V. Ryan 2010-2022
Potentiometers are used in circuits, when it is necessary to alter the resistance. Dark/light and temperature sensors usually have these components, as the potentiometer / variable resistor allows the circuit to be made more or less sensitive (they can be turned up or down - reducing or increasing resistance).
The long handle on the potentiometer / variable resistor can be turned clockwise or anticlockwise, altering the value of the resistance.

The range of resistance varies, for example:
0 to 100 ohms
0 to 1M ohms
This can be seen when using circuit simulation software such as Circuit Wizard. The resistance value of a potentiometer / variable resistor can be altered.

The animations starts with a 4.67% resistance level and reaches a resistance level of 28.33%. It can go all the way up to 100% resistance, preventing current flow, in a circuit.

The resistance is altered by using the computer’s mouse, to turn the potentiometer’s dial.
The circuit below is a temperature sensor. When the temperature drops below 3 degrees centigrade, an LED illuminates. Increasing the resistance value of the potentiometer / variable resistor, by turning its handle, makes the circuit less sensitive. For instance, the temperature sensor would require a higher temperature (e.g. 8 degrees centigrade) before the LED illuminates.

Circuit explanation in detail:

When the thermistor is warmed up by an hair drier its resistance drops, this will take a few seconds. As its resistance drops current begins to flow from positive 9volts to negative 0 volts. Current flows into the base of the transistors allowing the LED to light.

The handle of the potentiometer / variable resistor can be turned up or down, to increase or decrease resistance, in this way it can make the circuit more or less sensitive.
The circuit below, shows a potentiometer being used an input device, connected to input A/D 1 of a programmable microcontroller circuit. When the resistance of the potentiometer rises to a particular level (approximately to 27K resistance), the LED illuminates and the solenoid energises.

Diagram ‘C’ shows the complete circuit and the resistance value of the potentiometer, analogue sensor, as the LED illuminates.
The 3D circuit (below) is a GENIE E18 PIC microcontroller circuit, similar to the one above. The potentiometer is the only input. When the handle is turned either clockwise or anticlockwise, its resistance varies immensely. This means that a potentiometer can have a large range of settings, making it ideal as an analogue sensor.