I'm working on the design of the "Sensor head" for a metal detector. Schematically it is:


It is a set of Tx and Rx coils arranged inside a shield. Some photos of the prototypes that I have built:

Tx + Rx + Cabinet

Summary of the constructive parameters and electrical characteristics obtained:


The results obtained are not bad, but I want to find the design with which this "stage" is optimized.

That's why the questions:

  • How much increase the number of Rx turns?
  • How much increase the number of Tx turns?
  • How to choose inductance value Tx and Rx?
  • Any extra information will be welcome!

1 Answer 1


You've over complicated things - the two receive coils just need to be series wound antiphase and fed through a non-centre-tapped transformer although you could get-away without a transformer in some applications.

For the transmit coil you have used two with each located quite close to each receive coil. You only need one transmit coil and that can be positioned centrally between the two receive coils for this to work effectively. And, you don't have to use a transformer to drive the single central coil.

I see you are running at 300 kHz so I'm guessing this is some kind of food/pharmaceutical drop-thru detector because that tends to be the default operating frequency. If so then scrap the idea of multiturn coils and go for single turn coils. For the oscillator, this will produce a typical inductance of several hundred nano henry and can be "driven" and tuned to produce several amps. More current means a stronger magnetic field and better sensitivity.

Fewer turns on the receive windings means less problem when trying to phase-out product effects (but do use a graphite shield).

If you are concerned about my credentials in giving this advice and, assuming that this is the type of application I mentioned above, here's one I designed earlier (circa 2006): -

enter image description here

  • \$\begingroup\$ Hi Andy! Thanks for your answers. I do not know if it's done correctly, but I'll post a answer based on your comments. \$\endgroup\$
    – PepoXII
    Commented May 17, 2018 at 16:57
  • \$\begingroup\$ I can't understand how this works! If I want to add clarifications to my first publication and request clarification on an answer, how should I do it? \$\endgroup\$
    – PepoXII
    Commented May 17, 2018 at 17:01
  • \$\begingroup\$ I think you should be able to edit your original post but, in case your reputation level has not risen to those dizzy heights that require it, you can post as an answer but just temporarily and make it clear that you are trying to amend your original question and some kind heart (possibly me) will amend your question and then you can "scrub" your answer. I don't get asked this so I may be offering a cheat that is frowned upon but try it and I'll take the rap. You can request answer clarifications by adding a comment as you have just done. \$\endgroup\$
    – Andy aka
    Commented May 17, 2018 at 17:06
  • \$\begingroup\$ @PepoXII But, be aware that moving the question in a big way tends to be frowned upon unless you make it clear where you have amended things in your question. Reason: it may invalidate answers already given (mine is the only one so that's less of an issue other than causing me tiresome work!) \$\endgroup\$
    – Andy aka
    Commented May 17, 2018 at 17:08
  • \$\begingroup\$ I do not want, in any way, to "invalidate" your answer, on the contrary, I appreciate it. And that's why I DO NOT WANT to change my first question, but I want to publish more things, to which I hope you and other people can contribute. If the idea is that there are few published entries, I will edit my first question! \$\endgroup\$
    – PepoXII
    Commented May 17, 2018 at 17:20

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