Can be used as end stop, rotary counter etc. Needs a suitable magnet, for fischertechnik I recommend 4x4mm cylindrical neodym. Pins are labeled O for Out, ⏚ for GND and V for Vin. Don't forget a pullup resistor on the output, the microcontroller's internal pullup will do fine. Designed for Micronas HAL 525, HAL 526 Hall-Effect Switches: https://www.mouser.com/ds/2/735/HAL525_HAL526_Hall-effect_Sensor_Switches_3EN-1003331.pdf However, the form factor is quite common so I assume it will fit for similar sensors, too. Bend back the sensor's legs, clip them to length and solder them to a 5 pin male pin header, remove the two outer pins and you're ready for assembly.