Hall effect sensors output a voltage that varies in proportion to the presence of a magnetic field, which is typically provided by a nearby magnet.

The remote hall effect sensor. The idea is that you just need to stick a small magnet to your extruder, add a bit of extra G-code that will park it over the sensor at the end of each layer, and ...

Linear Hall-Effect Sensor According to the datasheet, SS49E is a linear hall-effect sensor that is operated by the magnetic field from a permanent magnet or an electromagnet. The linear sourcing ...