A Simple Vehicle Loop Detector

Created "Feb 20 2007"
Updated Oct 12 2008

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: Numerous persons have now built this and are Really happy with there results. However a few people have had problems with Sensitivity and Stability. PLEASE READ THE "IMPORTANT NOTE" Below.


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 Shape, Size or diameter are possible. For a Car, I would suggest a 4 to 6 foot diameter, or Square Shape Coil. 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 0.47uF.

This Circuit is for a Real Car or Truck, Maybe a motorcycle. It is NOT for detecting Small "Model Cars". It does Not work well on Small Objects.

Important Note: This circuit can have numerous "Harmonic" resonant frequencies, using any given capacitor for "CX", and with your particular coil.

For "Best Sensitivity and Stability", you need to tune it to the "Actual Resonant frequency", Not one of these Harmonic Frequencies.

So finding the Correct Value for "CX" and the True Resonant point, "May be somewhat Frustrating".

One other point: Your Coil should be wrapped, taped or bundled in some way, so the coil wires don't move. If the wires are not stable, it will cause inductance changes resulting in frequency drift.

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.

"Back to my Projects Page"

Loop Detector Schematic.

Loop Detector Schematic, Modified to drive a Relay.

Loop Detector PCB.

Overlay Picture. Note: This Tiny Test Coil was for testing only, "on my work bench" and although it works, detection distance is not very good. Detection Distance is MUCH BETTER with a Large 6 foot Coil.

All Imformation in this Article is "Copyright protected",
And All Rights Reserved.

Chemelec *Copyright © 2007 & 2008*