diagram shows the construction of screened cable. The central wire carries the signal and the screen is connected to 0V (common) to shield the signal from electrical interference. Screened cable is used for audio signals and dual versions are available for stereo.

Construction of a screened cable

Photograph © Rapid Electronics

Co-axial cable

This type of screened cable (see above) is designed to carry high frequency signals such as those found in TV aerials and oscilloscope leads.

Mains flex

Photograph © Rapid Electronics

Flex is the proper name for the flexible cable used to connect appliances to the mains supply. It contains 2 cores (for live and neutral) or 3 cores (for live, neutral and earth). Mains flex has thick insulation for the high voltage (230V in UK) and it is available with various current ratings: 3A, 6A and 13A are popular sizes in the UK. Mains flex is sometimes used for low voltage circuits which pass a high current, but please think carefully before using it in this way. The distinctive colours of mains flex should act as a warning of the mains high voltage which can be lethal; using mains flex for low voltage circuits can undermine this warning.

Capacitors

2008-06-30T04:28:00.000-07:00

Capacitors

Polarised (> 1µF) | Unpolarised (<> | Real Values | Variable & trimmers

Also see: Capacitance and Uses of Capacitors

Function

Capacitors store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.

Capacitance

This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

µ means 10^-6 (millionth), so 1000000µF = 1F
n means 10^-9 (thousand-millionth), so 1000nF = 1µF
p means 10^-12 (million-millionth), so 1000pF = 1nF

Capacitor values can be very difficult to find because there are many types of capacitor with different labelling systems!

There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol.

Polarised capacitors (large values, 1µF +)

Examples: Circuit symbol:

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.

It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply voltage. 25V is a sensible minimum for most battery circuits.

Tantalum Bead Capacitors

Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size.

Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.

For example: blue, grey, black spot means 68µF
For example: blue, grey, white spot means 6.8µF
For example: blue, grey, grey spot means 0.68µF

Unpolarised capacitors (small values, up to 1µF)

Examples: Circuit symbol:

Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!

Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!

For example 0.1 means 0.1µF = 100nF.

Sometimes the multiplier is used in place of the decimal point:
For example: 4n7 means 4.7nF.

Capacitor Number Code

A number code is often used on small capacitors where printing is difficult:

the 1st number is the 1st digit,
the 2nd number is the 2nd digit,
the 3rd number is the number of zeros to give the capacitance in pF.
Ignore any letters - they just indicate tolerance and voltage rating.

For example: 102 means 1000pF = 1nF (not 102pF!)

For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).

Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

Capacitor Colour Code

A colour code was used on polyester capacitors for many years. It is now obsolete, but of course there are many still around. The colours should be read like the resistor code, the top three colour bands giving the value in pF. Ignore the 4th band (tolerance) and 5th band (voltage rating).

For example:

brown, black, orange means 10000pF = 10nF = 0.01µF.

Note that there are no gaps between the colour bands, so 2 identical bands actually appear as a wide band.

For example:

wide red, yellow means 220nF = 0.22µF.

Polystyrene Capacitors

This type is rarely used now. Their value (in pF) is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene!) so you should use a heat sink (such as a crocodile clip). Clip the heat sink to the lead between the capacitor and the joint.

Real capacitor values (the E3 and E6 series)

You may have noticed that capacitors are not available with every possible value, for example 22µF and 47µF are readily available, but 25µF and 50µF are not!

Why is this? Imagine that you decided to make capacitors every 10µF giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits and capacitors cannot be made with that accuracy.

To produce a sensible range of capacitor values you need to increase the size of the 'step' as the value increases. The standard capacitor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E3 series (3 values for each multiple of ten)
10, 22, 47, ... then it continues 100, 220, 470, 1000, 2200, 4700, 10000 etc.
Notice how the step size increases as the value increases (values roughly double each time).

The E6 series (6 values for each multiple of ten)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how this is the E3 series with an extra value in the gaps.

The E3 series is the one most frequently used for capacitors because many types cannot be made with very accurate values.

Variable capacitors

Variable Capacitor Symbol

Variable Capacitor
Photograph © Rapid Electronics

Variable capacitors are mostly used in radio tuning circuits and they are sometimes called 'tuning capacitors'. They have very small capacitance values, typically between 100pF and 500pF (100pF = 0.0001µF). The type illustrated usually has trimmers built in (for making small adjustments - see below) as well as the main variable capacitor.

Many variable capacitors have very short spindles which are not suitable for the standard knobs used for variable resistors and rotary switches. It would be wise to check that a suitable knob is available before ordering a variable capacitor.

Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor if it is necessary to vary the time period.

Trimmer capacitors

Trimmer Capacitor Symbol

Trimmer Capacitor
Photograph © Rapid Electronics

How to Solder

2008-06-30T04:22:00.000-07:00

How to Solder

First a few safety precautions:

Never touch the element or tip of the soldering iron.
They are very hot (about 400°C) and will give you a nasty burn.
Take great care to avoid touching the mains flex with the tip of the iron.
The iron should have a heatproof flex for extra protection. An ordinary plastic flex will melt immediately if touched by a hot iron and there is a serious risk of burns and electric shock.
Always return the soldering iron to its stand when not in use.
Never put it down on your workbench, even for a moment!
Work in a well-ventilated area.
The smoke formed as you melt solder is mostly from the flux and quite irritating. Avoid breathing it by keeping you head to the side of, not above, your work.
Wash your hands after using solder.
Solder contains lead which is a poisonous metal.

Preparing the soldering iron:

Place the soldering iron in its stand and plug in.
The iron will take a few minutes to reach its operating temperature of about 400°C.
Dampen the sponge in the stand.
The best way to do this is to lift it out the stand and hold it under a cold tap for a moment, then squeeze to remove excess water. It should be damp, not dripping wet.
Wait a few minutes for the soldering iron to warm up.
You can check if it is ready by trying to melt a little solder on the tip.
Wipe the tip of the iron on the damp sponge.
This will clean the tip.
Melt a little solder on the tip of the iron.
This is called 'tinning' and it will help the heat to flow from the iron's tip to the joint. It only needs to be done when you plug in the iron, and occasionally while soldering if you need to wipe the tip clean on the sponge.

You are now ready to start soldering:

Hold the soldering iron like a pen, near the base of the handle.
Imagine you are going to write your name! Remember to never touch the hot element or tip.
Touch the soldering iron onto the joint to be made.
Make sure it touches both the component lead and the track. Hold the tip there for a few seconds and...
Feed a little solder onto the joint.
It should flow smoothly onto the lead and track to form a volcano shape as shown in the diagram. Apply the solder to the joint, not the iron.
Remove the solder, then the iron, while keeping the joint still.
Allow the joint a few seconds to cool before you move the circuit board.
Inspect the joint closely.
It should look shiny and have a 'volcano' shape. If not, you will need to reheat it and feed in a little more solder. This time ensure that both the lead and track are heated fully before applying solder.

Using a heat sink

Some components, such as transistors, can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the component body. You can buy a special tool, but a standard crocodile clip works just as well and is cheaper.

Soldering Advice for Components

It is very tempting to start soldering components onto the circuit board straight away, but please take time to identify all the parts first. You are much less likely to make a mistake if you do this!

Stick all the components onto a sheet of paper using sticky tape.
Identify each component and write its name or value beside it.
Add the code (R1, R2, C1 etc.) if necessary.
Many projects from books and magazines label the components with codes (R1, R2, C1, D1 etc.) and you should use the project's parts list to find these codes if they are given.
Resistor values can be found using the resistor colour code which is explained on our Resistors page. You can print out and make your own Resistor Colour Code Calculator to help you.
Capacitor values can be difficult to find because there are many types with different labelling systems! The various systems are explained on our Capacitors page.

