Naked Science Forum
General Science => General Science => Topic started by: askey on 27/09/2009 14:18:32
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hello ,i would like to make my own electro magnetic coil and build my own high powered metal detector ,but dont now where to start.would be interested in talking about coil making.whoever answers this thanks for your time.
[MOD EDIT - PLEASE PHRASE YOUR THREAD TITLES AS QUESTIONS IN FUTURE PLEASE. THANKS. CHRIS]
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One way to start would be to buy a kit metal-detector (perhaps from Velleman (U.S./international) or Maplin (UK) or Tandy/Radio-Shack type stores) - although this will still require some prior experience of basic soldering techniques.
Realistically though, it's unlikely that any kit would have the sensitivity and stability of a high-quality ready-built product.
I suspect operator-technique (and a good helping of luck) are also important.
While the principle of metal-detecting is fairly straightforward, the implementation can be quite a fine art, particularly when you're trying to really aim at the high end. Something which was genuinely educational about the principles to set up in the lab or spare-room probably wouldn't be practical to carry around in a field - especially in cold, drizzle and wind!
Coils are probably quite simple although obviously they need to be wound on non-metal formers, and they need to have very good mechanical stability. To sense very small pieces of metal your electronics needs to detect very small changes in the characteristics of the signals flowing in the coils, and any slight movement due to loose wires or thermal expansion and contraction could cause changes at least as big - resulting in false-sensing, or you having to turn the sensitivity down too much.
From the underlying physics, geometry determines that (all else being equal) a larger coil will tend to probe deeper into the ground than a small one. On the other hand, a small fragment of metal near the surface would produce a proportionally smaller (therefore harder to detect) change in characteristic in a large coil than in a small one. So there will be compromises, not necessarily one machine or design "better" than the other.
I'm not really familiar with commercial metal-detectors, but I'm sure good ones will distinguish between ferromagnetic (iron, nickel, chromium and many alloys of those) and non-ferromagnetic (everything else) metals. Technically you can do this by testing with both high- (10's-100's kHz) and low-frequency (perhaps 50-500Hz) magnetic fields. Using the higher frequency response as a baseline, ferromagnetic metals will interact to enhance low-frequency fields whereas non-ferromagnetic metals will not (I'm simplifying somewhat here - I'm referring to the metal samples increasing or decreasing the coil inductance at a given frequency).
A well-designed detector will also be nicely balenced, not too heavy, and comfortable to use and hold for several hours at a time.
I can well believe that with recent advances in signal processing in low-cost, low-power microprocessors/microcontrollers that the best detectors available today in a given price range will be vastly superior to products of 15 years ago.
I'm sure all sorts of genuinely useful technical features are possible, but watch out because there will also be many unfounded marketing claims!
I wish you luck, but finds like that in the news recently are very rare!
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I'm nit picking ...
The Tandy name was dropped in May 2000
http://en.wikipedia.org/wiki/Tandy_Corporation
There are cheap (£~10) metal detectors available used to detect pipes ...
http://cgi.ebay.co.uk/3-in-1-DETECTOR-pipe-stud-wire-metal-joist-live-cable_W0QQitemZ180396516735QQcmdZViewItem
Their range is only a couple of inches for a coin sized object.
BTW
In the UK you have to have permission from the landowner to use a metal detector on their land,
(the usual deal is a 50:50 split with the landowner)...
The Treasure Act 1996 and Rewards
The Act is accompanied by a code of practice which gives advice on the discovery, ownership and valuation of treasure, and on the identification and care of finds. A key provision of the Act relates to the payment of rewards in relation to treasure, with the relevant extract as follows:
“Those eligible to receive rewards are the finder(s), landowner and / or occupier. Where the finder has a valid permission from the occupier or landowner… he will receive his full share of the reward. It is normal practice to divide rewards equally between the finder and landowner on a 50:50 basis.”
This is our normal practice where metal detectorists are searching with our consent. The Act also states that if a finder is searching without permission they may receive a reduced or no reward.
http://www.thecrownestate.co.uk/metal-detecting
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It is nice to have a metal detector if your hobby is in metal finding because it can really help you dig out treasure. But of course there are provision and other laws you should consider. Try to ask around from your friends who own one and see if you can fabricate your personal metal detector.
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Can of help here. iT is nice to see someone trying to make a metal detector on his own. i have done a bit metal detecting. It is fantastic activity to expect the mysterious possibly metal detecting finds showing up. but not very familar to its working theory and construction. Wish you luck