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

[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  , 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.

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.

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).

[Haenk]

My dad is a (licensed) detector user - and uses one of the cheapest available (branded, not chinese) models. It's the legwork that counts.
Most spectacular find: only a couple of minutes away from his home, after finding dozens of bronze age "safety pins" (like these https://www.penn.museum/sites/expedition/ancient-safety-pins/ ), a field day by the states official group of detector users collected about 300 of them, one of them gold plated. These were buried with the ashes of the dead, so obviously he discovered a formerly unknown massive graveyard from the bronze age.
Recently added new satelitte images revealed a number of building post marks, so the obviously were buildings as well. A "Totenhaus" (house of the dead") might be a possible explanation.

[Electrodynamic]

I built a few Arduino based metal detectors like these,
https://www.instructables.com/Simple-Arduino-Metal-Detector/
https://simplemetaldetector.com/arduino-projects/arduino-metal-detector-with-metal-discrimination/?cn-reloaded=1

Short range, accuracy and tuning is really dodgy. I'm aware the internet is littered with good and inexpensive metal detectors but that's not the point. I like building stuff and think a person learns more by doing things first hand. The Arduino inductor-capacitor differential charge time circuit using interrupts is really dodgy. Basically useless and the two coil setup is the way to go.

Metal detection has come a long way,
https://modernmetaldetectors.com/collections/3d-imaging-metal-detectors/products/nokta-makro-invenio-pro-pack-metal-detector-with-3d-imaging-11000802

I started experimenting with imaging metal detectors using a Raspberry Pi 4, GPS sensor, magnetometer and three axis gyro/accelerometer. This project has a really big learning curve but it's interesting stuff.

[janoc]

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.

That - and also the likelyhood is pretty good that, depending on where you search, instead of an archeological artifact or something interesting you will find an unexploded munition from WWI/WWII and die.
There is even stuff like artillery shells with mustard gas from WWI still being unearthed every year in northern France. Not something one would want to find with a detector ...

In most EU countries you need a permission of the land owner to conduct such activity. Often even a permit from authorities in charge of preservation of historical artifacts/archaeology - and if you do find something (regardless whether with a detector, with a permit or without), you must call the authorities to it, otherwise one could easily end up charged with a crime.

Detectorists are not welcome in most places because there are entire organized gangs using these things plundering the cultural heritage and historical artifacts (e.g. militaria from the many wars and archaeological finds) for sale to collectors. All the while destroying the locations where they found them, making eventual official research there difficult/impossible.

And another large group is metal/cable theft - there are bands that will look for and dig up even underground cables in the fields. So if someone is running around with a metal detector in Europe, they are likely to get checked on by the police sooner than later.

[Overspeed]

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.

That - and also the likelyhood is pretty good that, depending on where you search, instead of an archeological artifact or something interesting you will find an unexploded munition from WWI/WWII and die.
There is even stuff like artillery shells with mustard gas from WWI still being unearthed every year in northern France. Not something one would want to find with a detector ...

In most EU countries you need a permission of the land owner to conduct such activity. Often even a permit from authorities in charge of preservation of historical artifacts/archaeology - and if you do find something (regardless whether with a detector, with a permit or without), you must call the authorities to it, otherwise one could easily end up charged with a crime.

Detectorists are not welcome in most places because there are entire organized gangs using these things plundering the cultural heritage and historical artifacts (e.g. militaria from the many wars and archaeological finds) for sale to collectors. All the while destroying the locations where they found them, making eventual official research there difficult/impossible.

And another large group is metal/cable theft - there are bands that will look for and dig up even underground cables in the fields. So if someone is running around with a metal detector in Europe, they are likely to get checked on by the police sooner than later.

Hello

Regarding the :
There is even stuff like artillery shells with mustard gas from WWI still being unearthed every year in northern France. Not something one would want to find with a detector ...

In France the Gov ( very clever clever ) think about to forbid farm tractors and plow .... as a lot of '' WWI souvenir''  are extracted from the soil by farm tractors each year since 1918 ....

In France the Government purpose is to make laws + forbid + control citizens , in very strict countries as UK or Sweden or Norway detectors are allowed and a lot of interesting historic discoveries are done .

 Regards
OS

[Overspeed]

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.

Hello

Flea market is full of used detectors , cost to design and built one is far over the cost of a used one , top brand as garett or Minelab are not so costly a good medium range / beginner detector as the Minelab Vanquish 340 is in the 300 Euro range

Regards
OS

[Kleinstein]

.... in very strict countries as UK or Sweden or Norway detectors are allowed and a lot of interesting historic discoveries are done .

A problem with detecting for historic artifacts is that quite some of the discoveries are not made public, but end up in dark markts. With many finds the historic value is not in the parts / find, but the context, that is usually lost when someone goes around to look for artifacts with a dector an shovel. It would be impractical to ban the shovels.  It needs a suitable culture on how to handle such finds to allow metal detecting without doing much damage to historic sites.

In finding WW1 shells is rather limited to specific areas. For those heavily effected areas banning plowing (or need for a professional seach) may indeed be reasonable. In Germany the more common danger is finding WW2 AA shells that did not explode. These can be scattered over quite some areas.

[Overspeed]

.... in very strict countries as UK or Sweden or Norway detectors are allowed and a lot of interesting historic discoveries are done .

A problem with detecting for historic artifacts is that quite some of the discoveries are not made public, but end up in dark markts. With many finds the historic value is not in the parts / find, but the context, that is usually lost when someone goes around to look for artifacts with a dector an shovel. It would be impractical to ban the shovels.  It needs a suitable culture on how to handle such finds to allow metal detecting without doing much damage to historic sites.

In finding WW1 shells is rather limited to specific areas. For those heavily effected areas banning plowing (or need for a professional seach) may indeed be reasonable. In Germany the more common danger is finding WW2 AA shells that did not explode. These can be scattered over quite some areas.

Hello
Yes and no regarding some points

Time erosion destroy a lot of buried objects in metal , that depend of the nature of the soil , in some clay soil metallic objects looks as buried some week ago in other soil as black rich soil anything is converted in oxide so what is not discovered is lost for ever this problem is amplified by farm products as fertilizer which generate hard chemical reaction .

Illegal acts are illegals and the law don't really stop illegal operations if ''easy money '' is the purpose .

Regarding ordnance unexploded WWII is different of WWI as WWI battle fields in France  have been covered by millions of rounds and due to primitive fuze systems a lot of shell have not exploded on impact , during the 50/60/70's a lot of local resident have harvested steel and iron in the battle fields to resale as scrap metal . The area is truly huge as that a quarter of France ( North East ) plus some other area used as storage and forgotten after war .

an interesting article in french :
 http://dandelotmije.over-blog.com/article-dechets-de-guerre-un-autre-visage-du-centenaire-123648504.html

Actually in France even 100 years after complete unexploded shell are discovered each year in quantity.

Some very interesting historic discovery have been done in Norway Rennesoey island to the north of the Norwegian city of Stavanger other in UK In 2015 in Oxfordshire, for example, detectorist James Mather discovered the ‘Watlington Hoard’ of nearly 200 9th century coins, which are now owned by Oxford’s Ashmolean Museum. More recently, in 2021, a couple discovered a 600-year-old engraved figurine of a bible, made of solid gold, in North Yorkshire.

Metal detection is an interesting spare time any way but design and built at reasonable cost a high performance metal detector is a great challenge

Regards
OS

[Overspeed]

Hello

a nice video on discrimination in metal detection



Regards
OS