An encoder is used to measure rotational speed of motors or wheels.
For light based encoders a series of markings or holes are made around the rotating disk that are used to break a light beam or change reflectivity that is detected using an infrared sensor.
Magnetic based encoders use a disk with alternating magnetic orientation as markings around it that are read by a Hall effect sensors.
As the wheel rotates these markings or magnetic changes can be counted and computed against time to determine the rotational speed.
Encoders can be supplied fixed to motors or as separate items for adding to anything that rotates.
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