Some components require special care when soldering. Many must be placed the correct way round and a few are easily damaged by the heat from soldering. Appropriate warnings are given in the table below, together with other advice which may be useful when soldering.

For more detail on specific components please see the Components page or click on the component name in the table.

For most projects it is best to put the components onto the board in the order given below:

	Components	Pictures	Reminders and Warnings
1	IC Holders (DIL sockets)		Connect the correct way round by making sure the notch is at the correct end. Do NOT put the ICs (chips) in yet.
2	Resistors		No special precautions are needed with resistors.
3	Small value capacitors (usually less than 1µF)		These may be connected either way round. Take care with polystyrene capacitors because they are easily damaged by heat.
4	Electrolytic capacitors (1µF and greater)		Connect the correct way round. They will be marked with a + or - near one lead.
5	Diodes		Connect the correct way round. Take care with germanium diodes (e.g. OA91) because they are easily damaged by heat.
6	LEDs		Connect the correct way round. The diagram may be labelled a or + for anode and k or - for cathode; yes, it really is k, not c, for cathode! The cathode is the short lead and there may be a slight flat on the body of round LEDs.
7	Transistors		Connect the correct way round. Transistors have 3 'legs' (leads) so extra care is needed to ensure the connections are correct. Easily damaged by heat.
8	Wire Links between points on the circuit board.	single core wire	Use single core wire, this is one solid wire which is plastic-coated. If there is no danger of touching other parts you can use tinned copper wire, this has no plastic coating and looks just like solder but it is stiffer.
9	Battery clips, buzzers and other parts with their own wires		Connect the correct way round.
10	Wires to parts off the circuit board, including switches, relays, variable resistors and loudspeakers.	stranded wire	You should use stranded wire which is flexible and plastic-coated. Do not use single core wire because this will break when it is repeatedly flexed.
11	ICs (chips)		Connect the correct way round. Many ICs are static sensitive. Leave ICs in their antistatic packaging until you need them, then earth your hands by touching a metal water pipe or window frame before touching the ICs. Carefully insert ICs in their holders: make sure all the pins are lined up with the socket then push down firmly with your thumb.

What is solder?

Solder is an alloy (mixture) of tin and lead, typically 60% tin and 40% lead. It melts at a temperature of about 200°C. Coating a surface with solder is called 'tinning' because of the tin content of solder. Lead is poisonous and you should always wash your hands after using solder.

Solder for electronics use contains tiny cores of flux, like the wires inside a mains flex. The flux is corrosive, like an acid, and it cleans the metal surfaces as the solder melts. This is why you must melt the solder actually on the joint, not on the iron tip. Without flux most joints would fail because metals quickly oxidise and the solder itself will not flow properly onto a dirty, oxidised, metal surface.

The best size of solder for electronics is 22swg (swg = standard wire gauge).

Desoldering

At some stage you will probably need to desolder a joint to remove or re-position a wire or component. There are two ways to remove the solder:

Using a desoldering pump (solder sucker)

1. With a desoldering pump (solder sucker)

Set the pump by pushing the spring-loaded plunger down until it locks.
Apply both the pump nozzle and the tip of your soldering iron to the joint.
Wait a second or two for the solder to melt.
Then press the button on the pump to release the plunger and suck the molten solder into the tool.
Repeat if necessary to remove as much solder as possible.
The pump will need emptying occasionally by unscrewing the nozzle.

2. With solder remover wick (copper braid)

Apply both the end of the wick and the tip of your soldering iron to the joint.
As the solder melts most of it will flow onto the wick, away from the joint.
Remove the wick first, then the soldering iron.
Cut off and discard the end of the wick coated with solder.

After removing most of the solder from the joint(s) you may be able to remove the wire or component lead straight away (allow a few seconds for it to cool). If the joint will not come apart easily apply your soldering iron to melt the remaining traces of solder at the same time as pulling the joint apart, taking care to avoid burning yourself.

First Aid for Burns

Most burns from soldering are likely to be minor and treatment is simple:

Immediately cool the affected area under gently running cold water.
Keep the burn in the cold water for at least 5 minutes (15 minutes is recommended). If ice is readily available this can be helpful too, but do not delay the initial cooling with cold water.
Do not apply any creams or ointments.
The burn will heal better without them. A dry dressing, such as a clean handkerchief, may be applied if you wish to protect the area from dirt.
Seek medical attention if the burn covers an area bigger than your hand.

To reduce the risk of burns:

Always return your soldering iron to its stand immediately after use.
Allow joints and components a minute or so to cool down before you touch them.
Never touch the element or tip of a soldering iron unless you are certain it is cold.

How to Read a Schematic

2008-06-30T04:15:00.000-07:00

How to Read Schematic

In general terms, a circuit can be described as any group of electrical or electronic devices connected together by conductors. Conductors are most often metallic, and wires were the conductor of choice in the past. Old radios and other electronic equipment were often a rat's nest of wires. Today, it's more common to find metallic pathways, often called traces, on a board constructed of a mixture of fiberglass and epoxy. The terms board and card are interchangeable.

A schematic in electronics is a drawing representing a circuit. It uses symbols to represent real-world objects. The most basic symbol is a simple conductor, shown simply as a line. If wires connect in a diagram, they are shown with a dot at the intersection:

Conductors that do not connect are shown without a dot, or with a bridge formed by one wire over the other:

Among the connections are power and ground, the high and low system voltages respectfully. The 5 volt system power in the schematic is shown simply as 5V. There is also a +12V supply and a -12V supply. Ground, or 0 volts, has its own symbol:

A switch is a device that is capable of allowing the user to break the circuit as if the wire had been broken. Its symbol reflects this characteristic:

The three switches in the diagram are grouped in a Dual In-line Package (DIP).

A resistor is a device that resists the flow of charge. Its symbol reflects this characteristic by making the line jagged:

Just in case you have seen "flow of current" elsewhere rather than "flow of charge", see "Science Myths" in K-6 Textbooks and Popular culture and the definition of current below.

The unit of resistance is the ohm, pronounced om with a long o. The K in the schematics stands for kilohm or thousands of ohms. 10K means the same as 10,000. Meg and sometimes M mean megohm or million ohms. 4.7Meg or 4.7M is the same as 4,700,000.

You will see two variations on resistors in the schematic. One is the resistor array or network. It is a Single In-line Package (SIP) containing several resistors connected together. They can be found in many configurations. The one used here simply connects one end of the resistors to each other and brings them out to a common connection. The other end of each resistor is left free. Another variation is the variable resistor. It has a third contact that can move along the resistor element to permit the values at that point to be variable. The moveable part is called the wiper and is shown as an arrow.

There is a relationship between voltage, current and resistance that is expressed by Ohm's Law, which says that Voltage is equal to Current times Resistance, or:

V = I * R

V is voltage (often referred to as Electromotive Force where E rather than V is used), I is current and R is resistance. Current is expressed in Amperes, or amps for short. Very little current is used in typical electronic circuits, so milliamps, which means 1/1000 amp, is used. One milliamp = .001 amp. It's abbreviated ma, or sometimes MA.

To paraphrase a definition of charge from whatis.com :

"The coulomb (symbolized C) is the standard unit of electric charge in the International System of Units (SI). It is a dimensionless quantity. A quantity of 1 C is equal to approximately 6.24 x 10¹⁸, or 6.24 quintillion."

