Any ideas on how to accomplish this?

[Archangel]

Love those bureaucratic words, Traffis lights around town use a large coil of wire under the street to sense the metal vehicles presence. I believe a smaller coil is there too as the tank coil of an oscillator. I remember seeing metal detector circuits over the years that used 2 coils, and you would null out the circuit, when metal appeared it would throw the circuit out of Null and it would oscillate. I believe the coils were supposed to be at right angles to each other.

Another suggestion: Radio Shack sells a cheap / toy metal detector, it might be the basis of what you need.

Last Idea, bury a sheet metal plate,or install overhead or on side, and hook to a darlington transistor switch circuit. the vehicle might induce enough stray capacitance to trigger switch.
Cheap enough to experiment, anyway.

edit : Doe! sorry Ron, I didn't see your mention of metal detector.

Melanie: I like it!
JS
http://www.ssec.honeywell.com/magnet...eets/an218.pdf
http://home.clara.net/saxons/bfo.htm
http://www3.telus.net/chemelec/Projects/Metal/Metal.htm
http://home.skif.net/~yukol/MetalE.htm
http://electronics.howstuffworks.com...-detector3.htm
http://www.absoluteautomation.com/dr...rms/index.html