The Magnetic Sensor is a very simple device which uses "Hall Effect" principles to detect the presence of magnetic fields. Whenever the sensor is in the presence of a magnetic field strong enough, an on-board LED turns on and the sensors digital output is activated. Although this can be used for general purpose applications it is very often used in conjunction with a small magnet fitted to a moving part of some apparatus. It will then detect whenever the "moving part" comes within the range of the sensor. This can be very useful in control and automation projects.