"In terms of SI base units, the coulomb is the equivalent of one ampere-second. Conversely, an electric current of 1 amp represents 1 C of unit electric charge carriers flowing past a specific point in 1 second. The unit electric charge is the amount of charge contained in a single electron. Thus, 6.24 x 10¹⁸ electrons have 1 C of charge. This is also true of 6.24 x 10¹⁸ positrons or 6.24 x 10¹⁸ protons, although these two types of particles carry charge of opposite polarity to that of the electron."

Since we deal mostly with electrons in electronics, 1 amp represents the effect of 6,240,000,000,000,000,000 electrons flowing past a point per second. Thus, since current is already defined as something flowing, to say "current flow" would be to say "..... flowing flow" which is incorrect because it is redundant.

Now let's say we have a 10K resistor and 2 milliamps of current. The voltage across the resistor will be:

V = 10,000 * .002 = 20 volts

We can use the above equation to generate an equation for each of the three variables. It requires remembering just two things:
1. It's ok to do something to one side of an equation as long as the same thing is done to the other side. The two sides will remain equal.
2. Anything divided by itself is equal to 1.

Start with the original equation:
V = I * R
Now divide both sides by R. Since R/R = 1, the right side now becomes I * 1 which is simply I, giving us V/R = I. If we switch sides and put the I on the left we end up with:
I = V/R

Again, start with the original equation:
V = I * R
Now divide both sides by I. Since I/I = 1, the right side now becomes R * 1 which is simply R, giving us V/I = R. If we switch sides and put the R on the left we end up with:
R = V/I

Thus, all three equations are:
V = I * R
I = V/R
R = V/I

One way to remember the three equations is to say, "The Vulture looks down and sees the Iguana and the Rabbit side by side (V = I * R), the Iguana sees the Vulture over the Rabbit (I = V/R) and the Rabbit sees the Vulture over the Iguana (R = V/I)."

A very common circuit is a voltage divider. It looks like the following:

Two resistors connected end-to-end are said to be connected in series. The total resistance is simply the sum of the two. In this case, it would be 22000 + 33 = 22033 ohms. If 1 volt is applied to the open end of the 22K resistor, the current through the whole circuit would be
I = V/R = 1/22033 or .00004538646576 amps, or about .05 milliamps.

The voltage across the 33 ohm resistor is then
V = I * R = .00004538646576 * 33 = .00149775337 volts, or about 1.5 millivolts (1/1000 volt).

Resistors are also often connected in parallel , such as below:

The value of the above parallel network is:
R = 1/(1/R1 + 1/R2 + 1/R3)
The equation is good for any number of resistors.

Capacitors are devices which have metal plates separated by an insulator. They are used to temporarily store an electrical charge. Their symbol reflects their construction:

The unit of capacitance is the Farad, but it's so large that the microfarad is used in practice. Microfarad means millionths of a Farad. It's often abbreviated mf, MF or some variation, although the correct abbreviation is µF. A value without a designator is assumed to be in microfarads. For example, in the schematic you will see several capacitors simply designated .1. They are actually .1µF capacitors.

Some capacitors must have their leads connected to the positive or negative side of a circuit. They are polarized capacitors. When that is the case, one side will be shown with a + sign where the positive side must be, or a - sign where the negative side must be, or both.

It's also very common to see picofarads abbreviated pf in some schematics. A picofarad is 10^-12 Farad, and is sometimes called a micromicrofarad.

A diode permits the flow of charge in only one direction. Its symbol reflects this characteristic, but with a slight problem:

Anode Cathode

The slight problem comes from the fact that flow of charge, at least in a wire, is from where there are a greater number of electrons to where there are fewer. Electrons are negatively charged. Thus, electrical flow of charge is from negative to positive in a wire. The problem with the symbol is that the cathode, not the anode, is the negative side. Electrical flow of charge is from the cathode to the anode, against the direction of the arrow.

Integrated Circuits contain many individual components. They, in turn, usually form several functional blocks. For example, the following is a pinout for the 74LS08 Quad 2 Input AND gate, along with its truth table. VCC is the 5 volt supply, and GND is ground. Sometimes ground is shown as VSS. The gate inputs are the As and Bs, and the outputs are the Ys. Thus, the inputs to gate 1 are 1A and 1B, and the output is 1Y. You will see variations on these conventions, but they hold true in many cases.

An Operational Amplifier also contains many individual components, but is not a digital circuit. It looks a little like a buffer, but has 2 inputs:

You can find a more detailed treatment of operational amplifiers at Professor Douglas M. Gingrich's site at The University of Alberta. For a simplified coverage of the subject, look at the circuit below.

An Op-Amp has many important characteristics. One of them is that the above circuit, called an inverting amplifier, attempts to prevent any current through the inverting input. In this circuit, R1 connects to the inverting input. R2 also connects to the inverting input, with its other end connected to the output. R2 is called the feedback resistor. Let's attempt to drive a current through the inverting input by placing 1V on the unconnected end of R1 and assume that the right end has 0 volts on it. The current will be
I = V/R = 1/1K = 1ma

The output will try to counter this by driving a current of the opposite polarity through the feedback resistor into the inverting input. The required voltage to do that will be
V = -(I * R) = -(1ma * 10K) = -10V.

Thus, we get a voltage to current conversion, a current to voltage conversion, a polarity inversion and, most importantly, amplification. Amplification or gain is commonly labeled G. In the case of the inverting amplifier,
G = -(Feedback Resistor / Input Resistor)
In this case, it's G = -(R2/R1)

Since the feedback cancels out the input, there is no voltage at the inverting input. It is said to be at virtual ground .

Now look at the circuit below from the schematic you will see in the hardware section.

The gain is a little over -1000 in order to provide enough amplification for the low output of a microphone. The signal is not only amplified but inverted because we are going into the inverting input. The inversion however, is not quite the same as it is in a digital device. Here, we are talking about an audio analog signal that, once transformed into an electrical signal by the microphone, moves much more smoothly and continuously in the negative and positive voltage directions. Inversion here means that when the input moves in the positive direction, the output moves in the negative direction. When the input goes toward negative, the output goes toward positive. C1 prevents DC voltages from even getting into the circuit. This blocking action will be discussed in a future section.

The non-inverting side is designated by the +. It is there that a positive offset voltage is applied. If R1 were not connected to C1 but rather to ground, the non-inverting side would exhibit a gain of (R2/R1)+1 for the bias voltage. With C1 however, there is no DC gain for the non-inverting side, and AC is shorted to ground by C2. The result is a gain of 1 on the non-inverting side for DC voltages. The purpose of the bias circuit will be covered in the next section.

The following is a self-test over this section. It would be a very good idea to make sure you know the answers to all of the questions since the sections that follow will build on this one.

1) _____ is a drawing that represents a circuit.

A) Switch
B) Schematic
C) Ground
D) Diagram

2) A _____ is a device that allows the user to break the circuit.

A) Scissors
B) Schematic
C) Resistor
D) Switch

3) A _____ is a device that resists the flow of charge.

A) Resistor
B) Buffer
C) Diode
D) Microfarad (or µF;)

4) The unit of resistance is the __1__ . The relationship between voltage, current, and resistance is expressed by __2__ .

A) Buffer, Amplifier
B) Capacitors, Diode
C) Ohm, Ohm's Law
D) Circuits, Switch

5) The __1__ is the unit of current. If there is very little current, it is expressed as __2__, which means 1/1000th.

A) Amperes (or Amps), Milliamps (or Ma or ma)
B) Volts, Milliavolts
C) Picofarads (or pf), Microfarads (or µF;)
D) Amplifier, Circuits

6) _____ are devices which have metal plates separated by an insulator. They temporarily store an electrical charge.

