The 41F is a bipolar hall effect latch, and that word is the whole story. A south pole on the marked face switches the output on, and it stays on after the magnet leaves. Only a north pole switches it back off. That memory is exactly what you want for anything spinning past alternating poles, and exactly what confuses people who expected a plain switch. Runs from 3.5V to 24V.
- Bipolar latch: south sets it, north resets it
- Holds its state with no magnet present
- Clean edges, ideal for RPM counting at speed
- Wide 3.5V to 24V supply range
- Open collector output, needs a pull-up
- Reads LOW once latched on
- Power-up state is undefined by design
- Works with Arduino, ESP32 and Raspberry Pi
- TO-92 package, breadboard friendly
- Marked “41F 551” on the flat face
How it works
Specifications
| Model | 41F-551, marked “41F 551” |
| Sensor type | Bipolar hall effect latch |
| Supply voltage | 3.5V to 24V DC |
| Supply current | Typically 4mA |
| Output type | Digital, open collector (active low) |
| Output state | Goes LOW on a south pole and stays LOW until a north pole |
| Magnetic polarity | South sets the output, north resets it. Removing the magnet does nothing. |
| Power-up state | Undefined until it sees a pole |
| Pull-up resistor | Required, or use the microcontroller's internal pull-up |
| Package | TO-92, 3-pin, 1.27mm lead pitch |
| Pinout | Marked face toward you, legs down: 1 VCC / 2 GND / 3 OUT |
| Sensing face | Flat (marked) face of the TO-92 body |
| Operating temp | -40°C to +85°C |
| Body size | Approx. 4.1mm wide, leads 14mm+ |
| SKU | 41F-HALL-TO92 |
Wiring & getting started
- Hold the sensor with the flat marked face toward you, legs pointing down. Pins 1, 2, 3 run left to right.
- Pin 1 -- VCC: 3.5V to 24V. On an ESP32 use the 5V rail, since 3.3V is below the minimum.
- Pin 2 -- GND: common ground of your circuit.
-
Pin 3 -- OUT: to a digital input, with a pull-up.
pinMode(pin, INPUT_PULLUP)saves fitting a resistor. - Establish a known state at startup. A latch powers up undefined, so sweep a known pole past it before you trust the reading.
- Sweep both poles to test. South pulls it LOW and it holds. North releases it back HIGH.
Great for
Alternating north and south segments give one clean transition per pole, with no jitter near a release threshold.
A BLDC rotor presents alternating poles, which is precisely what a bipolar latch is built to read.
One magnet swipe latches on, the other latches off, with no power needed to hold state.
Rotary position sensing where you need an unambiguous edge rather than a proximity window.
Attach to a hardware interrupt pin and count edges without debouncing.
Contactless state detection that survives dust, damp and vibration.
Good to know: if the output latches on and never releases, nothing is broken. That is what a latch does, and it is waiting for the opposite pole. If you want an output that releases as soon as the magnet moves away, use a switch such as the A3144 instead. The sensing element sits in the flat marked face, so point that side at your magnet. Shipped from our Te Awamutu stock.