A Hall effect sensor exploits the Lorentz force on charge carriers in a thin semiconductor strip. Pass a bias current through the strip, apply a perpendicular magnetic field, and electrons deflect sideways, creating a measurable voltage across the strip's edges. That voltage is linear with field strength — typically tens of microvolts per gauss — which is why every commercial Hall IC bundles the sensing element with a chopper-stabilized amplifier, offset cancellation, and often a temperature compensation block on the same die.

There are three flavors you'll encounter:

• Linear (analog) Hall sensors like the Allegro A1324 output a voltage proportional to field: typically VCC/2 at zero field, with a sensitivity around 5 mV/gauss. Use these for current sensing and proportional position feedback.
• Switch (digital) Hall sensors like the A3144 are essentially a Hall element plus comparator with hysteresis. They trip ON above ~100 gauss and release below ~50 gauss — perfect for end-stop detection or RPM counting.
• Latching Hall sensors need a south pole to set and a north pole to reset. These drive brushless DC motor commutation, where three latches read the rotor's magnet position 120° apart.

Real-world example — contactless current sensing: The ACS712 puts a Hall sensor next to an integrated copper conductor. Current through the conductor produces a magnetic field, the Hall element measures it, and you get a galvanically isolated current reading with a 1.2 mΩ insertion resistance — far better than a shunt's voltage drop and isolation hassle. A ±20A ACS712 outputs 100 mV/A centered at VCC/2, so reading 2.7V on a 5V supply means roughly +2A flowing through.

Rule of thumb for positioning a Hall switch: a small neodymium disc magnet (6mm × 3mm, ~N42) produces about 2000 gauss at its face but drops as roughly 1/d³ along the axis. To reliably trip a 100-gauss switch, keep the air gap under ~8mm. Always orient the magnet's pole face perpendicular to the sensor's marked face — most digital Hall switches are unipolar and only respond to one polarity.

Watch out for: stray fields from nearby motors or transformers can bias your reading; shield with mu-metal if needed. Hall sensors also have offset drift over temperature (typically ±1 gauss/°C), which is why precision applications use spinning current techniques internally — rotating the bias current direction through the Hall plate at tens of kHz to chop out offset, much like a chopper-stabilized op-amp.