A) In Series
B) Cathode
C) Capacitors
D) Microfarad

7) What permits the flow of charge in only one direction?

A) Anode
B) Diode
C) Cathode
D) Schematic

8) _____ contain many individual components and usually form several functional blocks.

A) Schematics
B) Diodes
C) Amplifiers
D) Integrated Circuits

9) The _____ also contains many components, but is not a digital device.

A) Inverting Amplifier
B) Operational Amplifier
C) Volt
D) Electron

How to Read Schematic Answer:

1) B) Schematic

2) D) Switch

3) A) Resistor

4) C) Ohm, Ohm's Law

5) A) Amperes (or Amps), Milliamps (or Ma or ma)

6) C) Capacitors

7) B) Diode

8) D) Integrated Circuits

9) B) Operational Amplifier

Source: http://www.learn-c.com/

Oscillators, Pulse Generators, Clocks...

2008-06-30T03:46:00.000-07:00

Oscillators, Pulse Generators, Clocks...
- Capacitors and the 555 Timer IC
.

Introduction

As electronic designs get bigger, it becomes difficult to build the complete circuit. So we will use prebuilt circuits that come in packages like the one shown above. This prebuilt circuit is called an IC. IC stands for Integrated Circuit. An IC has many transistors inside it that are connected together to form a circuit. Metal pins are connected to the circuit and the circuit is stuck into a piece of plastic or ceramic so that the metal pins are sticking out of the side. These pins allow you to connect other devices to the circuit inside. We can buy simple ICs that have several inverter circuits like the one we built in the LED and Transistor tutorial or we can buy complex ICs like a Pentium Processor.

The Pulse - More than just an on/off switch

So far the circuits we have built have been stable, meaning that the output voltage stays the same. If you change the input voltage, the output voltage changes and once it changes it will stay at the same voltage level. The 555 integrated circuit (IC) is designed so that when the input changes, the output goes from 0 volts to Vcc (where Vcc is the voltage of the power supply). Then the output stays at Vcc for a certain length of time and then it goes back to 0 volts. This is a pulse. A graph of the output voltage is shown below.

The Oscillator (A Clock) - More than just a Pulse

The pulse is nice but it only happens one time. If you want something that does something interesting forever rather than just once, you need an oscillator. An oscillator puts out an endless series of pulses. The output constantly goes from 0 volts to Vcc and back to 0 volts again. Almost all digital circuits have some type of oscillator. This stream of output pulses is often called a clock. You can count the number of pulses to tell how much time has gone by. We will see how the 555 timer can be used to generate this clock. A graph of a clock signal is shown below.

The Capacitor

If you already understand capacitors you can skip this part.

A capacitor is similar to a rechargable battery in the way it works. The difference is that a capacitor can only hold a small fraction of the energy that a battery can. (Except for really big capacitors like the ones found in old TVs. These can hold a lot of charge. Even if a TV has been disconnected from the wall for a long time, these capacitors can still make lots of sparks and hurt people.) As with a rechargable battery, it takes a while for the capacitor to charge. So if we have a 12 volt supply and start charging the capacitor, it will start with 0 volts and go from 0 volts to 12 volts. Below is a graph of the voltage in the capacitor while it is charging.

.

.

We can control the speed of the capacitor's charging and discharging using resistors.

Capacitors are given values based on how much electricity they can store. Larger capacitors can store more energy and take more time to charge and discharge. The values are given in Farads but a Farad is a really large unit of measure for common capacitors. Common capacitors use measurements of pf and uf. Pf means picofarad and uf means microfarad. A picofarad is 0.000000000001 Farads. So a 33pf capacitor has a value of 33 picofarads or 0.000000000033 Farads. A microfarad is 0.000001 Farads. So a 10uf capacitor is 0.00001 Farads and a 220uF capacitor is 0.000220 Farads. If you do any calculations with formulas using the value of the capacitor you have to use the Farad value rather than the picofarad or microfarad value.

Capacitors are also rated by the maximum voltage they can take. This value is always written on the larger can shaped capacitors. For example, the 220uF capacitor in this kit has a maximum voltage rating of 25 volts. If you apply more than 25 volts to them they will die.

The 555 Timer

Creating a Pulse

The way the 555 timer works is that when you flip the first switch, the Output pin goes to Vcc (the positive power supply voltage) and starts charging the capacitor. When the capacitor voltage gets to 2/3 Vcc (that is Vcc * 2/3) the second switch turns on which makes the output go to 0 volts.

The pinout for the 555 timer is shown below

Deep Details

Pin 2 (Trigger) is the 'on' switch for the pulse. The line over the word Trigger tells us that the voltage levels are the opposite of what you would normally expect. To turn the switch on you apply 0 volts to pin 2. The technical term for this opposite behavior is 'Active Low'. It is common to see this 'Active Low' behavior for IC inputs because of the inverting nature of transistor circuits like we saw in the LED and Transistor Tutorial.

Pin 6 is the off switch for the pulse. We connect the positive side of the capacitor to this pin and the negative side of the capacitor to ground. When Pin 2 (Trigger) is at Vcc, the 555 holds Pin 7 at 0 volts (Note the inverted voltage). When Pin 2 goes to 0 volts, the 555 stops holding Pin 7 at 0 volts. Then the capacitor starts charging. The capacitor is charged through a resistor connected to Vcc. The current starts flowing into the capacitor, and the voltage in the capacitor starts to increase.

Pin 3 is the output (where the actual pulse comes out). The voltage on this pin starts at 0 volts. When 0 volts is applied to the trigger (Pin 2), the 555 puts out Vcc on Pin 3 and holds it at Vcc until Pin 6 reaches 2/3 of Vcc (that is Vcc * 2/3). Then the 555 pulls the voltage at Pin 3 to ground and you have created a pulse. (Again notice the inverting action.) The voltage on Pin 7 is also pulled to ground, connecting the capacitor to ground and discharging it.

Seeing the pulse

To see the pulse we will use an LED connected to the 555 output, Pin 3. When the output is 0 volts the LED will be off. When the output is Vcc the LED will be on.

Building the Circuit

Before you start building the circuit, use jumper wires to connect the red and blue power rows to the red and blue power rows on the other side of the board. Then you will be able to easily reach Vcc and Ground lines from both sides of the board. (If the wires are too short, use two wires joined together in a row of holes for the positive power (Vcc) and two wires joined together in a different row of holes for the ground.)

Connect Pin 1 to ground.
Connect Pin 8 to Vcc.
Connect Pin 4 to Vcc.
Connect the positive leg of the LED to a 330 ohm resistor and connect the negative end of the LED to ground. Connect the other leg of the 330 ohm resistor to the output, Pin 3.
Connect Pin 7 to Vcc with a 10k resistor (RA = 10K).
Connect Pin 7 to Pin 6 with a jumper wire.
Connect Pin 6 to the positive leg of the 220uF Capacitor (C = 220uF). (You will need to bend the positive (long leg) up and out some so that the negative leg can go in the breadboard.
Connect the negative leg of the capacitor to ground.
Connect a wire to Pin 2 to use as the trigger. Start with Pin 2 connected to Vcc.

Now connect the power. The LED will come on and stay on for about 2 seconds. Remove the wire connected to Pin 2 from Vcc. You should be able to trigger the 555 again by touching the wire connected to pin 2 with your finger or by connecting it to ground and removing it. (It should be about a 2 second pulse.)

