Variable Resistors (only two pins are used)

These have a circular or linear track made of resistive material. As the resistor is adjusted, a slider moves along the track varying the resistance. Two of the three pins are used.

Potentiometers (all three pins are used)

These devices are also called potentiometers or just pots.

They can be pre-set and adjusted with a small screw driver or adjusted with a knob.

Uses include light dimmers, volume controls, tone controls and voltage adjusters.

Here is an animation to show how a potentiometer works. Use the mouse to turn the knob to move the slider along the resistive track. The position of the slider determines the resistance.

 

Volume Control

By adjusting the slider, it is possible to send between 0 and 100% of the preamplifier output to the power amplifier.

The coupling capacitors are used to block DC voltages at the preamplifier output and power amplifier input. At the same time they allow the AC signals to pass.

Position Measurement

If a potentiometer is connected to a rotating shaft, the voltage output from the potentiometer depends on the shaft position. This arrangement is best for shafts that do not spin continuously. Examples include the rudder position on a boat or aircraft, the position of a steerable antenna or satellite dish and the position of a game joystick. A comparator circuit is used to compare the measured position with the wanted position. The comparator output is used to control a servo, motor or hydraulic valves which adjusts the shaft position. Both model and full size ships and aircraft use systems like this.