Handheld Metal Detector

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

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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.

kensmith@rahul.net   forging knowledge
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Ken Smith

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Homer J Simpson

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 If they patented it when I designed it, the patent should be well out of date by now.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,


Reply to
Chuck Olson

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