Making it Oscillate

Next we will make the LED flash continually without having to trigger it. We will hook up the 555 so that it triggers itself. The way this works is that we add in a resistor between the capacitor and the discharge pin, Pin 7. Now, the capacitor will charge up (through RA and RB) and when it reaches 2/3 Vcc, Pin 3 and Pin 7 will go to ground. But the capacitor can not discharge immediately because of RB. It takes some time for the charge to drain through RB. The more resistance RB has, the longer it takes to discharge. The time it takes to discharge the capacitor will be the time the LED is off.
To trigger the 555 again, we connect Pin 6 to the trigger (Pin 2). As the capacitor is discharging, the voltage in the capacitor gets lower and lower. When it gets down to 1/3 Vcc this triggers Pin 2 causing Pin 3 to go to Vcc and the LED to come on. The 555 disconnects Pin 7 from ground, and the capacitor starts to charge up again through RA and RB.

To build this circuit from the previous circuit, do the following.
Disconnect the power.
Take out the jumper wire between Pin 6 and Pin 7 and replace it with a 2.2k resistor (RB = 2.2K).
Use the jumper wire at pin 2 to connect Pin 2 to Pin 6.
Now reconnect the power and the LED should flash forever (as long as you pay your electricity bill).
Experiment with different resistor values of RA and RB to see how it changes the length of time that the LED flashes. (You are changing the amount of time that it takes for the Capacitor to charge and discharge.)

Formulas

These are the formulas we use for the 555 to control the length of the pulses.

t1 = charge time (how long the LED is on) = 0.693 * (RA + RB) * C
t2 = discharge time (how long the LED is off) = 0.693 * RB * C
T = period = t1 + t2 = 0.693 * (RA + 2*RB) * C
Frequency = 1 / T = 1.44 / ((RA + 2 * RB) * C)

t1 and t2 are the time in seconds. C is the capacitor value in Farads. 220uF = 0.000220 F. So for our circuit we have:

t1 = 0.693 * (10000 + 2200) * 0.000220 = 1.86 seconds

t2 = 0.693 * 2200 * 0.000220 = 0.335 seconds

T = 1.86 + 0.335 = 2.195 seconds

Frequency = 0.456 (cycles per second)

All the parts in this kit are included with the Beginners Kit and the Microcontroller Beginner Kit. If you already have a breadboard and a power supply, and you just want the parts for this kit, order the 555Kit.

The 555Kit includes:

2 - 555 ICs

5 - 10K ohm Resistors

5 - 2.2K ohm Resistors

5 - 510 ohm Resistors

5 - 330 ohm Resistors

1 - 220 uF Capacitor

5 - LEDs

Jumper Wires

Learning About Transistors and LEDs

2008-06-30T03:44:00.001-07:00

Learning About Transistors and LEDs

The LED

An LED is the device shown above. Besides red, they can also be yellow, green and blue. The letters LED stand for Light Emitting Diode. If you are unfamiliar with diodes, take a moment to review the components in the Basic Components Tutorial. The important thing to remember about diodes (including LEDs) is that current can only flow in one direction.

To make an LED work, you need a voltage supply and a resistor. If you try to use an LED without a resistor, you will probably burn out the LED. The LED has very little resistance so large amounts of current will try to flow through it unless you limit the current with a resistor. If you try to use an LED without a power supply, you will be highly disappointed.

So first of all we will make our LED light up by setting up the circuit below.

Step 1.) First you have to find the positive leg of the LED. The easiest way to do this is to look for the leg that is longer.

Step 2.) Once you know which side is positive, put the LED on your breadboard so the positive leg is in one row and the negative leg is in another row. (In the picture below the rows are vertical.)

Step 3.) Place one leg of a 2.2k ohm resistor (does not matter which leg) in the same row as the negative leg of the LED. Then place the other leg of the resistor in an empty row.

Step 4.) Unplug the power supply adapter from the power supply. Next, put the ground (black wire) end of the power supply adapter in the sideways row with the blue stripe beside it. Then put the positive (red wire) end of the power supply adapter in the sideways row with the red stripe beside it.

Step 5.) Use a short jumper wire (use red since it will be connected to the positive voltage) to go from the positive power row (the one with the red stripe beside it) to the positive leg of the LED (not in the same hole, but in the same row). Use another short jumper wire (use black) to go from the ground row to the resistor (the leg that is not connected to the LED). Refer to the picture below if necessary.

The breadboard should look like the picture shown below.

Now plug the power supply into the wall and then plug the other end into the power supply adapter and the LED should light up. Current is flowing from the positive leg of the LED through the LED to the negative leg. Try turning the LED around. It should not light up. No current can flow from the negative leg of the LED to the positive leg.

People often think that the resistor must come first in the path from positive to negative, to limit the amount of current flowing through the LED. But, the current is limited by the resistor no matter where the resistor is. Even when you first turn on the power, the current will be limited to a certain amount, and can be found using ohm’s law.

Revisiting Ohm's Law

Ohm's Law can be used with resistors to find the current flowing through a circuit. The law is I = VD/R (where I = current, VD = voltage across resistor, and R = resistance). For the circuit above we can only use Ohm's law for the resistor so we must use the fact that when the LED is on, there is a 1.4 voltage drop across it. This means that if the positive leg is connected to 12 volts, the negative leg will be at 10.6 volts. Now we know the voltage on both sides of the resistor and can use Ohm's law to calculate the current. The current is (10.6 - 0) / 2200 = 0.0048 Amperes = 4.8 mA

This is the current flowing through the path from 12V to GND. This means that 4.8 mA is flowing through the LED and the resistor. If we want to change the current flowing through the LED (changing the brightness) we can change the resistor. A smaller resistor will let more current flow and a larger resistor will let less current flow. Be careful when using smaller resistors because they will get hot.

Next, we want to be able to turn the LED on and off without changing the circuit. To do this we will learn to use another electronic component, the transistor.

1.6.1 The Transistor

Transistors are basic components in all of today's electronics. They are just simple switches that we can use to turn things on and off. Even though they are simple, they are the most important electrical component. For example, transistors are almost the only components used to build a Pentium processor. A single Pentium chip has about 3.5 million transistors. The ones in the Pentium are smaller than the ones we will use but they work the same way.

Transistors that we will use in projects look like this:

The transistor has three legs, the Collector (C), Base (B), and Emitter (E). Sometimes they are labeled on the flat side of the transistor. Transistors always have one round side and one flat side. If the round side is facing you, the Collector leg is on the left, the Base leg is in the middle, and the Emitter leg is on the right.

Transistor Symbol

The following symbol is used in circuit drawings (schematics) to represent a transistor.

Basic Circuit

The Base (B) is the On/Off switch for the transistor. If a current is flowing to the Base, there will be a path from the Collector (C) to the Emitter (E) where current can flow (The Switch is On.) If there is no current flowing to the Base, then no current can flow from the Collector to the Emitter. (The Switch is Off.)

Below is the basic circuit we will use for all of our transistors.

To build this circuit we only need to add the transistor and another resistor to the circuit we built above for the LED. Unplug the power supply from the power supply adapter before making any changes on the breadboard. To put the transistor in the breadboard, seperate the legs slightly and place it on the breadboard so each leg is in a different row. The collector leg should be in the same row as the leg of the resistor that is connected to ground (with the black jumper wire). Next move the jumper wire going from ground to the 2.2k ohm resistor to the Emitter of the transistor.

Next place one leg of the 100k ohm resistor in the row with the Base of the transistor and the other leg in an empty row and your breadboard should look like the picture below.

