There are a variety of sensors that can be used for speed feedback. The most common are optical encoders and Hall effect sensors. The optical encoder consists of multiple components. Install a sheave on the shaft at the non-drive end of the motor. An infrared LED provides a light source on one side of the wheel, and a phototransistor detects light on the other side of the wheel. Light passing through the slot in the wheel turns on the phototransistor. When the rotating shaft rotates, the phototransistor will turn on and off according to whether the light passes through the wheel groove or not. The frequency of transistor on and off characterizes the speed of the motor. In applications where the motor is shifted, an optical encoder will also be used to feedback the motor position.
Hall effect sensors are also used to provide speed feedback. Similar to an optical encoder, a Hall-effect sensor requires a rotating element connected to the motor and a stationary element. The rotating element is a wheel with one or more magnets mounted on its outer edge. The stationary sensor detects the passing magnet and generates TTL pulses.