The units I have used have been pre-built. The coil specs are for about two-three turns, and then doing a twisted pair back to the sensor.

They use an LC circuit, and a frequency counter - when a vehicle enters there is a delta Hz due to the change in L, above a certain threshold and fiddle with an output.

Search for vehicle loop detector...

http://emxinc.com/vehicle-loop-detector-d-tek.html 
http://www.neverfail.com/NewFiles/Products.html 
http://renoae.com/traffic/files/AX/Model%20AX%20Operating%20Instructions%20%2003-30-04%20(551-0106-01).pdf




>>> brad_lists@greenepa.net 15 Oct 07 13:38:50 >>>
Hello all,

   I am working on a little project to help secure my home... I have a long 
driveway and would like to know when someone is approaching my house. I used 
to use a driveway monitor that had an IR motion detection sensor. This 
worked, but produced more false alarms than true alarms. Not that it was 
malfunctioning, but since I live in a rural area, it would detect every deer, 
groundhog, dog, whatever...

   I would like to design something similar to:

	http://www.smarthome.com/790908.html 

   It uses magnetic probes to detect vehicles approaching. 

   Unfortunately, I know nothing about metal detection. I've never even used a 
metal detector, let alone know how one works well enough to design one.

   Can anyone give me some pointers on how to go about making magnetic probes 
such as the ones used in the above system? Maybe a website explaining the 
basics of ferrous material detection and some simple schematics for a basic 
metal detection probe? Basically I just want to find a circuit that upon 
detection of the metal, it sends out a digital pulse. From there I'll use 
that to activate the alarm be it over wireless, wired, whatever...

   I'd appreciate any advice.

Thanks,
Brad
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