A Hall effect sensor is a sensor what varies its output when it meets a magnetic field. The sensor contains three wires and produces a digital signal. The sensor consists a thin piece of metal what passes a continuous current through it. When the sensor is placed in a magnetic field the electrons move to one side of the metal and protons move to the other side of the plate. Then you will be able to get a voltage.
The sensor only produces around seven volts. When the electrons move to one side a potential difference is produced between the two sides of the metal.