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I am currently working on the design of a metal detector circuit. I am relying on the method that the coil's inductance would change when a metal is close to it (due to eddy currents.) I have tried several methods using op-amps such as transimpedance amplifiers, etc., but none of them worked.

I saw this question posted

It shows a very nice method to detect small variations in resistances using the Wheatstone bridge and instrumentation amplifier. Unfortunately, this method wouldn't work for inductances, and is not linear like the Wheatstone bridge for resistances.

To summarize, I want a method to detect small variations in an inductor/impedance in an AC circuit.

JRE
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Mora
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1 Answers1

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If you make the coil resonant with a parallel tuning capacitor and use the coil/capacitor in an oscillator circuit, you should be able to detect small frequency changes as metal passes near the search head coil.

This is the principle of the BFO metal detector; you have a fixed frequency oscillator and the search head coil built into an oscillator. What you get is the difference frequency out and that can be made to be an audible signal. So, when you pass the search head over a piece of metal you get a sudden rise in frequency in your headphones.

Of course, you can use an MCU to detect that change in frequency too.

Andy aka
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    Thank you very much, this answer extremely helped me!!!!! The BFO is an amazing method and I am currently working on it. Thanks again <3 !!! – Mora Dec 08 '22 at 07:27