>   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
>
>
Dave and Tom, here in Mankato they still use the same system on the signal
light's and at the D Q shop's. in the Twincitys they also use a sensor for
the police and fire Veicles that respondes to there SOUND's also to control
the signals.

regards

http://ic.mankato.mn.us/~douglas
douglas@ic.mankato.mn.us