Basic Electronic Components

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.

by: http://www.iguanalabs.com/

ELECTRICITY BASIC

Electricity is the flow of electrons in a conductor and there are four quite intuitive quantities help to characterize it. voltage, current, resistance and power

The first is voltage. This term refers to the level of energy electrons have relative to some reference point (often called ground in a circuit). The higher the voltage, the more energy electrons have to do work as they travel through the circuit. In general, if two points are at a different voltage relative to each other, electricity will flow from one to the other if they are connected by something that conducts electricity. The unit of measurement of voltage is the Volt (V). As a sample of voltages our wall outlets at home (at least in the US) are 110V, the AA, C & D cells we buy at the corner store are all rated at 1.5V, and the electronics on Teleo Modules requires 5V.

The next quantity is current. This is an expression of how much charge is travelling through the conductor per second. The unit of measurement for current is the Amp (A). You can see that voltage and current are separate things: you can have a very small current at a very high voltage, a huge current at a very high voltage and so on.

The next quantity is resistance. Resistance is an expression of the degree to which electron flow will be impeded through a conductor. The unit is the Ohm (). In simple circuits resistance determines the relation between voltage and current. At the extremes, a short piece of wire will have a resistance of nearly zero Ohms, while an air gap (for example in an open switch) has very large resistance (millions of Ohms). Intuitively a couple of relationships will hold: in a conductor, a voltage difference between the two ends will cause a current to flow. How much current will be determined by how much resistance the conductor offers. If there's less resistance more current will flow. In fact, given a power source of high enough capacity, if you half the resistance, you will double the current. Conversely, if you double the resistance, you will half the current.

The final quantity is power. The unit of power is the Watt. It's an expression of the overall energy consumed by a component. It is worked out by multipling the voltage and the current together - P = VI. For example if a motor was running at 12V and the current it was drawing was 2A, the power it would be dissipating would be 24W.

CIRCUIT BASICS

For electricity to flow, there needs to be a path that connects all the elements together. In the diagram below, you can see how electricity can travel from the cell around in a loop through the lamp and back to the cell again provided all the wires are in their proper places.

It should be noted that we show electricity travelling from the positive (+) side of the cell around the circuit to the negative side. This is called conventional current. The slightly odd thing about this is that the electrons that constitute electrical current are negatively charged and actually travel in the opposite direction. The fact that we depict current travelling from the positive to the negative is an historical accident. Fortunately unless you're doing something esoteric like semiconductor physics, this extra layer of complexity need never worry you.

Lamp and Cell Circuit

Circuit diagrams provide a very efficient way to describe an electronic circuit. They use a small set of symbols and conventions that need to be learned but the benefits of their form over a more pictorial style are so definitive that they are used universally.

The circuit above can be diagrammed more efficiently. In order to illustrate this, here are some symbols used to depict cells and lamps.

Cell. This is often called a battery, but technically a battery is multiple cells. This is what C, D, AA and AAA cells we can get at the corner store all are. This kind of cells is rated at 1.5V.

Lamp. Like you might find in a flashlight. Lamps have voltage ratings like many things. This rating indicates the voltage that the lamp is designed to run at. It will be the highest voltage the lamp can withstand without getting too hot and burning out. Lamps may also state their wattage - the power they consume. From this and the a re-arrangement of the equation for power (I = P / V) the likely current consumed can be calculated.

With these symbols we can now construct a circuit diagram with a cell and a lamp.

Simple Lamp and Cell Circuit Diagram

This circuit diagram expresses only the essential features of the circuit. The abstract symbols hide all the myriad details of appearance that are actually irrelevant to the circuit. For example, the circuit is not influenced by the colors of the wires so they're not indicated, curves in the wire are irrelevant so the lines in the diagram are all straight, the actual physical design of the lightbulb is from an electrical point of view, immaterial, etc.

