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Simple Metal Detector Circuit

The metal detector circuit shown here must represent the limits of simplicity for a metal detector, yet the design works surprisingly well. It uses just one 40106 hex Schmitt inverter IC, a capacitor and a search coil – and of course the batteries. A lead from IC1b pin 4 needs to be attached to a medium wave radio aerial, or it should be wrapped around the radio. It can be used even like those hand held metal detectors.

As shown, the metal detector gives a respectable range for beat frequency operation (bfo) up to 90mm for a bottle-top. In fact, for the ultimate in simplicity, capacitor C1 may be omitted. In this way the author achieved am amazing 150mm range for the bottle-top. However, with the frequency then being raised to more than 4MHz, instability becomes a significant problem.

Metal Detector circuit diagram

metal detector diagram

As shown, the circuit oscillates at around 230kHz. One may also experiment with the frequency by changing the value of C1. A Faraday shield may be added to reduce ground effect and capacitive coupling, and this is wired to 0V.

Metal detector video with a funny song

Since the inductor resists rapid changes voltage, the charging of C1 is slightly delayed as the logic level at IC1a pin 2 changes. This sets up a rapid oscillation, which is picked up by a MW radio. Any changes in the inductance of the search coil (through the presence of metal) bring about a change to the oscillator frequency. Although 230kHz is out of range of the Medium Wave band, an MW radio will clearly pick up harmonics of this frequency.

Metal detector calibration
The making of search coil L1 allows a lot of room for error and is far from critical. The author used seventy turns 30 s.w.g. (0,315mm) enamelled copper wire on a 120mm diameter former.
The metal detector is set up by tuning the MW radio to pick up a whistle. Not every such harmonic works well, and the most suitable one needs to be found. The presence of metal will clearly change the tone of the whistle.

Metal detector faq
This is not an industrial or security metal detector and is not even close to loma or eriez metal detection products. It’s just a portable but not a hand held metal detector.

60 Comments

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  • Me

    hey
    I made this circuit with 2 capacitors and different Ic and the maximum distance that the frequency changes is about 15 cm .
    I wanna improve the maximum distance, so my question is can I do it by this circuit or should I change my circuit entirely…
    I tried different capacitors , does it works or not?
    If I give the ic more voltage, does the current of coil increase or not?…
    can you help me…

  • samyak jain

    plz let me aware of use of hcf40106 ic in metal detection

  • seif helal

    how can i make it work to detect metals under ground
    range about 1m

  • gaurav

    i m trying it but its not working ,,,,,,,,,,,plzzzz help n send me more details

  • muskoka_survey_tech

    Mr. Marion,

    Hello, I am survey technician with some training in computer technology. I want to build this project as a protoype with my sons. Also, wanted to thank you for posting it.

    Could you please verify the following:
    – that 470p is 470 picofarads?
    – is the project in the video about this detector because they seem to have alot more parts in their circuit than shown in the diagram
    – is this detector particularily for ferrous metals?
    – would you be interested me keeping you posted with with what we come up with? (I noted your email from above)

    I’m sure I could build other more elaborate and more expensive ones, but I think its good to start small and work my way up. I liked the video with the kids, looks like they had lots of fun!

    Looking forward to your replies,
    Sincerely,
    mst

    • Jim Keith

      Watched the video clip. Wish they had something like that when I was a kid. I complain about how poor an attention span children have today, while forgetting what it was like for me as a kid. I now remember that I was in the Radio Club when in Jr. High and I was the only one that had an interest in the subject. Everyone else was goofing off.

      I got a glimpse of the schematic capture on the screen. It is indeed more complex as it has a multivibrator that is probably used as a local oscillator. The other stages probably were the search oscillator, mixer and headphone amp. That would make a good project for electrcircuits.com.

      They also put a good deal of effort into the search coil.

    • Jim Keith

      470 picofarads, and perhaps it could be tweaked with a trimmer capacitor to make the frequency what you want. If your receiver has a BFO, it will be a great help.

      Ferrous materials tend to reduce frequency while non-ferrous tend to increase frequency. By setting the BFO tone at perhaps 400hZ, you can experiment with different materials to see if the tone increases or decreases frequency.

      Ground unused inputs of the 40106 as there are 4 more sections. Ungrounded CMOS inputs can cause sections to oscillate at several mHZ.

  • Morgothen

    Dont work at all

  • mithilesh kumar sinha

    please send me detail circite digram ofgold metal detector with guide line fo aprox one ft defth capacity .I will be thank full to you .

  • RAMBABU,GAVARA

    kits pricelist wanted

  • Nate Youngblood

    1. This is a one-way website, it’s for comments. How the website works should be obvious to anyone with even one year of university.

    2. If you can’t spell out “please” or “good” but prefer to use the EZScript abbreviations, you’re insulting other readers with your laziness.

    3. Among the students, there are Indian and Arabic names. I have worked with Indians and Arabs, in the States and in Europe, who believe in integrity and self-reliance. What happened to that?

    4. It’s an easy circuit, 3 components and a battery case. I have never seen simpler.

    5. If there is a person who needs such a device — maybe you have land mines in your village — this device is not sophisticated enough to help you. There is a “Metal detector faq” at the top that explains this.

  • sudhu

    plz send me the details of this project..

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