Until now, our circuits have been static. Once soldered, they always did the same thing. The potentiometer is the component that allows us to interact with electronics.
Technically, it is nothing more than a variable resistor. But its three-leg construction makes it the perfect tool for generating analog voltages.
What’s Inside
If you open a potentiometer, you’ll see a very ingenious mechanical design:
- The Resistive Track: It’s a horseshoe made of carbon or cermet that has a fixed resistance (e.g., 10k
) from end to end. - The Wiper: It’s a metal leg connected to the rotating shaft. This leg physically rubs against the track.
The Three Pins
- Pin 1 and Pin 3 (Ends): They are connected to the ends of the track. If you measure the resistance between them, you will always get the total value (e.g., 10k), no matter how much you turn it.
- Pin 2 (Center): This is the Wiper. When you turn it, it moves closer to either Pin 1 or Pin 3.
The Variable Voltage Divider
Although you can use it as a “Rheostat” (using only 2 legs to vary the series resistance), 99% of the time you will use it as a Voltage Divider.
We connect:
- Pin 1: To 5V (VCC).
- Pin 3: To 0V (GND).
- Pin 2 (Output): To the analog pin of your Arduino (A0).
By turning the knob, Pin 2 electrically travels from 0V to 5V.
This is the standard way to tell a microcontroller a desired position, volume, or speed.
Linear and Logarithmic Potentiometers
You go to buy a potentiometer and you see it says “B10K” or “A10K”. Which one do you choose? If you get it wrong, your project will feel “off”.
Type B: Linear
- Behavior: If you turn the knob 50%, the resistance is 50%. If you turn it 25%, it’s 25%.
- The Curve: It’s a straight line.
- Use: Sensors and Controls. For reading angles in a robotic arm, adjusting screen brightness, or setting parameters on an Arduino. You want the movement to be proportional.
Type A: Logarithmic
- Behavior: At first, the resistance changes very slowly. At the end, it changes all at once. At 50% of the turn, you might only have 15% of the resistance.
- The Curve: It’s exponential.
- Use: Volume and Audio.
- Why? The human ear is not linear; it’s logarithmic. For us to perceive that the volume increases smoothly from 1 to 10, the electrical power has to increase exponentially. If you use a Linear one for volume, it will seem like the volume jumps from 0 to 100 as soon as you touch it a little.
In the old days (and with some rare manufacturers), it was the other way around (A=Linear, B=Log). But the modern global standard is A=Audio (Log), B=Linear. When in doubt, measure it with a multimeter at 50% rotation.
Digital Potentiometers (DigiPots)
There are chips that are potentiometers controlled by SPI or I2C. They have a resistor ladder and transistors inside that switch the steps. They are very useful for automatically calibrating circuits without needing an operator with a screwdriver.
Potentiometers and Power ⚠️
Classic Mistake: “I want to slow down this toy motor, so I’ll put a potentiometer in series with the red wire.”* Result: Smoke and a burning smell in 5 seconds.
Why? Common potentiometers are for signals, not power. Their permissible power rating is usually low and can also be reduced when we only use a part of the track.
A motor, even a small one, can draw 1 Ampere. Passing through the carbon track of the potentiometer dissipates much more power than the plastic can handle.
The Solution: The potentiometer should only give the command (reference voltage signal). This weak signal goes to the base of a Transistor (or a PWM driver), and it is the transistor that handles the motor’s power.
