Theory - Magnetic Field Sensor

The sensor uses a Hall effect transducer that produces a voltage that is linear with magnetic field. The sensor measures the component of the magnetic field that is perpendicular to the flat sensor covered with black heat-shrink tubing. Maximum output occurs when the white dot on the sensor points towards a magnetic north pole. When no magnetic field is present, the sensor will read an offset voltage of about 2 volts.

A magnetic field will cause the voltage to increase or decrease depending on the direction of the field. The minimum is 0 volts, maximum is 4 volts. If the offset voltage is set incorrectly, or if the magnetic field is beyond the range of the sensor, the voltage will reach one of these limits. An amplifier used with the sensor has a switch for low and high amplification. 

Low amplification is used to measure relatively strong magnetic fields around permanent magnets and electromagnets. Each volt represents 32 gauss (3.2x10^-3 tesla).

High amplification is used mainly to measure the magnetic field of the earth and very weak fields. It can be used for other magnets, but the sensor must remain in one position so that the reading is not affected by the background field of the earth. It is 20 times more sensitive than the low amplification. Each volt represents 1.6 gauss (1.6x10^-4 tesla). The range of the sensor is ±3.2 gauss.