This Hall Sensor Module used
Hall Sensor - A3144E. The Hall Sensor measures the Hall Effect, which is a production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor as well as a magnetic field perpendicular to the current. The output of the Hall sensor switches low and turns on when a magnetic field perpendicular to the Hall sensor exceeds the BOP threshold, and it switches high and turn on when the magnetic field disappears. And the comparator circuit that make the output signal more reliability.