Clayton said:
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
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patent should be well out of date by now.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