Now put one end of a yellow jumper wire in the positive row (beside the red line) and the other end in the row with the leg of the 100k ohm resistor (the end not connected to the Base). Reconnect the power supply and the transistor will come on and the LED will light up. Now move the one end of the yellow jumper wire from the positive row to the ground row (beside the blue line). As soon as you remove the yellow jumper wire from the positive power supply, there is no current flowing to the base. This makes the transistor turn off and current can not flow through the LED. As we will see later, there is very little current flowing through the 100k resistor. This is very important because it means we can control a large current in one part of the circuit (the current flowing through the LED) with only a small current from the input.

Back to Ohm's Law

We want to use Ohm's law to find the current in the path from the Input to the Base of the transistor and the current flowing through the LED. To do this we need to use two basic facts about the transistor.

1.) If the transistor is on, then the Base voltage is 0.6 volts higher than the Emitter voltage.

2.) If the transistor is on, the Collector voltage is 0.2 volts higher than the Emitter voltage.

So when the 100k resistor is connected to 12VDC, the circuit will look like this:

So the current flowing through the 100k resistor is (12 - 0.6) / 100000 = 0.000114 A = 0.114 mA.

The current flowing through the 2.2k ohm resistor is (10.6 - 0.2) / 2200 = 0.0047 A = 4.7 mA.

If we want more current flowing through the LED, we can use a smaller resistor (instead of 2200) and we will get more current through the LED without changing the amount of current that comes from the Input line. This means we can control things that use a lot of power (like electric motors) with cheap, low power circuits. Soon you will learn how to use a microcontroller (a simple computer). Even though the microcontroller can not supply enough current to turn lights and motors on and off, the microcontroller can turn transistors on and off and the transistors can control lots of current for lights and motors.

For Ohm’s law, also remember that when the transistor is off, no current flows through the transistor.

1.6.2 Introduction to Digital Devices - The Inverter

In digital devices there are only two values, usually referred to as 0 and 1. 1 means there is a voltage (usually 5 volts) and 0 means the voltage is 0 volts.

An inverter (also called a NOT gate) is a basic digital device found in all modern electronics. So for an inverter, as the name suggests, it's output is the opposite of the input (Output is NOT the Input). If the input is 0 then the output is 1 and if the input is 1 then the output is 0. We can summarize the operation of this device in a table.

Input	Output
1	0
0	1

To help us practice with transistors we will build an inverter. Actually we have already built an inverter. The transistor circuit we just built is an inverter circuit. To help see the inverter working, we will build a circuit with two inverters. The circuit we will use is shown below.

First Inverter (already built) Second Inverter

To build the circuit, use the transistor circuit we just built as the first inverter. The first inverter input is the end of the 100k ohm resistor connected to the yellow jumper wire. Build another circuit identical to the first (the basic transistor circuit from Section 1.6.1) except leave out the yellow jumper wire connected to the 100k ohm resistor (the inverter input). This circuit is the second inverter.

Connect the output of the first inverter to the input of the second inverter by putting one end of a jumper wire in the same row of holes as the 2.2k ohm resistor and the Collector of the transistor (the output of the first inverter) and putting the other end in the same row of holes as the leg of the 100k ohm resistor of the second inverter (the input to the second inverter).

Here is how to check if you built it correctly. Connect the first inverter input (the yellow jumper wire) to 12V (the positive row). The LED in the first inverter should come on and the LED in the second inverter should stay off. Then connect the first inverter input to 0V (the ground row). (You are turning off the switch of the first inverter.) The first LED should go off and the second LED should come on. If this does not happen, check to make sure no metal parts are touching. Check to make sure all the parts are connected correctly.

The input can either be connected to 12V or 0V. When the Inverter Input is 12V, the transistor in the first inverter will turn on and the LED will come on and the Inverter Output voltage will be 0.2V. The first Inverter Output is connected to the input of the second inverter. The 0.2V at the input of the second inverter is small enough that the second transistor is turned off. The circuit voltages are shown in the diagram below.

When the Inverter Input is connected to 0V, the transistor in the first inverter is turned off and the LED will get very dim. There is a small amount of current still flowing through the LED to the second inverter. The voltage at the first Inverter Output will go up, forcing the second inverter transistor to come on. When the second inverter transistor comes on, the second inverter LED will come on. To find the voltage at the output of the first inverter (10.4V), use Ohm's law. There is no current flowing through the transistor in the first inverter so the path of the current is through the first LED, through the 2.2k resistor, through the 100k resistor, through the second transistor to ground. The voltage at the negative side of the first LED is fixed at 10.6V by the LED. The voltage at the second transistor base is fixed at 0.6V by the transistor. Then given those two voltages, you should be able to find the voltage at the point in the middle (10.4V) using Ohm’s law. (Hint: First find the current and then work through Form 1 of ohm’s law to find the voltage at the point between the 2.2k resistor and the 100k resistor.)

Switch the input back and forth from 0V to 12V and you can see that when the first stage is on, the second stage is off. This demonstrates the inverting action of the Inverter.

The next project in the series is called Pulses, Oscillators, Clocks...
It introduces capacitors and the LM555 timer. With these you can make circuits with LEDs that will continually flash! Click here to go to the Pulses, Oscillators, Clocks... page.

For more details on digital logic, go to Truth Table Page.

You can order the parts to build this circuit. The kit includes the parts for this project and the parts needed for the 555 project.

Beginner's Kit, includes:

5 - 330 ohm Resistors

5 - 510 ohm Resistors

5 - 2.2K ohm Resistors

5 - 10K ohm Resistors

5 - 100K ohm Resistors

1 - 220 uF Capacitor

5 - LEDs

5 - NPN Transistors

2 - 555 Timer ICs

1 - Small Breadboard

1 - 12 Volt DC Power Supply for Breadboard

Jumper Wires

Using the Breadboard

2008-06-30T03:41:00.001-07:00

Using the Breadboard
(Socket Board)

The bread board has many strips of metal (copper usually) which run underneath the board. The metal strips are laid out as shown below.

These strips connect the holes on the top of the board. This makes it easy to connect components together to build circuits. To use the bread board, the legs of components are placed in the holes (the sockets). The holes are made so that they will hold the component in place. Each hole is connected to one of the metal strips running underneath the board.

Each wire forms a node. A node is a point in a circuit where two components are connected. Connections between different components are formed by putting their legs in a common node. On the bread board, a node is the row of holes that are connected by the strip of metal underneath.

The long top and bottom row of holes are usually used for power supply connections.

The rest of the circuit is built by placing components and connecting them together with jumper wires. Then when a path is formed by wires and components from the positive supply node to the negative supply node, we can turn on the power and current flows through the path and the circuit comes alive.

For chips with many legs (ICs), place them in the middle of the board so that half of the legs are on one side of the middle line and half are on the other side.

A completed circuit might look like the following. This circuit is from our Microcontroller Beginner Kit.

We have three sizes of breadboards and jumper wire sets for sale. Click here for more information

Finding Voltage and Current Using Ohm's Law

2008-06-30T03:37:00.000-07:00

Finding Voltage and Current Using Ohm's Law

There is a simple relationship between current, voltage and resistance. This relationship is called Ohm’s Law. The formula is the following.

Difference in Voltage = Current * Resistance

or DV = I * R

This is Form 1 of Ohm's Law.

To find current and resistance the following forms can be used. They are the same as the above formula but in a different form.