The lines on the diagram need not even be wires. They can be traces of copper on a printed circuit board (PCB) or pretty much anything that conducts. In a car, for example, wires that go from parts of the motor, headlights and other devices back to the negative terminal of the battery are not needed. The metal of the car's chassis and body are the return path to the battery and complete the circuit.

Another very important abstraction is the connection of wires. People are often confused by this. The diagram below shows the same circuit as before except for the addition of another lamp. The problems arise when people try to figure out exactly how to connect the things together. How literally does the diagram need to be followed?

Dual Lamp Circuit

Take the top section of the circuit. It depicts a long section of conductor with one lamp getting its power from it about half way along and another getting its power at the far end. As illustrated below, this shouldn't be taken literally. Since the wires in this circuit are pretty much perfect conductors, it really doesn't matter. As long as all the things that are shown to be connected together are connected, the circuit will work.

Dual Lamp Circuit Wiring Alternatives

ELECTRICAL AND ELECTRONICS DEVICES

ELECTRICAL AND ELECTRONIC DEVICES

To move beyond cells and lamps, we need to introduce some more electrical and electronic devices and their symbols.

	Push button. A normally open push button conducts electricity when it is being pressed, otherwise it's an open circuit.
	Switch. Has an on and an off position. Conducts when it's on and is an open circuit when off.

To see how devices combine, the cell and lamp circuit from above is recreated below with the addition of a switch to turn the lamp on and off. The switch works, just like it looks like in the diagram, by making or breaking a connection which completes the circuit or leaves it open. An important observation is that it doesn't matter whether the switch is on the connection from the positive side of the battery to the lamp or on the negative side. As long as it can disrupt the circuit somewhere, it will work as a switch.

Lamp Circuit with switch

More sophisticated circuits require more complex components. Some more are presented below.

	Resistor. Device that resists the flow of electricity equally in both directions. The two main important values associated with resistors are their resistance and their power rating. Resistance is measured in Ohms (). An Ohm is quite a small measurement for a lot of electrical applications so the k (or just k) is often used. 1k is 1000. The other value is power. Resistors dissipate energy so its important that exactly how much energy they can dissipate is known. Most applications for resistors require only fractional Watts of power.
	Capacitor. Device that temporarily stores electric charge. There are two main important values that characterize a capacitor. The first is the capacitance - measured in Farads. It turns out that a Farad is a huge amount, so capacitors are often measured in micro-Farads (F) or pico-Farads (pF). The other important quantity is the rated voltage. This value must never be exceeded in a circuit.
	Diode. Semiconductor device that conducts electricity in only one direction. Exist in different varieties. Zener diodes permit conduction in the reverse direction only when the reverse voltage exceeds a certain amount. TVS diodes are like Zeners except capable of much higher currents.
	MOSFET. (very short for metal-oxide silicon field-effect transistor). Special kind of transistor switch. When the Gate (G) terminal voltage is brought sufficiently high, current will flow from the Drain (D) to the Source (S) terminal. Usually require the Gate voltage to be 12V above the Source, but logic-level MOSFET's can work directly from a microprocessor output. MOSFETs are characterized by several values: their maximum voltage, their resistance when they're on and their maximum power dissipation. Be wary of manufacturer's claims about maximum current often these are exaggerated and require the device be kept at 25°C which often requires liquid nitrogen cooling...

	Potentiometer. A variable resistor. Often connected as a voltage divider to create variable voltages when used as a rotational position sensor.
	LED. Light Emitting Diode. Common indicator in electronics. Produces a lot of light for not much current. But will very quickly (perhaps instantaneously) burn out if too much current is allowed to flow in it. Like any diode, has very low resistance in its conducting direction, so a resistor in series with it to limit the current is usually a requirement.
	Photoresistor. A resistor with the useful property that its resistance changes depending on how much light it is receiving. Photoresistors can have a quite impressive resistance range, for example from a few million ohms (M) in the dark to under a hundred ohms in bright light. One possible disadvantage is that their reaction time is in the order of 100ms - too slow for many applications.

