Variable Resistors

Variable resistors are another fundamental component of electronics. They allow us to interact with an electronic circuit by using outside stimuli from the real world. These components change their resistive value based on the conditions of the surrounding area or engagement. They come in many different shapes and sizes and come in even more varieties of types of variable resistors. These could be rotational or linear potentiometers, Light Dependent Resistors, Temperature-Dependent Resistors or Thermistors, and more.

The Potentiometer

A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick.

Potentiometer Schematic Symbol and Anatomy

The schematic symbol for a potentiometer looks like the symbol for a traditional resistor with an additional arrow coming from it. This is because it is a resistor but can be varied by spinning a dial or slide- this is what the arrow represents.

A traditional resistor has two terminals, an input and an output, and are nonpolarized (which means it doesn't matter which is which.) A potentiometer has three legs or terminals:

First leg (left or right end): This connects to one end of a resistor inside the potentiometer, where the total resistance begins. On a schematic, this leg is usually labeled A. Resistors are not polarized, so you decide which does not matter. However, typically, we assign the "left" leg as A since we read left to right.
Second leg (middle): This is called the "wiper." It moves along the resistor as you turn the knob, allowing you to select a specific resistance. This leg provides the output or the adjustable voltage. On a schematic, this leg is usually labeled C.
Third leg (opposite end): This connects to the other end of the resistor inside the potentiometer, representing the maximum resistance when the wiper is turned to that side. On a schematic, this leg is usually labeled B. B can also be used as a second input for specialized circuits. These circuits may or may not act as a ground or reference voltage. Examples are as follows:

Ungrounded:

Voltage Divider: In most cases, the potentiometer is used as a voltage divider. Terminal B doesn’t have to be grounded because the wiper provides a variable voltage between V+ (at terminal A) and whatever voltage is at terminal B. If terminal B is unconnected or connected to a fixed voltage (not ground), the wiper’s output will still vary between these two points.

Resistive Element: The potentiometer’s resistive element provides a variable resistance as the wiper moves, regardless of whether terminal B is grounded. The purpose of the potentiometer is to tap into the voltage potential across the resistive element, which doesn’t necessarily require grounding.

Grounded:

Rheostat Configuration: If the potentiometer is being used as a variable resistor (rheostat) rather than a voltage divider, it is common to ground terminal B. In this setup, only one side of the resistive element (the wiper) is used in conjunction with terminal A, and terminal B is grounded to complete the circuit. A common example of a rheostat might be a dimmer switch in a home circuit.

Ground Reference for the Output: If the circuit needs the potentiometer’s output to have a ground reference (such as in audio volume control), terminal B might be grounded to ensure the output is referenced to 0V when the wiper is at one end of its range.

By turning the knob, the wiper moves between the first and third legs, changing the resistance between the wiper (middle leg) and each end. This is how the potentiometer adjusts voltage or resistance in a circuit.

The Light-Dependent Resistor (LDR)

A light-dependent resistor, LDR, or photo-conductive cell is a passive component that decreases or increases resistance with respect to receiving luminosity (light) on the component's sensitive surface. The resistance of a photoresistor will decrease with an increase in light intensity; in other words, it exhibits photoconductivity. A photoresistor can be applied in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as a resistance semiconductor. In the dark, a photoresistor can have a resistance as high as several megaohms (MΩ), while in the light, a photoresistor can have a resistance as low as a few hundred ohms. If incident light on a photoresistor exceeds a certain frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electrons conduct electricity lowering resistance. The resistance range and sensitivity of a photoresistor can substantially differ among different devices. Most LDRs will start with high resistance in the dark and have low resistance in the light- however, some LDRs can be built to have the opposite effect.

LDR Schematic Symbol

The schematic symbol for a potentiometer looks like the symbol for a traditional resistor with an additional circle around it and arrows pointing into it. This is because it is a resistor but can be varied when exposed to light or darkness. The arrows symbolize the light rays that are reaching the LDR while the circle symbolized the glass or plastic clear casing that surrounds the photoresistor.

An LDR only has two terminals, an input and an output, and is nonpolarized (which means it doesn't matter which is which.)

Thermistor

A thermistor is a resistance thermometer or a resistor whose resistance is dependent on temperature. The term is a combination of “thermal” and “resistor”. It is made of metallic oxides, pressed into a bead, disk, or cylindrical shape, and then encapsulated with an impermeable material such as epoxy or glass. Depending on the temperature, the resistance will change because of the material properties a thermistor is made of as it heats and cools.

Thermistor Schematic Symbol

The schematic symbol for a potentiometer looks like the symbol for a traditional resistor with a bent line crossing through it. This is because it is a resistor but is varied when exposed to different ranges of temperatures. The bent line symbolizes the varying temperature.

A Thermistor only has two terminals, an input and an output, and is nonpolarized (which means it doesn't matter which is which.)

Digital Resistor

A digital variable resistor is a type of variable resistor where the change of resistance is not performed by mechanical movement but by electronic signals. They can change resistance in discrete steps and are often controlled by digital protocols such as an IC or bys tepping up and down simple signals.

See our page on Integrated Circuits for the schematic and additional diagrams.

For more information on Variable Resistors and a few experiments to build to further understand them, check out our page on Variable Resistors in a Circuit.

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