Form 2: Current = Difference in Voltage / Resistance

or I = DV / R

Form 3: Resistance = Difference in Voltage / Current

or R = DV / I

These formulas are always used for situations where there are two points with a resistor between them. DV is the difference in voltage between the two points and current is what flows between the two points. These simple relationships allow us to calculate many things. Given any two of the three values (Current, Resistance, and Difference in Voltage) the third can be found. The most common calculation is for current. Voltage is easy to measure and the resistance can be found from the resistor (see color codes). Once these values are known, current can be calculated using Form 2 of Ohm’s law, I = DV / R. For example, consider the problem shown in Figure 1. One side is at 0 volts (ground) and the other side is at 5 volts (with a multimeter, black probe on right side, red probe on left side).

Figure 1

The voltage difference between Point A and Point B is 5 - 0 = 5 volts (DV=5). There is a resistor between the two points which has a value of 500 ohms (R=500). We know that current flows from a point of high voltage to a point of low voltage so we can draw an arrow from the higher voltage to the lower voltage.

Figure 2

Now we can find the current flowing through the resistor using Form 2 of Ohm's Law.

I = DV / R

DV / R = 5 / 500

5 / 500 = 0.01 Amps

0.01 Amps = 10 milliAmps

10 milliamps can be abbreviated as 10 mA

This means the current is 10 mA. ( I = 10mA )

Now to check our answer we can use Form 1 and Form 3 of Ohm’s law. We have to use the value of current in Amps for these formulas. So if we have I = 0.01 Amps and Resistance = 500 ohms then by using Form 1 of Ohm’s law we can find:

Difference in Voltage = DV

DV = I * R

I * R = 0.01 * 500

0.01 * 500 = 5 volts

This is the voltage we started with so the value we found for the current must be right.

We can also check the answer with Form 3 by using I = 0.01 Amps and DV = 5 volts.

Resistance = R

R = DV / I

DV / I = 5 / 0.01

5 / 0.01 = 500 ohms.

So R = 500 ohms

Now consider the problem shown in Figure 3. The voltage on one side is 10 volts and the voltage on the other side is 3 volts. Therefore the voltage difference between the two points is 10 - 3 = 7 volts (DV = 7 V). The resistor is 400 ohms (R = 400).

Figure 3

Then the current flowing from left to right is

I = DV / R

DV / R = 7 / 400

7 / 400 = 0.0175 Amps

0.0175 Amps = 17.5 milliAmps

17.5 milliAmps = 17.5 mA

This means the current flowing from the left to the right is 17.5 mA.

Now suppose we have two points with a voltage difference of 5 volts. Point A is at 5 volts and Point B is at 0 volts (ground). (Notice that the voltage difference is the important part. If Point A is at 7 volts and Point B is at 2 volts then the voltage difference is the same, 7 - 2 = 5 volts.) Now suppose we want a current to flow between Points A and B and we want the current to be 0.02 Amps ( I = 0.02 Amps = 20 mA).

Now we need to find the value of the resistor so we use Form 3 of Ohm’s Law.

Resistance = Difference in Voltage / Current or R = DV / I

DV / I = 5 / 0.02 = 250 ohms

This means that putting a resistor with a value of 250 ohms between Points A and B will make a current flow from Point A to Point B and the current will be 0.02 Amps (20 mA). Now using the values of voltage and resistance, check the value of the current using Form 2 of Ohm’s law.

Basic Electrical Components

2008-06-30T03:35:00.001-07:00

Basic Electrical Components

Resistors

Resistors are components that have a predetermined resistance. Resistance determines how much current will flow through a component. Resistors are used to control voltages and currents. A very high resistance allows very little current to flow. Air has very high resistance. Current almost never flows through air. (Sparks and lightning are brief displays of current flow through air. The light is created as the current burns parts of the air.) A low resistance allows a large amount of current to flow. Metals have very low resistance. That is why wires are made of metal. They allow current to flow from one point to another point without any resistance. Wires are usually covered with rubber or plastic. This keeps the wires from coming in contact with other wires and creating short circuits. High voltage power lines are covered with thick layers of plastic to make them safe, but they become very dangerous when the line breaks and the wire is exposed and is no longer separated from other things by insulation.

Resistance is given in units of ohms. (Ohms are named after Mho Ohms who played with electricity as a young boy in Germany.) Common resistor values are from 100 ohms to 100,000 ohms. Each resistor is marked with colored stripes to indicate it’s resistance. To learn how to calculate the value of a resistor by looking at the stripes on the resistor, go to Resistor Values which includes more information about resistors.

Variable Resistors

Variable resistors are also common components. They have a dial or a knob that allows you to change the resistance. This is very useful for many situations. Volume controls are variable resistors. When you change the volume you are changing the resistance which changes the current. Making the resistance higher will let less current flow so the volume goes down. Making the resistance lower will let more current flow so the volume goes up. The value of a variable resistor is given as it’s highest resistance value. For example, a 500 ohm variable resistor can have a resistance of anywhere between 0 ohms and 500 ohms. A variable resistor may also be called a potentiometer (pot for short).

Diodes

Diodes are components that allow current to flow in only one direction. They have a positive side (leg) and a negative side. When the voltage on the positive leg is higher than on the negative leg then current flows through the diode (the resistance is very low). When the voltage is lower on the positive leg than on the negative leg then the current does not flow (the resistance is very high). The negative leg of a diode is the one with the line closest to it. It is called the cathode. The postive end is called the anode.

Usually when current is flowing through a diode, the voltage on the positive leg is 0.65 volts higher than on the negative leg.

LED

Light Emitting Diodes are great for projects because they provide visual entertainment. LEDs use a special material which emits light when current flows through it. Unlike light bulbs, LEDs never burn out unless their current limit is passed. A current of 0.02 Amps (20 mA) to 0.04 Amps (40 mA) is a good range for LEDs. They have a positive leg and a negative leg just like regular diodes. To find the positive side of an LED, look for a line in the metal inside the LED. It may be difficult to see the line. This line is closest to the positive side of the LED. Another way of finding the positive side is to find a flat spot on the edge of the LED. This flat spot is on the negative side.

When current is flowing through an LED the voltage on the positive leg is about 1.4 volts higher than the voltage on the negative side. Remember that there is no resistance to limit the current so a resistor must be used in series with the LED to avoid destroying it.

To learn about LEDs through an interactive kit, look at LED and Transistor Kit

Switches

Switches are devices that create a short circuit or an open circuit depending on the position of the switch. For a light switch, ON means short circuit (current flows through the switch, lights light up and people dance.) When the switch is OFF, that means there is an open circuit (no current flows, lights go out and people settle down. This effect on people is used by some teachers to gain control of loud classes.)

When the switch is ON it looks and acts like a wire. When the switch is OFF there is no connection.

Basic Definitions and Concepts

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Basic Definitions and Concepts

Introduction

Welcome to the exciting world of electronics. Before we can build anything we need to look at a couple of things. Anytime you have an electrical circuit, you have voltage and current. We build circuits to control voltage and current.

Current

Current is what flows through a wire. Think of it as water flowing in a river. The current flows from one point to another point just like water in a river. Current flows from points of high voltage to points of low voltage. Current can be shown in circuit diagrams by using arrows as in Figure 1. The arrow shows which way the current is flowing. An I is usually included beside the arrow to indicate current.

Figure 1

The unit of measurement for current is the Ampere, or Amp for short, and abbreviated as A. (The name Ampere comes from Mr. Ampere who played with electricity as a small boy in Vermont.) Common currents are 0.001 Amps (0.001A) to 0.5 Amps (0.5A). Since currents are usually small, they are usually given in the form of milliAmps (abbreviated mA.) The milli means divided by 1000, so 0.001 Amps equals 1 milliAmp (1 mA) since 1 / 1000 = 0.001. Also, 0.5 Amps equals 500 milliAmps (500mA) since 500 / 1000 = 0.5.