	Supply. The Supply symbol is a diagrammatical shortcut used to indicate that the wire is connected back to the power. It saves having to draw a wire from the power source to every point in the circuit that uses it.
	Ground. The Ground symbol is a diagrammatical shortcut of the same kind as the supply. It is used to indicate that whatever is connected should be considered to be connected to the Ground of the power supply.

	Motor. Conventional DC motor. When deciding how to control a given motor there are several important issues: what voltage was the motor designed to work with and how much current does it draw when it's running. Commonly available DC motors can draw anything from 10mA to more than 100A. Motor selection is a huge topic. You'll need to consider voltage, power consumption, RPM, torque, start and stall current, mounting requirements, heat dissipation, etc.
	Coil. Can represent a relay, a pneumatic or hydraulic valve or solenoid. The principle is the same in all cases: when the coil is energized, it creates a magnetic field which attracts some metal part. Some coils can heat up if left on for a long time so they are often given a duty-cycle meaning that their designer specifies whether they can be left on indefinitely or whether they're designed only to switch on and off again rarely.
	Battery. The idea is that there is one wide and narrow line (cell) for each cell in the battery. When it becomes onerous to draw all the cells, an ellipsis is added before the last cell. The common rectangular 9V battery we buy at the store is in fact 6 1.5V cells stacked together.

OHM'S LAW

OHM'S LAW

We mentioned earlier that there is a relationship between voltage, current and resistance. It turns out that this relationship is a mathematical one and it can be expressed very simply by way of Ohm's Law.

Ohm's Law observes that in a simple resistive circuit, voltage (V) , resistance (R) and current (I) are related in the following way:

V = IR

This expression can be rearranged algebraically to find other ways to use it as follows:

I = V / R (dividing both sides by R)
R = V / I (dividing both sides by I)

The idea is that if you know two of the quantities, you can work out the third by using one of the equations.

Resistor

In the circuit fragment above, a resistor is connected between a 5V supply and ground (0V). We can use this to check the assertions we made earlier about the effects of doubling the resistance on a circuit and so on. If the resistor's value is 10, we can work out what current will flow as follows. We know the voltage (5V) and the resistance (10), so the form of the equation we need is:

I = V / R

Substituting our values in we have:

I = 5 / 10

The current I in our circuit will be 0.5A which can also be expressed as 500mA.

Now if we were to double the resistance (to 20), let's confirm that we halve the current:

I = V / R

Substituting our values in we have:

I = 5 / 20

The current I in our new circuit will be 0.25A (or 250mA), which is indeed half the previous current.

Ohm's Law is a very important relation to remember how to use. Finding current from resistance and voltage or finding voltage from current and resistance is something that is required very frequently in electronics.

The energy of the electricity passing through the resistor is being converted by the resistor into waste heat. The ability of the resistor to do this depends on how big it is. If it's too small it will not be able to get rid of the heat fast enough and it may start to overheat and eventual burn.

The resistor's power rating is stated in Watts. Recall that the power is calculated by multiplying the voltage by the current:

P = VI

In this last case, the voltage across the resistor was the same, 5V, but the current was halved to 0.25A. Putting those values into the equation gives:

P = (5)(0.25)

P = 1.25W

Thus, the resistor chosen must be rated at least 1.25 W - which is rather a large resistor in the context of digital electronics.

As a side note and another nice use of Ohm's Law, the V term in the power expression P = VI can be substuted by IR, making P = IR, which a very convenient way to calculate power when the voltage drop across something is not known.

VOLTAGE DIVIDERS

VOLTAGE DIVIDERS

One of the most important concepts when interfacing sensors is the concept of the voltage divider.

Consider the circuit we were looking at in the last section. If we were to take a look at the voltage at the high side of the resistor, we would measure approximately 5V. If we were to measure the voltage at the low side we'd see approximately 0V. What would we measure in the middle of the resistor? Presumably with half the resistive material above and half below we'd measure 2.5V.

