Hi Gil,

There are a lot of people who can explain how motion detectors work better than me. I would suggest you go to the library and check out Poptronics magazine for the last 12 months. You can find at least 4 articles on motion detectors. I have done a lot of security work using them and one of the biggest demand items I have used is something which can be interfaced with a motion detectors and call a beeper using an external modem. I have built a circuit using an 89C2051 and a Maxtor 232 chip to interface to an external modem. Whenever the motion detectors senses something it calls a beeper, gives a 7 digit code hangs up the phone and waits 15 minutes to repeat if motion is still detected. I also included an extra input for whatever you want. I have used it on detecting water flooding on many building. The software for this device is at http://members.com.aol/cpu8052/index.html.

Regards,

Charles Bannister