Hall-effect sensors can be used for tilt switches, counters, proximity detectors, metal detectors ,pressure sensors, current sensors and gauss meters. A basic low-cost Hall-effect sensor is the Sprague UGN3020T. This sensor includes a Hall cell, voltage regulator, signal amplifier, and Schmitt trigger. The circuit shown can protect instruments, antiques, or windows/doors by placing a small magnet on the item you wish to protect. The sensor is then placed in close proximity to the magnet. When the magnet is moved past the sensor, an output activates a relay. The sensor drives a 2N5812 transistor for an on-off type output. A DPST relay could be used to sound a local alarm, or the circuit could be connected to a central alarm panel. The same basic Hall sensor could be used for counting moving objects or as a wind-speed indicator.

Parts List

NOs   Part     Description

1      R1        1k, Resistor
1      R          256 E, Resistor
1      D1        IN4001 silicon diode
1      Q1        2N5812 transistor
1      Q2        2N3055 transistor
1      U1        UGN3120 Hall cell (Allegro Microsystems)
1      RY-1     9-12-V relay
1                  MAGNET Bar or cylinder magnet

   Schematic