Preset resistors are used in circuits when it is necessary to alter the resistance. Dark/light and temperature sensors usually have these components as the preset resistor allows the circuit to be made more or less sensitive (they can be turned up or down - reducing or increasing resistance).

A small screwdriver can be used to turn the centre part of the preset resistor, altering the value of the resistance.

The range of resistance varies, for example:
0 to 100 ohms
0 to 1M ohms

The two circuits below are sensor circuits. The one of the left is a temperature sensor and the one on the right is a light sensitive circuit. Increasing the value of the preset resistor by turning the centre with a small screwdriver makes the circuit less sensitive. For instance, the temperature sensor would require a higher temperature and the light sensitive circuit would need more intense light before they activated.

TEMPERATURE SENSOR

LIGHT SENSOR

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