He built this set of no-contact limit switches using a hall effect sensor. The small black package sticking out past the end of the protoboard is the sensor. It is used to detect a magnetic field.

Delft University's inverted pyramid Hall-effect sensor achieves improved sensitivity and offset reduction, enhancing ...

Among the changes made by Apple to its laptops over the years, the transition from a Hall sensor-based sleep sensor to an angle sensor that determines when the lid is closed is a decidedly ...