many thanks to John Tibbits for his discussion of the operating principles
of metal detectors.  I was somewhat curious about the finer points, myself.

        My one experience with a metal detector was when a neighbor brought
one to our house in the early seventies and asked me to install the
add-on earphone jack which the manufacturer supplied.  They had a suitable
hole in the case of the detector and a little bag containing the jack and
the mounting nut and probably a little schematic and that was pretty much it.

        After getting the jack connected, I remember playing with it a bit.
It was one of the heterodyne-type metal detectors which probably operated
around 100 KHZ or so.  I remember tracing the reinforcing bars in the
concrete slab of our house.  The one thing that I think I recall that sort
of peaked my curiosity was that when I took it outside and swept it over the
grass, the oscillator pulled a little in the opposite direction that it had
gone for the iron bars and bolts I had been putting near the coil.  I wasn't
sure if this opposite pulling was due to the grass or the dirt underneath,
but one could definitely hear a little rise in the tone if ferrous metal
made it drop in pitch or vice versa.  If I sound a little vague, it is
because a heterodyne-type metal detector will give opposite results
depending upon which side of the center frequency one has tuned the search
oscillator in reference to the detector's oscillator.  This should be a hint
or slight warning to anybody who wants to use a PIC.  Be sure you know
the relationship between the two oscillators or design your frequency
counting routine to either read the search oscillator directly or
self-calibrate for a period of time to get a base-line reading before going
treasure-hunting.:-)

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group