A Complete Guide to the Incremental Rotary Encoder

If you look around your house, and think of how things work, remember the word, rotary encoder.

Take a look at your home stereo, your computer mouse and trackball, and your photographic lenses. These objects along with many other robotics use a rotary encoder.

Below you will find out exactly what a rotary encoder does, the parts that make it work, and why its a great thing.

What a Rotary Encoder Is

A rotary encoder is a device that converts the angular position of a shaft to a digital code. A rotary encoder makes a series of waves when it rotates, using two output channels; A and B. It works by rotating a code disk in the path of a light source. It is also known as a quadrature encoder and contains a light source, optical encoder disc, and light sensors. 

What Are They Used for

This encoder is used to track motion and determine position and velocity. Most all home and car stereos use a rotary encoder for volume control.

Output Channels

As stated above, a rotary encoder uses two output channels which are A and B. If it is detected that A is ahead of B the disk is rotating clockwise, however is B is ahead of A then the opposite is true, and the disk is rotating in a counter clockwise way. Both A and B detect the direction, and the position of the the rotation.

Encoder Resolution

The encoder resolution is the term used for the number of wave cycles produced per one turn of the shaft. These waves are also sometimes known as pulses.

Light Source

The light source provides the light to several light sensors within.

Optical Encoder Disc

This source blocks light from the light source from going to the light sensor. A resolution of 360 means that the encoder disc will have 360 lines in it, and one turn of the encoder shaft will create 360 waves.

First State

When the light sensor is receiving light, then it is in the first state of electronic circuit.

Second State

When the light from the light source going to the sensor is blocked by the optical encoder, and the sensor is not receiving light, this is when it is in the second state of the sensor output.


All sensors have 2 outputs. These outputs are high and low. One is called a bit, and then for the 4 sensors, it is rightly called a 4-bit output.

Light Encoder

The light encoder is a plastic disc with areas of transparency, and opaque areas as well. This sends light to some sensors, and at the same time blocks light to any remaining sensors.

Why This Type of encoder

An incremental rotary encoder is very accurate. It uses only 2 sensors, but they have been known to have up to 10,000 counts plus per revolution. 

Understanding an incremental rotary encoder, or any encoder for that matter is not an easy task. You have to look at what you are dealing with for the object, and go from there. There are many encoders, with the rotary encoder being rated one of the top encoders.