Try using a magnetic sensor.  Honeywell has some very good ones.  They are
sensitive enough to detect a disturbance in the earth's magnetic field from
a large metal object (car, truck, etc.).  From what the data sheet says that
vehicle detection is one of its uses.  Check out their data sheets at
http://www.ssec.honeywell.com/products/magsensor_index.html

Steve


-----Original Message-----
From:   pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On
Behalf Of H. Smit
Sent:   Wednesday, July 21, 1999 2:00 PM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        help wanted on detection loop

hello,

I'm working on a project to operate my driveway gate by using satellitedish
motors. for the mechanical part it went ok, but for the electronical part i
have some difficulties. I want to operate my gate by driving over an
detection loop in my driveway, the kind they use by traffic lights. maybee
one of you can give me a hint of how these detection loops are working.

Please advise,
Regards,
Henk Smit.