Magnetic contact switch (door sensor)

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. You can also pick up some double-sided foam tape from a hardware store to mount these, that works well without needing screws.
Technical Details
    •    Normally open reed switch
    •    ABS enclosure
    •    Rated current: 100 mA max
    •    Rated voltage: 200 VDC max
    •    Distance: 15mm max
Dimensions:
    •    Box size (each side): 29mm x 14mm x 9mm / 1.1" x 0.6" x 0.4"
    •    Cable Length: 305mm ± 12mm / 12" ± 0.5"
    •    Weight (per side): 5.4g
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.

 

Features:  
Switch Type: NC (normally closed type), circuit is connected when magnet closes to the switch  
Mounting Nethod: SMD (by screws or double-sided tapes)  
Voltage: 12V DC  
Rated Power: 10W  
Maximum working current: 0.5Amp  
Actuation distance:18mm ±6mm  
Material: Nd magnetic  
Housing Material: ABS  
Wire lead: 22 Awg, 300mm long  
Color: white