Variable and light dependent

The last two kinds are the interesting ones, because their resistance is not fixed — one you change, and one changes itself.

Variable resistors

Resistors made so that their value can be changed as required are called variable resistors.

The value is changed by turning or sliding the component, either by hand or with a screwdriver. There are several kinds.

Preset resistors Rheostats Volume controllers

Figure to be added

Figure 19.27 — (a) several kinds of variable resistor: a rheostat, a preset resistor, and a rotating volume controller; (b) the circuit symbols used for a variable resistor, a rectangle and a zigzag each with an arrow through it.

Variable resistors are used for controlling the volume of a radio, for adjusting various electronic circuits, and for changing the current in laboratory experiments — which is exactly what the rheostat did in the Ohm's law activity.

Light dependent resistors

Light dependent resistors (LDRs) are made using special chemical substances such as cadmium sulphide. Their resistance changes according to the intensity of the light falling on them.

In the dark

When there is little light, the resistance of these resistors is high.

In the light

When light is present, the resistance of these resistors is low.

Figure to be added

Figure 19.28 — (a) the circuit symbols for a light dependent resistor, a resistor symbol in a circle with two arrows pointing in at it; (b) the appearance of an actual LDR, a small disc with a wavy track on its face.

A resistor that senses

Because its resistance follows the light, an LDR turns "how bright is it?" into "how much current flows?" — which is why they are used in circuits that have to switch a device on and off according to the light level, such as a street lamp.