The US1881 is a bipolar Hall effect latch IC fabricated from mixed-signal CMOS technology with advanced chopper stabilisation for accurate, stable magnetic switch points. Unlike a simple unipolar switch, it latches its output and holds state when the magnet is removed — making it the go-to choice for BLDC motor commutation, angular position sensing, and non-contact toggle switching across a wide 3.5V–24V supply range.
- Bipolar latch — south pole sets output ON (BOP), north pole sets output OFF (BRP)
- 3.5V–24V supply range, up to 50 mA output current
- Chopper-stabilised amplifier — stable thresholds across temperature
- Mixed-signal CMOS — optimised for 5V and 12V BLDC motor commutation
- SOT-23 package (output polarity reversed vs UA package)
- −40°C to +150°C operating temperature range
- 100 mW max power dissipation, RoHS compliant
- Compatible with Arduino, ESP32, Raspberry Pi and more
Specifications
What’s in the pack
Quantity as selected. TO-92 flat (UA) package, through-hole, ready to use.
Great for
Getting started
- Orient the sensor
Hold the TO-92 (UA) package with the flat branded face toward you, legs pointing down. Pins 1, 2, 3 run left to right. The flat face is the active sensing surface.
- Wire power and ground
Connect Pin 1 (VDD) to your supply — 5V for Arduino, or use 5V Vin on ESP32 for adequate headroom above the 3.5V minimum. Connect Pin 2 to GND. Add a 100 nF bypass capacitor between VDD and GND close to the sensor.
- Connect the output with a pull-up
Connect Pin 3 (OUT) to a digital GPIO. Use
pinMode(pin, INPUT_PULLUP)in Arduino, or add a 10 kΩ resistor from Pin 3 to VDD. - Test the latch behaviour
Power-up state is undefined. Sweep a known magnetic pole past the sensor at startup to establish a known state. South pole facing the marked face = output ON. North pole = output OFF. Remove the magnet and the output holds its last state.
How it works
Common questions
What is the difference between bipolar latch and unipolar switch?
A unipolar switch only responds to one pole (usually south). A bipolar latch responds to both — south sets the output ON, north sets it OFF, and removing the magnet holds the last state. This makes the US1881 ideal for motor commutation and toggle applications where you need definite on/off control from alternating poles.
Does the output hold state when the magnet is removed?
Yes. This is the key feature of a latch. Once set by a magnetic field, the US1881 holds its output until the opposite pole is applied. Power-up state is undefined, so your firmware should detect pole position at startup to establish a known state.
Is the SOT-23 package the same polarity as the TO-92 (UA)?
No — the SOT-23 output polarity is reversed relative to the UA (TO-92 flat) package. Always check your package marking. For the UA package: south pole facing the flat face = output ON (low). For SOT-23 it is the opposite.
What supply voltage does it need?
The US1881 operates from 3.5V to 24V, making it compatible with 5V Arduino systems and 12V motor driver circuits alike. The output voltage mirrors the supply range (3.5V–24V), and maximum output current is 50 mA.
Do I need a pull-up resistor on the output?
Yes. The output is open-drain, so it needs a pull-up to VDD to read a HIGH state. A 10 kΩ resistor works well, or use INPUT_PULLUP mode on an Arduino GPIO to use the internal pull-up.
Package note: The US1881 UA (TO-92 flat) and SOT-23 packages have reversed output polarity — always verify your package type before wiring. For a simpler south-pole-only switch, see the OH137 unipolar Hall switch.