a Hall-effect sensor
/ hawl /
A Hall-effect sensor feels magnetic fields. It is a small slab of semiconductor with a current flowing through it; bring a magnet near and the field pushes that flowing current sideways, piling up charge on one edge and creating a tiny voltage across the slab. No magnet, no voltage; stronger field, bigger voltage. It is a way to sense a magnet — and therefore position, rotation, or current — without anything ever touching.
The physics is the Hall effect: a magnetic field perpendicular to a current-carrying conductor deflects the moving charges, producing a Hall voltage across the sides proportional to both the current and the field strength. The raw Hall voltage is small, so practical Hall sensors are integrated chips with the amplifier built in. Two flavors exist: a linear Hall sensor outputs a voltage proportional to field strength (good for measuring how strong or how close); a switch-type Hall sensor flips its output high or low when the field crosses a threshold, with hysteresis built in so it does not chatter (good for counting or detecting presence). Because the device is contactless, the magnet can be sealed away behind a wall or spinning on a shaft.
Why this matters: Hall sensors are everywhere you need rugged, wear-free, non-contact sensing — brushless motor commutation, wheel-speed and crankshaft sensors in cars, current sensing (measure a wire's magnetic field to read its current without breaking the circuit), proximity switches, and the lid-closed detector in a laptop. The honest caveats: they need a magnet and a controlled gap, so mechanical placement matters; linear types drift with temperature and have offset that good designs trim out; and a switch-type gives you only on/off, not a value. For magnetic and contactless position sensing, though, they are tough, cheap, and reliable.
A linear Hall sensor powered from 5 V outputs 2.5 V (mid-supply) with no magnet, and swings about 1.3 mV per gauss of field. Bring a magnet that produces 500 gauss at the chip and the output rises to about 2.5 V + 500 times 1.3 mV = roughly 3.15 V. Move the magnet away and it falls back toward 2.5 V — distance becomes voltage.
A magnetic field pushes the current sideways, making a voltage — contactless sensing.
Pick the right type: a linear Hall sensor gives a proportional voltage (a value), while a switch-type gives only on/off. Linear parts have temperature drift and offset, so for precision measurement they need calibration or a part with built-in compensation.