I found a cool page that shows how motion detectors work.
www.glolab.com/pirparts/infrared.html
Apparently the parts to make this stuff cost under
$5 per detector. Add in a box of cat 5 and strip down to individual pairs and you have 4000 feet of coverage (note signal cross talk isn't an issue given that your sending on or off not an analog signal). For $100 bucks you can cheat hardcore at bow hunting. (using motion detectors to map migration paths)
