Tuesday, June 24, 2008



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.

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