[JamesBrown]

Originally Posted by RyanCahoon

Taking a different approach, you might could use some sort of proximity sensor. Options include PIR sensors (what they use in automatic doors), or ultrasonic or infrared rangers. You should be able to get any of these for less than $25. If you only want a proximity alarm, you should only need one-three (depending on the desired monitoring angle and sensor type). You may have to go to a higher number if you want to actually aim accurately. You'll have to be careful if you attempt this with the range finders, as they rely on sensing reflections, and crosstalk between the sensors can invalidate the readings.

These types of sensors would only require a fairly low-power processor. The common choice would be an Arduino (as is the case in the linked PIR sensor tutorial), but if you want to go really low cost, TI just came out with a new controller board that costs less than $5 that looks interesting.

--Ryan

I have seen a Sentry that works using PIR and an Arduino, it is accurate and pretty cost effective (the Arduino is overkill for this application but with all of the resources the community provides I would still recommend it)