I have recieved Numerous Requests for a circuit to detect Vehicles
using a Buried Coil.
So this morning I Designed up this circuit.

This is about the Simplest you can get,
But still quite useful for many Detecting applications.
And although this one is particularly simple, I find it works very well.

While I haven't actually Buried the coil, I Did lay a large coil Flat
on a Cement floor and it works Fine.
UPDATE:
A Few  persons have now built this and are Really happy with there results.

NOTES:

The Precise Setting of the Controls can also Determine Sensitivity,
to Possibly just avoid detecting a Bicycle, if Needed.

Depending on HOW you Set the Trimpots,
You can make the LED, go EITHER "OFF" or "ON", When the car is over the coil.
So Adjust these Trimpots for the way you want the LED to Respond.

Coils of almost any diameter are possible.
For a Car, I would suggest a 4 to 6 foot diameter, or Square Shape.
Possibly between 3 to 6 turns.

DEPENDING ON YOUR COIL, Size/Shape/Turns, A Little Expermentation
may be necessary with a different capacitor value, in place of the .47uF.

The Coil gets Buried where its Needed and the Circuit should go inside
your House or Garage. No Special Wire is Needed to Join them together.

Additionally a PIZO BUZZER Could be used in Place of the LED, to give
an Audio Warning Alarm.

The PCB shown below is Available from me, If Needed.



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Loop Detector Schematic.

Loop Detector PCB.

Overlay Picture.
          Note this Tiny Test Coil was for testing only, "on my work bench".
          It works Equally Well with a Large 6 foot Coil.