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You built your own Metal Detector; What's your (interesting) STORY ?

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RJSV:
Hello:
   A bit of frivolous question.   About 8 years ago my friend's brother was in a 'terrible' divorce situation, (unfortunate, for any husband or wife).

    The angry spouse stole the dog, trashing the garage, in the estranged hus.  AND tossed a whole bag of valuable old coins, into murky pond in the local duck pond (public park) !

    I had contemplated what was possible, if having a standard METAL DETECTOR, the kind with disk shaped 'coil' on a long handle.

   Does anybody here have a story to relate, either of making a sweep metal detector, or weird / unusual findings ?

   One person related,  actually found a whole SAFE, 2 ft. by 1 ft. and used welding torch, but not much value !

   Come-on;   Entertain US, here.

ozcar:
I made a metal detector in 1969! I did not think of it as Dodgy Technology though.

I tried it out in an area that had been submerged for decades, but which had recently dried out due to change in mining activity in the area. What I found most of, was bicycles, or pieces thereof, and horseshoes.

I could add one of these here  :horse:, but I'm not sure if the horses were dead, or were just fitted out with new shoes.

RJSV:
   Thanks;   I did place this in Dodgy section, partially to give the regular section a break, for more 'balance', as it were.

   Serious thought, in regards to GOLD detection, being a non-ferrous metal, is to (contemplate) what sorts of energetic particles can be emitted, for some alternative detection.

   Since that avenue is a big motivation, also perhaps much of conventional jewelry has some ferrous backing or framing.   Thus there being an avenue for success, in probes underwater, near land.

   Of course, a SAFE would likely have ferrous walls, even if containing gold bars.

And, since I'm into limited, single purpose circuits, a metal detector enthusiast could tell an interesting tale.   Plus a circuit would be nice to read about, especially novel circuits.

T3sl4co1l:
I mean, it works? It's just an impedance bridge. Maybe it's at fixed frequency (or, tracking: equivalent / clone reference oscillator), maybe it's variable (L varies with load --> F_res shift --> detector --> ??? --> profit!!! (sometimes)), but whatever the case, you only have two degrees of freedom and nothing else: resistance and inductance.  Ferrous objects tend to increase L and R, nonferrous tend to reduce L and increase R.  Or reduce R in the parallel equivalent, whatever.  You can tell something's there, and the littlest bit about what it is, and beyond that, not really much.  The rest is digging and sampling -- and searching, in general.

With certain antenna geometries, or more of them, you can start to do some feeling-out of the environment around; measure reflected currents perhaps, or phase shifts, or get more specific directional information (if not to such an extent as to form an image as such; metal detector frequencies tend to be low, so imaging is almost N/A).  The spacial resolution is all near field (unless you're doing something like ground-penetrating radar), so you need a big coil to detect far-away changes, and you need extreme sensitivity (parts per thousand or better) to detect small objects at such distance.  And you don't have any spacial awareness [within the field of a given antenna], you can't know if that shift is due to a deposit of black sand or a Damascus sword.



--- Quote from: RJSV on August 09, 2024, 03:49:51 am ---Serious thought, in regards to GOLD detection, being a non-ferrous metal, is to (contemplate) what sorts of energetic particles can be emitted, for some alternative detection.

--- End quote ---

Well I mean... I guess if you had a neutron activation source handy?  You could probe with that?  And find some gold decomposition emissions as a result?  But good luck getting hold of much neutron intensity via legal methods.  Or maybe something with natural muons, if you don't mind soaking in the local environment via a bulky (and expensive) detector system.  But in any case, that's a much more general kind of element detection, not what "metal detection" is commonly understood to mean (eddy current and ferromagnetic phenomena).

Tim

Kleinstein:
There are plenty of metal detectors available, e.g. used to find coins and similar metal on beaches. They work reasonable OK and AFAIK they are leagal in the US. In some European countries there are restrictions because they are also used for illegal seach for archelogal artifacts.
For a bunch of coins one would not even need an especially sensitive one, just a water proof / mudd resistant version.

There are also fakes one of cause, claiming unreasonable dection of gold or similar. The real ones can destingush between magnetic and non magnetic materials and possibly a little about the size / conductitiy range within limits (e.g. get a good guess between a penny and a quarter - but real life has enough interference to not be sure once there are 2 parts or a background rock).

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