This sensor is essentially a reed switch, encased in an ABS plastic shell. Normally the reed is 'open' (no connection between the two wires). The other half is a magnet. When the magnet is less than 13mm (0.5") away, the reed switch closes. They're often used to detect when a door or drawer is open, which is why they have mounting tabs and screws. But these also have a sticky-foam-tape back so you can also place them in locations you cant attach with screws.
Netduino driver and Netduino sample. This sample shows how to use an interrupt handler to respond to a button press or a magnetic contact switch closing.
Tutorials on the Adafruit Learning System featuring this product:
Printable catalog (PDF)
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