Hall Effect Sensor
A Hall effect sensor is a transducer that varies its output voltage in response to changes in magnetic field density. Hall sensors are used for proximity switching, positioning, speed detection and current sensing applications. In its simplest form, the sensor operates as an analog transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced.
Basically, by placing a hall effect sensor near a magnet attached to your device that moves in some way, you can get a voltage proportional to the distance/orientation of the magnet. One commonly used configuration is placing the sensor directly above a dipol magnet attached to a rotating joint or shaft. The sensor (with some circuitry) will return an analog voltage proportional to the angle of the joint or shaft.
How It Works
As you can tell from the name, the Hall Effect Sensor takes advantage of the phenomenon known as the Hall Effect. The Hall Effect refers to the potential difference (Hall voltage) on opposite sides of a thin sheet of conducting or semiconducting material in the form of a 'Hall bar' (or a van der Pauw element) through which an electric current is flowing, created by a magnetic field applied perpendicular to the Hall element.