Speed sensors are devices which measure the velocity of a rotating shaft or other rotating object. They are used in all kinds of machinery for different purposes, such as controlling the speed of parts in a machine, determining the output of a motor, or even as part of a closed-loop control system to ensure that machines continue to operate within specified parameters. Generally, they measure the speed of a shaft or other rotating object, outputting a signal that is proportional to the speed. The most common types of speed sensors are optical, Hall-effect, magnetoresistive, and eddy current. As their name implies, optical sensors use light to measure the speed. Hall-effect sensors use a magnetic field to measure speed and have the advantage of being able to measure speed in either direction. Magnetoresistive sensors measure speed using the change of resistance within a magnetic field, while eddy current sensors measure the speed by detecting changes in the magnetic flux.