Voltage drop in resistor

If we have two resistors, this desire to peek inside a resistor can be faked. This stems from the observation that when two resistors are connected in line (called in series) their resistances add linearly. Two 1000 (1k) resistors in series have a total resistance of 2k, for example. Thus the point p between the two resistors can be thought of as the midpoint of a 2k resistor.

Two Resistors

Here intuitively, it can be seen that there's a good case for suspecting that the voltage at point p is indeed 2.5V. We can actually prove this with some simple math and using Ohm's Law. If math's not your strong point you can skip lightly over the next paragraph or two.

So in our the circuit above, we have two resistors, of 1k each. Their total resistance is 2k, so, by Ohms law, the current running through them is

I = V / R

I = 5 / 2000

I = 0.0025A or 2.5mA

Using Ohm's Law, we can actually work out what that voltage will be. The trick is that now we know the current running through both of the resistors, we can apply Ohm's law to each individual one to see what the voltage across it is. Taking the bottom resistor,

V = IR

V = 0.0025 * 1000

V = 2.5V

This means that at the point p there will be a voltage of 2.5V. So our intuition is correct.

Using two resistances (of potentially various kinds and values) a voltage somewhere between the supplied voltage and zero can be obtained. If the high side resistor is converted into a smaller value than the low side resistor, the voltage at p will be higher. If the high side resistor is converted into a higher value than the low side resistor the voltage at p will be lower.

Now if you have a way of measuring voltages (like we do on a number of our Teleo modules) this can be a very useful thing. We can see how useful in the next few example circuits.

Firstly there is the potentiometer. The potentiometer the device that used to control the volume and other things on your stereo before everything became digital and microprocessor controlled. You turned it and things got louder or quieter. The potentiometer is just a voltage divider. It has a resistive strip arranged in a circle and an arm (called a wiper) that can tap onto it at any point on the circle. Power is applied to one end of the resistive strip and ground to the other. The wiper can slide to any point in between.

In the circuit below, when the wiper is close to the 5V the point p will be at a value close to 5V. When it's halfway, it will be around 2.5V and when it's close to the other end it will be close to 0V.

Potentiometer

Another example of the usefulness of a voltage divider is in the use of a resistive sensor (like a photoresistor). The circuit below shows how this is wired. This time point p measures 2.5V only if the resistance of the photoresistor is almost exactly the same as the resistor below it (r). If the photoresistor's resistance falls (as happens when it is exposed to light) the voltage at p will rise. Conversely if the photoresistor's resistance rises (as happens when it is removed from the light), the voltage at p will fall. A device monitoring the voltage at line p will therefore be able to monitor the amount of light on the photoresistor.

Light Sensor

On final example to illustrate the usefulness of the voltage divider is the push button used as a digital input to electronics. The idea is to get a high voltage at p when the button is presses and a low one when it is not.

Push Button without a Voltage Divider

The first question to ask is why bother with a voltage divider at all? Without it, won't the push button connect p to 5V when it's pushed and leave it at 0V when un-pushed? This is true for the first part, but not true for the second part. In fact what happens is that the button does indeed connect p to 5V when it's pushed, but when it's released, the wire to p is actually unconnected (called floating). It is not getting pulled up to 5V, nor is it getting pulled down to 0V. Electrical noise from nearby wires and other devices will make the actual voltage on the wire fluctuate unpredictably. Sensitive electronics reading this voltage may get erroneous readings in this state.

The solution is to make a voltage divider. When the switch is open, its resistance is very very high compared to the resistor, so the voltage at p is very close to 0V. When the switch is closed, its resistance is close to zero: very low when compared to the resistor, so the point p reads a voltage close to 5V. In most modern digital circuits a fairly high value resistor (like 10k) can be used so that the current being wasted by flowing through the switch and through the resisitor is kept to a fairly low amount. (this can be calculated using Ohm's Law: I = V / R, I = 5 / 10000, I = 0.5mA).

