Hi,
It is probably wore cost effective to buy a motion sensor than build your own. Both Melanie and Mister_e are refering to passive infra red (PIR) sensors.
These are cheap and reliable but can produce false alarms due to warm air currents etc. There are more sophisticated units that have dual detectors, one PIR and the other either doppler radar or ultrasonic. An output is only produced when both sensors "see" a target. Even if you are working on an "intellegent" sensor type system (networked rather than just lots of contacts in series) I'd suggst you buy the sensors (you can't buy the detector, fresnel lens, tamper switch and case as parts for the cost of the unit) and add a PIC to them. You can pick up the signal that drives the relay. Removing the relay makes for a silent unit rather than one that clicks as you go by. A pic is a great choice for the control panel. I-buttons by Maxim-Dallas make great entry keys.

Robert G8RPI.