Voltage

Voltage indicates the power level of a point. Voltage is measured in volts. If we continue the river comparison, a point at the top of a hill would be at a high voltage level and a point at the bottom of a hill would be at a low voltage level. Then, just as water flows from a high point to a low point, current flows from a point of high voltage to a point of low voltage. If one point is at 5 volts and another point is at 0 volts then when a wire is connected between them, current will flow from the point at 5 volts to the point at 0 volts.

A measurement of voltage is much like a measurement of height. It gives you the difference in voltage between those two points. If point A is at 10 volts and point B is at 2 volts then the voltage measured between A and B is 8 volts (10 -2). This is similar to measuring height. We measure the height of hills the same way. We say the sea level is at zero feet and then compare other points to that level. On top of Mary’s Peak you are 4000 ft high (compared to sea level). In the same way we call the lowest voltage in a circuit zero volts and give it the name ground. Then all other points in the circuit are compared to that ground point. Rivers always flow towards sea level and currents always flow towards ground.

A battery is similar to a dam. On one side is a lot of stored up energy. When a path is formed from that side to the other side then current flows. If there is no path then current does not flow and the energy just stays there waiting for a path to form to the other side. The path can be a big path with lots of current flowing or a small path with just a little bit of current flowing. With a dam, a little bit of water flow could go on for a long time, but flow through a big path that lets all the water go at once would only last a short while. A battery is the same. If there is big path from the high voltage side to the low voltage side then the battery will not last long.

There are two special cases that we give names. One is when the current is zero (open circuit) and the other is when the voltage is zero (short circuit).

Open Circuit

An open circuit is when two points are not connected by anything. No current flows and nothing happens. If a wire in your vacuum cleaner breaks it can cause an open circuit and no current can flow so it does not do anything. There may be a voltage between those two points but the current can not flow with out a connection.

Short Circuit

A short circuit (or short) is when two points with different voltage levels are connected with no resistance (see resistors) between two points. This can cause a large amount of current to flow. If a short circuit happens in your house, it will usually cause a circuit breaker to break or a fuse to blow. If there is no device to limit the current, the wires may melt and cause a fire. This situation is something like a dam breaking. There is a large amount of energy suddenly free to flow from a high point to a low point with nothing to limit the current.

Series Connection

A series connection is when two components are joined together by a common leg and nothing else is connected to that point as shown in Figure 2.

Figure 2

Parallel Connection

A parallel connection is when two components are joined together by both legs as shown in Figure 3.

Figure 3

Finding the Value of Resistor Color Codes

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Finding the Value of a Resistor by Color Codes

To calculate the value of a resistor using the color coded stripes on the resistor, use the following procedure.

Step One: Turn the resistor so that the gold or silver stripe is at the right end of the resistor.

Step Two: Look at the color of the first two stripes on the left end. These correspond to the first two digits of the resistor value. Use the table given below to determine the first two digits.

Step Three: Look at the third stripe from the left. This corresponds to a multiplication value. Find the value using the table below.

Step Four: Multiply the two digit number from step two by the number from step three. This is the value of the resistor n ohms. The fourth stripe indicates the accuracy of the resistor. A gold stripe means the value of the resistor may vary by 5% from the value given by the stripes.

Resistor Color Codes (with gold or silver strip on right end)

Color	First Stripe	Second Stripe	Third Stripe	Fourth Stripe
Black	0	0	x1
Brown	1	1	x10
Red	2	2	x100
Orange	3	3	x1,000
Yellow	4	4	x10,000
Green	5	5	x100,000
Blue	6	6	x1,000,000
Purple	7	7
Gray	8	8
White	9	9
Gold				5%
Silver				10%

Follow the above procedure with the examples below and soon you will be able to quickly determine the value of a resistor by just a glance at the color coded stripes.

Examples

Example1:

You are given a resistor whose stripes are colored from left to right as brown, black, orange, gold. Find the resistance value.

Step One: The gold stripe is on the right so go to Step Two.

Step Two: The first stripe is brown which has a value of 1. The second stripe is black which has a value of 0. Therefore the first two digits of the resistance value are 10.

Step Three: The third stripe is orange which means x 1,000.

Step Four: The value of the resistance is found as 10 x 1000 = 10,000 ohms (10 kilohms = 10 kohms).

The gold stripe means the actual value of the resistor mar vary by 5% meaning the actual value will be somewhere between 9,500 ohms and 10,500 ohms. (Since 5% of 10,000 = 0.05 x 10,000 = 500)

Example2:

You are given a resistor whose stripes are colored from left to right as orange, orange, brown, silver. Find the resistance value.

Step One: The silver stripe is on the right so go to Step Two.

Step Two: The first stripe is orange which has a value of 3. The second stripe is orange which has a value of 3. Therefore the first two digits of the resistance value are 33.

Step Three: The third stripe is brown which means x 10.

Step Four: The value of the resistance is found as 33 x 10 = 330 ohms.

The silver stripe means the actual value of the resistor mar vary by 10% meaning the actual value will be between 297 ohms and 363 ohms. (Since 10% of 330 = 0.10 x 330 = 33)

Example3:

You are given a resistor whose stripes are colored from left to right as blue, gray, red, gold. Find the resistance value.

Step One: The gold stripe is on the right so go to Step Two.

Step Two: The first stripe is blue which has a value of 6. The second stripe is gray which has a value of 8. Therefore the first two digits of the resistance value are 68.

Step Three: The third stripe is red which means x 100.

Step Four: The value of the resistance is found as 68 x 100 = 6800 ohms (6.8 kilohms = 6.8 kohms).

The gold stripe means the actual value of the resistor mar vary by 5% meaning the actual value will be somewhere between 6,460 ohms and 7,140 ohms. (Since 5% of 6,800 = 0.05 x 6,800 = 340)

Example 4:

You are given a resistor whose stripes are colored from left to right as green, brown, black, gold. Find the resistance value.

Step One: The gold stripe is on the right so go to Step Two.

Step Two: The first stripe is green which has a value of 5. The second stripe is brown which has a value of 1. Therefore the first two digits of the resistance value are 51.

Step Three: The third stripe is black which means x 1.

Step Four: The value of the resistance is found as 51 x 1 = 51 ohms.

The gold stripe means the actual value of the resistor mar vary by 5% meaning the actual value will be somewhere between 48.45 ohms and 53.55 ohms. (Since 5% of 51 = 0.05 x 51 = 2.55)

Other Resistor Information

There are some more rules that may be useful when working with resistors. You do not need to know them but if you need a resistor with a value that you do not have, you my be able to use the following information to create the value of resistor you need.

First Rule for Resistors : Series Connection

When two resistors are connected in series, as shown in Figure 1, the new resistance between points A and B is R1 + R2.

AB
Figure 1

The resistors add together. For example if R1 = 500 ohms and R2 = 250 ohms then the resistance between points A and B would be R1 + R2 = 500 + 250 = 750 ohms.

Second Rule for Resistors : Parallel Connection

When two resistors are connected in parallel, as shown in Figure 2, the new resistance is smaller than either R1 or R2. The new resistance between points A and B is (R1 x R2) / (R1 + R2).

A

B

Figure 2

For example, if R1 = 500 and R2 = 250 then the resistance between points A and B = (500 x 250) / (500 + 250) = (125,000) / (750) = 167 ohms. If R1 = R2 then the new resistance is just R1 / 2.

Using these two rules, resistors can be combined to form new resistance values.