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Homemade metal (benchmark) detector?

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Has anyone built their own metal detector for finding buried benchmarks? If so, how did you do it?



I had saw this on youtube before, and was thinking about making a McGyver kind of multi-geocache placement based off of it. The searcher would find the objects to put together (as in the video) then use it to find a container with coordinates in it to the cache location. I never tested it in real-world applications to see if it really works....


Edited by LSUFan
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Do a Google search for How to build a metal detector


There are a number of web sites that have instructions to do that for example http://www.easytreasure.co.uk/bfo.htm

Most of these will work at a depth about equal to the diameter of the coil.


My father had one with two coils, one on each end of an eight foot pole that worked to a much greater depth. It would be nice to build one like that that would find benchmarks at a depth of about two or three feet.

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I built a metal detector while in high school - simple oscillator with a coil (inductor) is all that's necessary for the most basic metal detector. It worked well enough to find coins and bottle caps through a few inches of cover. I looked at some of the modern metal detectors and am flabbergasted by the prices of some of these. Seriously. I bet I could create a fairly simple oscillator with dual coils, output through an A/D converter and into an Android smart phone, where all the heavy lifting of results interpretation could be done with an app. Honestly, it's like mythology or something runs the metal detector market.

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The local Friends of the Library had their book sale last month and one of the books I picked up was this one. Second edition fourth printing August 2000. Cost me $2. This book is a major plug for the manufacturer, but it does explain some things.


The new metal detectors like I have are very complex, micro processor controled, and are specialized. Most of the ones found in stores like Radio Schlock are coin hunters. Mine will even tell you what coin it finds like a quarter, dime, or what ever.


There are others made special to detect gold nuggets and others to find coin caches (cans of coins hidden by someone long gone) in ghost towns.


While the coin finders will also detect steel, brass and any other metal they are optimized to find coins near the surface.


The book also goes into metal detectors for wanding passenger at the airport and detecting nails in wood before sawing ect. Also mentioned the two box deep seeking model. Interesting but long winded and always plugging their products. But well worth the $2.


I considered building my own BFO (beat frequency oscillator) type also, but maybe it is easier to buy the right one.

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I can understand some being priced around $200, which would be a pretty sophisticated unit. What I can't fathom are some which are priced $600 on up to over $1,000.


These units put me in mind of Citizens Band transceivers back in the late 1970's. They were all basically the same things, but there some with names like Golden Eagle with old school knobs and such and a fancy, larger desktop style cabinet. The guts were still about the same as you got from Radio Shack for $120, but they were priced at $500 on up - pure snake oil. All were still 4 watts input to the antenna, nothing more. A good antenna made far greater difference than transceiver. But the market was filled with units claiming to be more than they were, on appearances.


I'm inclined to believe a good set of coils is going to be more important than a BFO or one which scans using a spectrum of frequencies to see what resonates. $1,000 for a metal detector, though. Gads.

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I've happily used my Harbor Freight unit for several years and it works fine, but like all that type you're limited to a depth similar to the diameter of the search coil. If you allow for grass and rocks, that's not much. For the $25 on sale, I can't complain- it's found me several marks I wouldn't have found otherwise.


As 68-eldo says, there are/were detectors that put the coils on each end of a pole, and those can go much deeper. I have some ancient articles on how to build them, but they operate at frequencies and amplitudes that aren't technically legal these days- too much radio interference because they operate quite a bit higher than the usual ones today.


IMO, there are a lot of rip-off/snake-oil units out there for very high prices. I doubt they work much better than a simple design and a trained ear. Speaking of which, you get vastly better results if you use big old headphones that seal to the ear. Keeps out wind noise and saves batteries.


What we all want is a cheap ground penetrating radar!




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I rely heavily on my cheapie Harbor Freight detector--it works to about 6 inches when I am lucky, 4 pretty regularly, at least for disks. I haven't had any luck with the small rivets on railroads in my area. It also finds aluminum cans VERY well, and rebar in concrete bridges. I use it quite a bit, and it takes a beating. It is about 8 years old and, believe it or not, I am only on my second set of batteries! Everything is scratched and beat up, the battery door falls off from time to time, but it keeps on working reliably. One benefit is that the handle is extendable, which will allow me to shorten it and put it upside down in my backpack for longer hikes. Granted it looks like I have a halo, but I am pretty much of an angel most of the time anyway so that suits!

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Turns out I made that ancient metal detector article into a pdf file. It's from the 1950s in an application note from capacitor company Cornell-Dublier. The designs use tubes, but they give the coil designs (40 turns of #22 on a 1 foot diameter form) and operating frequencies. If one were to use a couple modern ICs to replace all the tube stuff, it wouldn't be hard to make the "pole type" detector and get several feet of depth. If anybody's interested let me know and I'll get it up on my web site. Size is 3MB.



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It would be interesting to see the article, but I can't imagine I'd use extreme depth capability very often. The RatShack unit several of us bought a few years ago has plenty of sensitivity for most places I've used it. The problem is that the world is so littered with pieces of metal that you find a dozen other things for every item you are looking for.



Side note on a slightly unusual application of a metal detector:

An 1875 church in town is about to be demolished, and there was speculation about if and where a time capsule might be located near the cornerstone. A hole cut from the inside (under the plaster is brick that you can carve with a fingernail) found nothing behind the cornerstone. I took my metal detector and found a strong signature on the buttress foundation stone under the engraved cornerstone, on one face only.


An abrasive wheel in a Skil-saw and hammer and chisel eventually got through 8 inches of stone and we removed a pure tin (not plated steel) box. Better than a geocache! The alternative to that work was to wait and hope the new owner's demolition crew would search the rubble for us as the building was removed.


Unfortunately it was not (or no longer?) sealed well and there is a pile of soggy, moldy paper inside. We don't know yet if an expert will be consulted to try to save any of the papers, or if there are coins underneath the paper. So it was a technical but not practical victory.

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