Hall Effect Sensor

GMW360ASM Angle Sensing Module Output:

Analog Output (Yellow): Outputs analog voltage. 0.5V when -180, 2.5 V when 0, and 4.5V when 180 degrees.

Magnet Output (White): Produces LOW signal if magnet is "out of range," HIGH if "in range."

To set sensor to 0 degrees, send a +5V pulse through the Analog Output wire for one second.

http://www.gmw.com/magnetic_sensors/sentron/2sa/GMW360ASM.html