Push ButtonVoltage Divider

When a resistor is used in this way - to make sure an input reads 0V when there is no circuitry driving the line either way - it is called a pull-down resistor. A resistor tied to +5v, for example, to make sure an input reads a positive voltage, when there is no driving circuitry is called a pull-up resistor.

DIGITAL LOGIC

DIGITAL LOGIC

Digital Logic is a whole subcategory of electronics stemming from the observation that by limiting signal values to two discrete values (5V and 0V, say) you can represent binary values: i.e. values made up of only the digits "0" and "1". Each one of these binary digits is called a bit (a contraction of "binary digit"). By convention, in a 5V system, 0V is assigned to the value 0 and 5V is assigned the value 1.

Strings of 0's and 1's can represent anything we like. We can have them represent numbers by using the binary number system. The binary number system is exactly like our base 10 numbering system except there are only the two numerals (0 and 1), not ten (0,1,2,3,4,5,6,7,8,9). In case you haven't done it for a while, in the decimal system we count like this (although normally we don't put the leading zero's in).

000, 001, 002, 003, 004, 005, 006, 007, 008, 009, 010, 011, 012, 013, ... 095, 096, 097, 098, 099, 100, 101, ...

The idea is that as we count, we go through all of our numerals in the rightmost column, then when we run out we increase the next column and go back to the 0 in the first column and repeat until the next column's numerals are all used up and so on. In the binary system we count exactly the same way, except we only have two numerals, so the next column thing happens a lot more often.

000,	001,	010,	011,	100,	101,	110,	111,	...
(0)	(1)	(2)	(3)	(4)	(5)	(6)	(7)

You might think that this is kind of wasteful, since we're already using three digits (bits) by the time we've counted to 4, but this is offset by the fact that each digit can be assigned to just one wire carrying a 0 or 1. With three bits we can represent up to 8 things. With 8 bits (which is called a byte) we can represent up to 256 things. This number grows quickly, by the time you get to 64 bits, you can represent 18446744073709551616 things.

As you might suspect, you can add, subtract, multiply, divide, binary numbers very easily using the same techniques you're familiar with in regular decimal arithmetic. You can even have binary decimal fractions (111.01, for example is 7.25) and do all kinds of heavy mathematics. The things you can represent by binary numbers can be bank balances, colors, calendar dates, distances, speeds, instructions - anything you like. This is where computers come in - computers work with and by manipulation of binary numbers, all trading on the idea that you can use wires carrying 0's and 1's to represent things in the real world and also to switch circuitry on and off.

Microprocessors use 0's and 1's to do everything, so it should not be surprising that getting 0's and 1's in and out of them is extremely easy. In a conventional 5V system, If an input pin is connected to 5V, it will read a "1". If it's connected to a 0V it will read a "0". Similarly, if an output is set internally to 1 it will have 5V on the pin or 0V if a 0 set. Note that outputs can sometimes be disabled. When this happens the output is said to be floating and could have any value.

A relatively new phenomenon is the presence of analog inputs on microprocessors. Analog inputs permit the reading of voltages between 0V and the value of the chip's power supply voltage. (Note that a microprocessor should never be directly connected to a voltage greater than it's power supply). Inside the microprocessor equipped to handle analog input, there is a device called an analog to digital converter (ADC) which samples the voltage on the pin and converts it into a binary number. The resolution of the ADC is just the number of binary digits that it resolves the voltage into. If it were a 1bit ADC, it's one digit would be able to resolve the voltage range into only two levels (i.e. <>2.5V). A 2bit ADC would resolve the voltage range into four levels (00, 01, 10, 11), and so on. The PIC18 microprocessors have 10bit ADC's, so they can resolve the 0 - 5V range into 1024 levels - each one being 5V / 1024 = about 0.0049V.

