Unipolar hall-effect sensor ics, often referred to as "unipolar switches, " are operated by a positive magnetic field. A single magnet presenting a south polarity (positive) magnetic field of sufficient strength (magnetic flux density) will cause the device to switch to its on state. After it has been turned-on, the unipolar IC will remain turned-on until the magnetic field is removed and the IC reverts to its off state. 

AH3131
The AH3131 is a high sensitivity digital unipolar Hall IC with single output.
The AH3131 Hall-effect sensor includes the following on a single silicon chip: reverse voltage protector, voltage regulator, temperature compensation circuit, Hall-voltage generator, signal amplifier, Schmitt trigger and opencollector output driver, etc. The integrated voltage regulator and temperature compensation circuit ensure that the sensor works stably in a wide voltage and temperature range. The reverse voltage protection circuit prevents the
sensor from being damaged by reverse voltage.
The AH3131, produced with bipolar technology, is more stable and reliable. It is available in three package types: SOT-23-3L (Type M), SOT-89 (Type S), and TO-92UA (Type UA). Each package is lead (Pb) free, with 100% matte tin plated leadframes.

Unipolar Hall IC
Note: the data in the table are typical valuesTest condition: the test voltage of supply current is 3.3V, and the ambient temperature is 25 ºC.