Vehicle Metal Detector Using Collpits Oscillator

Thread Starter

akash diwate

Joined Apr 7, 2016
Dear All,

I am trying to build up the circuit for vehicle detection using Collpits Oscillator (Inductive Loop). For that i have referred the circuit mention in figure. In this circuit L1 is get replaced by the inductive loop of size 4*8 Feet Having 5 Turns. The property of collpits that i am looking for , when metal comes closer to L1 (Inductive Loop) the frequency must need to change. While going through simulation part, as i am trying to change inductance value i.e L1 with value of 10 mH to 60 mH then frequency gets changed. It also proven by practical. Only the term change is inductive loop size. Loop size is small one 1*1.5 Feet.

As soon as metal get closer to that inductive loop, frequency gets changed and change in frequency get observed by using CRO. Now the query is, while connecting loop which having size of 4*8 then circuit didn't responds to frequency change. Please suggest suitable changes in schematic so that i can solve the issue. Ultimately goal is to design the circuit to detect vehicle presence on inductive loop.

Collpits Oscillator.JPG


Joined Sep 17, 2013
10mH seems way too high for a 4' x 8' air-cored 5-turn coil. An online calculator I tried gives the inductance as about 83uH. Try that in your simulator.


Joined Aug 21, 2017
For so large object like a car I cant see any sensitivity problem. or problem of frequency.
I would recommend two well approved circuits.
One is Clapp. The good thing over the Hartley inspite of they unpardoned similarity is that Clapp has Q-fold higher voltage on coil, so the irradiated EM field will be accordingly stronger, so the frequency may be diminished.
Another one I had used in many coil freq detection systems is very mild-oscillating in wide freq range the two transistor version of cascode - see here,72.jpg

I would begin from the last one and about 50 kHz.