PULSE WIDTH MODULATION (PWM)

PULSE WIDTH MODULATION (PWM)

It turns out that creating a variable voltage at more than a few hundred milliamps efficiently from a power supply is not that easy. Pulse Width Modulation (PWM) is a common technique for using simple on-off switching to produce the effect of a truly variable voltage. Motors, lights and a large variety of other devices can be controlled in this way. House light dimmers work the same way.

What happens is that instead of a device either solidly on or off, it is switched on and off many times a second. In the diagram below, voltage is plotted against time. The waveform in gray shows the rapid switching output of the PWM circuit, and the effective voltage is shaded in as an overlay.

PWM Concept Diagram

On the left portion of the diagram, the power is only on for a small amount of time, so the effective voltage the device under control experiences is very small. In the mid portion, the power is on for half of the total time, so the controlled device experiences approximately half the full voltage. Finally in the right portion of the diagram, the power is on for all the time, each cycle, in fact it never goes off at all, and so the voltage the device experiences is the full value of the supply.

The portion of each cycle for which the power is switched on is called the duty cycle, and is often expressed as a percentage ranging from 0% to 100%.

The effect this has on lights is like a dimmer - the light glows dimly and brightly depending on the duty cycle. If this technique is applied to a normal DC brushed motor, the motor speeds up and slows down.

POWER SUPPLIES

POWER SUPPLIES

There are many ways to provide power to a Teleo System. The best choice will depend on many things: the current requirements, portability, budget, etc.

In all cases, the power supply is presumed to have at least one output in the range 7.5V - 30V (the voltage range permitted for Teleo Systems). The positive line should be connected to the positive (+) connector on the Teleo Power Module. The ground (sometimes called "-") line should be connected to the other connector.

Connection to the Power Module

Batteries provide a wide range of voltages and current capacities. They are portable and sometimes rechargable. One of the major benefits of batteries (particulary NiCads and Lead Acid batteries) is their ability to deliver huge amounts of current. Care should be take with car batteries and many battery packs not to short circuit them. One obvious disadvantage is that they either need replacement or recharging before very long.

Linear Power Supplies used to be the main way to turn 110V AC (in the US) into DC power that most electronic devices require. They are typically heavy for their current output, but units can be found to supply huge amounts of current where this is necessary. The quality of the power obtained from a linear supply can vary tremendously from perfect to only half rectified (meaning the voltage will vary from 0 volts to some higher value 60 times a second). Unrectified power supplies have the additional problem that their voltage under light loads may be significantly higher than under heavy loads. These have to be monitored closely so their output voltage doesn't exceed the maximum permissible voltage.

Switch-mode Power Supplies can deliver a very high power output from a small package. They now form the bulk of laptop and other power supply equipment. These power supplies are a little quirkier than their linear cousins - sometimes generating electrical noise and sometimes in multiple output supplies requiring certain minimum currents in order for them to work.

WIRING CONVENTIONS

WIRING CONVENTIONS

Choosing a wiring color convention might seem unnecessary and may even run counter to the desired asthetic effect for the machine under construction, but we recommend that you adopt a color convention when wiring. It doesn't much matter what you choose, as long as you're consistent. Two that we try to always use are red for positive and black for ground. You always know that a red line has power and shouldn't be connected to just anywhere. We sometimes also adopt other conventions like green for inputs, orange for outputs, etc. The general idea is to use the same color for all wires that do roughly the same thing.

When it comes time to figure-out what is wired to what these conventions will really help.

Multimeter use

MULTIMETER USE

A multimeter (also known as a meter) is the first test instrument that we recommend you buy if you're just starting out in electronics. They range from US$20 up. Depending on which one you choose to get it will have a different set of features, but most will at least allow you to measure voltage, current and resistance.

Multimeter

There are almost always at least two possible sockets for the positive (red) probe on the multimeter. One will be for measuring voltage and resistance. The other(s) are for measuring current. It is important to put the probe in the correct socket before using the multimeter.

