The US1881 is an integrated Hall effect latched sensor.  Holding a magnet near the sensor will cause the output pin to toggle. This makes for a robust presence sensor. A reed sensor also works nicely, but can be limited by the glass encapsulation...

This kit includes US1881 and a magnet. The US1881 is an integrated Hall effect latched sensor.  Holding a magnet near the sensor will cause the output pin to toggle. This makes for a robust presence sensor. A reed sensor also works nicely, but can...