>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,

These systems work on the principle that a large ferro-magnetic object will
detune a nearby inductor coil.  Typically one or two turns of wire are embedded
in the concrete.  With no car on the sensor, the system  auto-tunes the system
to resonant the coil.  When a car drives over the sensor the coil falls out of
resonance and this event is detected.  The systems are fairly robust in that
they can work with many different sizes of coils due to the auto-tune feature.
(The cable driving the coil is a twisted pair.)

---- Steve