Magnetic Door Alarm Sensor Switch.

Magnetic door sensor and magnet each measure 27mm x 15mm x 9mm and each has a peel-off adhesive backing. Has 25cm of hard-wired cable included.

I believe the contacts are Normally Open - the circuit only gets completed when the magnetic contact is close to the reed switch contact.

If you want to reverse that, use this circuit: http://www.learningaboutelectronics.com/Articles/Transistor-inverter-circuit.php

UK 24 hour postage is £1. Just order before 1pm Monday-Friday and you should get it the next working day, if you're in the UK.

Price includes VAT, receipt provided.