Voltage measurement is achieved by making sure the probe is in the correct socket, selecting the voltage setting, putting the negative (black) probe on the circuit's ground and the positive probe on the point you want to test. You will get a reading of the voltage present. Some multimeters require that you pre-select a voltage range through a variety of methods to enable it to more accurately provide a reading. In these meters, care should be taken to not measure a large voltage when the meter is expecting a small one as damage to the meter can result.

Resistance is measured when a device is not in a circuit or at least the circuit is not powered up. To prepare the multimeter, select the resistance mode and make sure the positive probe is in the correct socket. In this mode the multimeter puts out a small positive voltage on the positive probe and deduces from the current being drawn, what the resistance must be using Ohm's Law . This mode is sometimes supplemented by a continuity function where the meter beeps when the probes are connected via a very low resistance. This is helpful when testing if two points on a device are connected as expected.

Current measurement is achieved by inserting the multimeter into a running circuit. To prepare the multimeter, make sure the positive probe is in the correct socket (often marked A), The circuit actually needs to be broken and re-routed through the multimeter. Be careful to make sure the multimeter can withstand the current you're going to measure. Most have fuses, but it still pays to be careful since who wants to fool around changing mutlimeter fuses all the time?

Wire Choice

Wire Choice

Wire comes in an variety of shapes and forms. There are several key considerations when choosing wire: current capacity (resistance) and mechanical strength being the primary ones that concern us here.

Above all, we recommend that you use stranded wire, rather than the solid wire. The stranded kind is much more tolerant of mechanical movement. It will permit many more flexes before breaking than the solid kind.

Wire Gauges 30, 22, 18, 16 Next to a Quarter.

In the US at least, wire is graded according to the American Wire Gauge (AWG) standard. You can see from the photo above that different wire gauges are considerably different in size. We recommend the following:

AWG 30 - for wiring of very low current electronic signals over short distances where there is absolutely no mechanical movement of the wire. Resistance is 4/1000'

AWG 22 - is excellent for all kinds of hook-up wiring. Not too small to break all the time, and not too heavy to restrict movement. Useful when the current doesn't exceed an amp or two. Very commonly used in long runs of low current twisted pair cable. Resistance is 16.20/1000'.

AWG 18 - quite heavy wire for high current use - up to 10A perhaps. Resistance is 6.3/1000'

AWG 16 - very heavy duty wire for high current use - beyond 10A. Resistance is 4/1000'

Source: http://www.makingthings.com/

Basic Electronics

Kirchhoff's Rules

Current Rule or Junction Rule (conservation of charge)

The algebraic sum of the currents entering any junction must equal the sum of the currents leaving that junction.

Therefore, in a series circuit the current is the same everywhere.

Voltage Rule or Loop Rule (conservation of energy)

The algebraic sum of the potential differences (voltage drops) around a closed conducting loop must equal zero.

Therefore, components connected in parallel have the same voltage across them.

Ohm's Law

where voltage V is in volts, resistance R is in ohms and current I is in amperes.

Joule's Law

where power P is in Watts, voltage V is in volts, resistance R is in ohms and current I is in amperes.

Thévenin's Theorem

Any two-terminal network of resistors and voltage sources is equivalent to a single resistor in series with a single voltage source.

Definitions

1 ampere = 1 coulomb/second

1 coulomb = 1 ampere · second

1 farad = 1 coulomb/volt

1 joule = 1 newton · meter

1 newton = 1 kg · meter/second²

1 ohm = 1 volt/ampere

1 volt = 1 joule/coulomb

1 Watt = 1 joule/second

Symbols, Dimensions and Units of Physical Quantities

Quantity	Common Symbol	Unit	Unit Symbol
Capacitance	C	farad	F
Charge, electrical	q, Q, e	coulomb	C
Current	I	ampere	A
Energy	E, U	joule	J
Force	F	newton	N
Frequency	f, v	hertz	Hz
Length	l, L	meter	m
Potential, electrical or Voltage	V	volt	V
Power	P	Watt	W
Resistance	R	ohm
Time	t	second	s
Work	W	joule	J