Metal detector schematic?

[cdev]

I'm looking for a way to build a semi-decent metal detector myself, I don't know how much I'll use it or if I will but its something I would like to have.

I see a lot of new and used ones online. But I dont want to spend much. If possible. I like winding coils, etc. I have a nice darsonval meter movement that I could use, (center reading)

I really don't know if it needs a center or side reading meter, I have a lot of parts, which I like using. So I am looking for interesting electromagnetic projects.

Could I (legally) use RF to peer into the ground? Now that would be interesting!

[ledtester]

You might find DiodeGoneWild's two videos detailing his experience building one from scratch interesting/informative:

DIY DD metal detector 1 - first success
https://youtu.be/eCRgNr6jj2g

DIY DD metal detector 2 - coin at 35cm, audio indicator, PCB
https://youtu.be/-czOV-hKSlA

[pqass]

See here for the first of two parts (schematic at 5:20):

[themadhippy]

https://worldradiohistory.com/UK/Practical-Electronics/00s/Everyday-Practical-Electronics-2002-10.pdf

https://worldradiohistory.com/UK/Everyday-Electronics/70s/Everyday-Electronics-1972-05-S-OCR.pdf

[tepalia02]

I once used Arduino UNO and metal-touch sensor for this project. https://sensorkit.joy-it.net/en/sensors/ky-036

[Gyro]

https://worldradiohistory.com/UK/Practical-Electronics/00s/Everyday-Practical-Electronics-2002-10.pdf

https://worldradiohistory.com/UK/Everyday-Electronics/70s/Everyday-Electronics-1972-05-S-OCR.pdf

For some reason, I remember being fascinated by the coil construcion and adjustment details of the 2002 one. It's one of those articles that are very well written.

[Conrad Hoffman]

This article is very old, in fact it uses tubes. Valves for some! I'm not suggesting it as a project, just as educational material. What's interesting is the design of the transmit and receive coils to get a large baseline. Metal detectors don't go much deeper than the diameter  of the coils, so I'm guessing this might do better. Naturally one would want to update it to solid state circuits. It might also not be legal these days.

[Kleinstein]

There are different was the metal detector work. I know of 4 types:
1) The target changes the resonance frequency of an LC resonator - so detection of frequency change
2) The target changes the coupling between 2 coils or set of coils - detected is the amplitude
    The coils could be at an angle or a round and figure 8 coil, so that in free space there is no coupling.
3) Pulsed operation and detection of the delayed part of the decay if the magnetif field
4) measurement of the effect on the earths magnetic field, so just a magnetic field probe. This system can go relatively deep, but only detects iron/steel.

[Gyro]

4.1) Proton Magnetometer...   https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1970-10.pdf

[RoGeorge]

Could I (legally) use RF to peer into the ground? Now that would be interesting!

Depends on where you live.  Here in Romania it is illegal to use metal detectors (not an EU law, it was illegal even before Romania joined EU).  Not sure if owning one without using is legal here, probably not.

[Gyro]

That's an odd law, I wonder whan the historical basis of that is.

In terms of using RF to peer into the ground, I think that comes under the heading of 'ground penetrating radar'. I don't know what the licencing and power restrictions are, but they are capable of mapping changes of ground density and material changes, revealing archiological features. I'm not sure whether you would get sufficient resolution for small metalic items [Edit: Ah, maybe, depending on frequency].

https://www.groundpenetratingradar.co.uk/about/how-does-gpr-work/
https://en.wikipedia.org/wiki/Ground-penetrating_radar

[MK14]

Depends on where you live.  Here in Romania it is illegal to use metal detectors (not an EU law, it was illegal even before Romania joined EU).  Not sure if owning one without using is legal here, probably not.

This website, doesn't seem to agree with you:
https://detectingschool.com/metal-detecting-in-romania/

The law there is pretty simple, you are not allowed to detect in archaeological sites or close to those and the private property has to be respected!

For the latter you should ask the owner of that private property for a written permission to do so. Searching in an archaeological site, without the prior written permission of the Ministry of Culture, is a crime and it is punished by prison time.

[RoGeorge]

I remember them differently, but I might be wrong.
Glad to hear they are lighter.



Later edit:
--------------
I've just checked.  You need a permit to buy/own a metal detector, no matter where you want to use it.
And if you use it in an area considered archeological site, registered detector or not, straight to jail! 

Ordinul Nr. 251/409/2275/M 115 din 31 mai 2004
https://legislatie.just.ro/Public/DetaliiDocumentAfis/54142

[n5al]

I don't know how accurate the list is, as laws are always changing, but this guy has made a big effort to document metal detecting legality on a country-by-country basis:
https://md-hunter.com/list-of-countries-where-metal-detecting-is-allowedbanned/


And just for fun...
Our forum members from Canada might remember this comedy skit, from a TV episode of "The Red Green Show".

[antenna]

Here are two metal detector schematics from a book (Encyclopedia of Electronic Circuits) I mailed home from a minimum security prison that was salvaged from a dumpster behind a library while doing community work.  The book was originally bought by a ham (SK) in 1985 when the book came out   Unfortunately, he passed at about the same time as I retrieved this book from the dumpster, so I can only assume the family had donated his books and the library sent them straight to the trash.  I'm glad I was able to rescue this one.