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I am trying to build a metal detector with double balanced coil, (the scheme source is the french magazine : L'ELECTRONIC POUR TOUS project: EN1465 Un détecteur de métaux très sensible, the magazine available on the internet), I had a problem with simulating the double balanced coil on PROTEUS marked by the red circle in the attached image. What electronic components should I use (only inductors) to simulate the receiver and sender coils? I saw some metal detector project they use an RLC circuit. Any ideas?

enter image description here

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  • \$\begingroup\$ Consider adding a screenshot of the Proteus circuit and any error message that application provided or any waveforms which indicates the problem that you are facing. \$\endgroup\$
    – AJN
    Aug 14, 2021 at 10:30

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The leftmost oscillator coil induces hopefully the same signal to the mid and rightmost (=R) coils. The mid and R coils are in series so that the caught sum signal is zero if there's no metal. If someone brings a piece of metal near the coils, it very unlikely affects (via eddy currents) identically to what's caught by mid and R coils. The balance vanishes and the detector line starts to get signal from the mid and R coil combination.

You can simulate the detector (a synchronous one) system simply by feeding the 5500 Hz oscillator signal to the +input of IC2-A. Do not cut its dc-path, R5 is a must.

To simulate the coil system you must know the coil inductances and their mutual inductances - total 6 different inductance values. I'm afraid that a direct measurement needs advanced instruments and calculating it needs advanced math knowledge, both beyond what I have. But if you know those inductances you simply change for ex the mutual inductance between the leftmost and mid coil.

You can measure from a built practical circuit how much mid and R coils catch.

How much a given piece of metal can cause mutual inductance imbalance at the operating frequency - it can be measured. It can be also calculated numerically with full 3D electromagnetic field simulation system. Again something quite advanced.

Not asked: There's many other metal detection systems. Some of them need only one coil.

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