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Handheld Metal Detector

Discussion in 'Electronic Design' started by Clayton, Nov 13, 2006.

  1. Clayton

    Clayton Guest

    Hi,
    I am looking for some plans/schematic for a Good
    Sensitive(hopefully)Handheld Metal Detector. It will be used for
    checking timber for nails and such before being re-milled.
    I was wondering if anybody could point me in the right direction?
    I know there are ones on the market for just this purpose but that
    takes all the fun out of it.

    If anyone here has any experience with building one I would very much
    appreciate your input.

    Thank You
     
  2. Ken Smith

    Ken Smith Guest

    How about a strong magnet and a compass? You run the magnet over the wood
    and then move it far away and then look for the magnetized nails.

    I think for this sort of use, a "frequency domain" metal detector is the
    better way to go. These are the designs where the coil is part of an
    oscillator that is beat against another oscillator. They can be fooled by
    an object that has just the right combination of electrical condutivity
    and magnetic permeability. People don't use silver coated nails so this
    won't be a real problem.

    You want the coil of the metal detector to be wider than the distance to
    the nail. When you get beyond the near field case, the signal drops off
    as the 6th power of distance, you you really don't want to be out that
    far.
     
  3. Chuck Olson

    Chuck Olson Guest

    About 40 years ago when I worked for a company that made "Proximity
    detectors" I designed a pickup coil using a ferrite rod, perhaps 6" to 8"
    long and I found that if the coil is wound so that the turn spacing
    increases from the center toward the ends, the proximity detection range
    along the rod can be made constant. (Without the variation in spacing, the
    detection range is greatest at the center of the rod). You may have to
    experiment with various windings, using close-wound for the center 1", and
    then loosening up the winding pitch each half inch out to the end. A little
    double-sided tape and a marker strip that shows the position of each turn
    should make it easy to control its characteristics. I believe it operated in
    the range for 50 to 100 KHz. If they patented it when I designed it, the
    patent should be well out of date by now.

    The circuit I used employed a transistor oscillator with feedback from the
    emitter to a tap on the capacitive divider that resonated the coil. It was
    essentially a series-tuned Colpitts configuration but with a potentiometer
    in series with the feedback path so that it could be adjusted to the edge of
    oscillation. The presence of metal near the coil would reduce the amplitude
    of oscillation. Oscillator output was obtained from the collector of the
    oscillator and delivered to a peak-to-peak rectifier, filtered and the
    resulting DC level was then detected by a Schmitt Trigger circuit which
    controlled a relay. Alternatively, instead of a relay, the filtered DC could
    bias an audio oscillator to produce an audible frequency change with the
    presence of metal.

    A search for proximity detection circuits should be productive.

    Good luck,

    Chuck
     
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