Dave, back in the 70's I was desinging these kind of detectors. We
buried a wire loop in concrete to detect vehicles. The loop was one part
of an LC tank whose frequency changed as a vehicle passed over. We simply
detected the frequency change. I have'nt thought about this since then so
I'm not sure what they use now days but I suspect it is similar.

   - Tom

At 03:51 PM 8/26/97 +1000, you wrote:
>Does anyone know how the car detector loops at fast food drive thru's
>and traffic lights work ? I saw a dud board from one and it seemed to
>have a 18 pin micro on it (not PIC). The micro seemed to connect to the
>loop via a small coupling transformer,etc. I assume it senses a change
>in a HF field in the loop. Any ideas ?
>Thanks...
>__________
>Dave Duffy