A rotary encoder is a special type of switch that converts the motion of the switch (clockwise or counterclockwise) into an output signal that can be used to determine what direction the knob is being rotated. They're capable of continuous rotation - compare that to potentiometers that are only about 3/4 of a circle!
Rotary encoders are not for situations where you need to know the exact position of the knob - for that, you'd want a potentiometer.
Rotary encoders are best in situations where you can use the change in position instead of the exact position. They're used in many applications such as the manual volume control or tuning on a car stereo.
This guide uses a mechanical, incremental rotary encoder. It has three pins: A, C, and B. C is the common ground for A and B. A and B are the signal pins. When you rotate the knob, A and B come into contact with the common ground pin, in a particular order depending on the direction you are rotating the knob. When each pin comes into contact with the common ground, they produce a signal. These signals are shifted out of phase with each other as one pin connects before the other pin. This is called quadrature encoding. You need to listen to those pins and the way they pulse in order to determine the direction and number of steps.
When rotating clockwise, the A pin connects first, followed by the B pin. When rotating counterclockwise, the B pin connects first, followed by the A pin. By tracking when each pin is connects to and disconnects from ground, we can use the signal changes determine which direction the knob is being rotated. The order in which they change is based on the direction the knob is being rotated. Tracking the signal changes allows us to use the rotary encoder as an incremental input.
Rotary encoders are really nice interface options but require special hardware or software support because of that high-speed pulsing.
In Arduino, you have to write code to manually track these signal changes and use that information to determine the direction of rotation and incremental changes. Luck for you, as of Circuit Python 3.0 we now have rotaryio support! Yay! It makes working with rotary encoders super easy!
This guide will show you how to use a rotary encoder with CircuitPython. We'll cover a basic example to get familiar with the rotary encoder. Then you'll learn how to use it as a volume control knob for your computer!