Sensors

Two very simple devices to use in sensor circuits are the LDR and the thermistor.

An LDR could be used to operate a burglar alarm or to switch on a light in ambient darkness (both requiring an increase in output pd when the light hitting the LDR goes down) or to open a garage door when the headlights hit it (requiring a rise in pd when the light intensity goes up) or to operate a diaphragm on a camera (small opening in bright light and vice versa).

A thermistor could be used to activate a sprinkler or a fire alarm when it gets too hot, or to switch a refrigerator unit on when the temperature in the freezing compartment rises (for all of these, the output pd needs to rise when the temperature goes up) or to switch a central heating system on when the ambient temperature falls below a set level (output pd needs to rise when the temperature goes down).

If you have a series combination of a fixed resistor R_F and the sensor resistor, R_S (the LDR or the thermistor), with a voltage source across the combination (ie a potential divider), the pd across one of the resistors will change most rapidly when the sensor resistance is about equal to the fixed resistor. A rapid change of output pd for a small change of sensory input constitutes a high sensitivity, so there is something to be said for making the fixed resistor adjustable, so that you can control the range of sensory input over which the circuit gives the greatest response.

The voltage you get across the output of a potential divider can give an indication of the size of the stimulus (by connecting a voltmeter); but it cannot operate anything, since the effective internal resistance of the cell/potential divider combination is too high. To interpret the voltmeter reading, you need calibration data in the form of a look-up table or graph, or you have to use an analogue instrument with a custom-made scale showing light intensity or temperature (or whatever) instead of potential difference. To operate something you need some sort of amplifier A 'follower' circuit is useful in this context. This circuit would enable you to make a fan blow more strongly, the weaker the light intensity was.

+Vs and -Vs are two of the three power rails (typically +15 V and -15 V), and there has to be a 0 V rail too, shown by the symbol at bottom right). These power rails provide the energy to drive the fan. The triangular thing is called an op-amp (operational amplifier). It's output (on the right) pretends to be a power source whose voltage is the same as the voltage presented to the + input, but without any internal resistance problems to worry about. And the brilliant thing is that its input resistance is so high that the potential divider doesn't know it is there at all.

If you want a device to switch on completely when the potentiometer output reaches a particular voltage, then you need a comparator first. This arrangement will switch a merry-go-round on when a bonfire is lit beneath it.

Suppose there is no fire. The thermistor resistance is high, so most of the pd is across it and little is across R. Hence the potential at A (relative to 0 V) is low. The potential at B is set to some middling value by the potentiometer to the left of B. If the potential at B is higher than that at A, then the output of the first op-amp is low, and vice-versa. So C and D are low, and the relay doesn't operate. Now let the fire be lit. The resistance of the thermistor goes down, the potential at A starts rising. When it gets just above B, the output at C suddenly flips high, D follows it, the relay operates and the merry-go-round goes round.