Omni Hall
An omnipolar Hall-effect sensor asserts its output whenever a magnet of either polarity is within range, and deasserts when the magnet is removed. This makes it ideal for applications where the magnet's orientation is unknown or varies, like lid-open detectors and rotational speed sensors.
The Hall element senses the component of magnetic flux perpendicular to the chip. Internal comparators use different trip thresholds for north and south poles (and hysteresis to prevent chatter). An open-drain output asserts low for either pole above the threshold, and releases when the field drops below the release threshold.
In plain terms
A magnetic door alarm that doesn't care which way you hold the magnet — north pole, south pole, either one triggers it. It outputs a clean logic-level switch, no analogue measurement needed.
Why designers use it
- Detect lid opening in phones and laptops regardless of which end of the magnet faces the sensor.
- Count gear-tooth or impeller rotations with a ring magnet of random pole orientation.
- Build tamper-detection switches on enclosures.
Best for
- Lid detection
- Rotation counting
- Tamper sensing
Key specifications
- Operate point: 1 mT – 100 mT
- Hysteresis: 0.1 mT – 5 mT
- Supply: 1.65 V – 5.5 V
- Sleep current: 1.5 µA – 50 µA (Sampled designs)
- Output: Open-drain or push-pull
When not to use it
- When you need to distinguish north from south pole — use a bipolar (latching) or unipolar Hall sensor instead.
- When continuous angle measurement is needed — use a Hall-effect angle encoder IC.
Common mistakes
- Placing the magnet too far from the sensor and seeing intermittent switching at the threshold boundary.
- Forgetting the pull-up resistor on the open-drain output — the output floats without it.
Where you will find it
- A laptop's lid-close sensor uses an omnipolar Hall IC embedded in the bezel: when the lid closes, a small magnet in the display frame comes within 5 mm and the IC asserts its output, signalling the SoC to enter sleep mode regardless of which corner of the display holds the magnet.
- A bicycle speedometer uses an omnipolar Hall sensor zip-tied to the fork: a spoke-mounted magnet triggers the sensor once per wheel revolution regardless of which pole faces the sensor, and the MCU converts the pulse period to speed using the wheel circumference.
- A coffee pod machine uses an omnipolar Hall sensor to detect the capsule drawer's closed position: the magnet is moulded into the plastic drawer, and the sensor's output gates the heating element so the machine can't brew with the drawer open.
A short history
An omnipolar Hall-effect switch is a magnetic sensor IC that activates in the presence of a magnetic field of either polarity — north or south pole — provided the field strength exceeds a threshold, and deactivates when the field is removed. This behaviour contrasts with unipolar Hall switches, which respond to only one pole, and bipolar latching switches, which require alternating poles to toggle. Melexis and Infineon (formerly Siemens Semiconductors) introduced omnipolar Hall switches in the mid-1990s to reduce assembly cost in consumer devices where precise magnet orientation was inconvenient. Omnipolar switches are now used in lid-close detection in laptops, stylus proximity sensing in tablets, and gear-tooth counting in appliance motors, where the polarity of the actuating magnet is difficult to guarantee during high-volume automated assembly.
Good to know
- Omnipolar Hall switches respond to either pole — handy when you don't know which way the user will install the magnet, like in a fridge door sensor.
- Modern sampled-Hall ICs sleep at 2 µA, wake every 25 ms to take a reading, and only fully power up if the field is near the threshold — coin-cell life of years.
- The Hall effect was discovered by Edwin Hall in 1879, 70 years before silicon could exploit it.