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Sensitive metal detector

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 28 December, 2008 » Sensitive metal detector « Previous Next »

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hackinblack
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Username: hackinblack

Post Number: 325
Registered: 09-2006

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Posted on Tuesday, 18 November, 2008 - 08:55 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

have any readers used,or adapted,a circuit for a sensitive metal detector?
i have searched for a suitable one without much luck.
i am trying to build a detector for scanning timber for metal fragments/nails prior to machining it,as i had to import the last blade for my saw i need to avoid the 'smashed carbide' caused by a buried nail.
although one commercial (U.S.)one pops up,it seems overpriced and has to be 'adjusted' with a screwdriver for sensitivity before use.
this seems like a cheap and nasty way to do it,as the thing is supposed to be 'advanced'
i need to scan reliably through 4"/10 CM softwood and even thicker hardwood (old beams!)
any ideas?
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terry
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Post Number: 505
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Posted on Wednesday, 19 November, 2008 - 02:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Sounds very easy, the hardest part is constructing the coils. Im sure any of the basic metal detector ciruits could be adapted.

can you give more details of how it needs to work please ?


Terry
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hackinblack
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Post Number: 328
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Posted on Wednesday, 19 November, 2008 - 05:30 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

the commercial ones ive seen are a paddle shaped 'wand' with a search coil around a foot long.they are waved over the length of wood one side for thin bits,both sides for bits 4" or more thick to scan for metallic objects hidden in the wood such as nails,staples,bullets;actually quite common!,

a freind of mine worked in a timber factory and as their timber came from remote areas close to MOD training land...they where constantly ducking as bullets literally flew about the factory when the cutter blades hit them)at several hundred pounds a go the bill for new blades was murderous.

from what i've read the minimum size of object detected increases with the detectors depth capability;so if you look for a fragment of staple in thick wood
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gajjer
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Post Number: 166
Registered: 05-2007

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Posted on Wednesday, 19 November, 2008 - 06:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi hackinblack

google 'geotech' there are loads of metal detector circuits there.
Remember that some of these are geared to finding gold and that you are primarily interested in iron/steel ( unless you have some very extravagent carpenters in your area! ).
I've read about detectors finding a nickle at 12 inches but not been able to make one quite that sensistive.
I would imagine that a staple would be on the surface so even though it is small it should be easy to detect. Bullets on the other hand . . .

Coil construction is probably the most important part. And you want a fairly small coil if you want to isolate the location on rough wood. With planed wood you should be able to see it easier but with old rough wood I would imagine it is more difficult.

I built a design based on the Mark Stuart design from the geotech website, but it will take some setting up. I'd probably start with a conventional BFO type.

Hope that helps
cheers
gaj
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gajjer
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Post Number: 167
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Posted on Wednesday, 19 November, 2008 - 06:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just had another thought.
I don't know if this is practical for your application but you could make the coil as a hoop - even square or rectangular. Then you could pass the wood through it. That way the target gets pretty close to the sensitive part or the coil. Different sized coils for different sizes of timber.

just a thought.

cheers
gaj
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hackinblack
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Post Number: 329
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Posted on Wednesday, 19 November, 2008 - 07:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

ive seen loop scanners,but for my purpose they aren't practical as some of the wood is squared-off oak trunks some 10ft long+ by 2ft square!

i found the geotech site after posting last time;naughty reverse engineering
did the one you made work well or so-so? the other ones on these sites are too complex,too many components,and inductors with no data other than value.simple is best,reliable is better
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gajjer
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Post Number: 169
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Posted on Wednesday, 19 November, 2008 - 08:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The one I built was a dual pick-up coil version of the Mark Stuart design. I didn't do any measurements of sensitivity. It was more an exercise in trying to improve on his design. Just playing really. It worked, but quantifying it would be difficult. It wasn't simple either. Its not a simple technique. Thats why I would go for the BFO type if you want to experiment. The sense coil is the important bit then the fixed oscillator just has to match that. You'll need a scope though.
Have a go. Got to be cheeper than a new blade.

cheers
gaj
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twintub
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Post Number: 14
Registered: 02-2007

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Posted on Wednesday, 26 November, 2008 - 09:18 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

For something cheap and cheerful (often the best soloution!) check out this sub £20 kit which has a detecting range upto 200cm!:
http://www.quasarelectronics.com/smart-kit/1022-metal-detector.htm

This website also sells ready-made a handheld wand (£65ish) and a 'kiddies' version (<£10) that may suffice.
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twintub
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Post Number: 15
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Posted on Wednesday, 26 November, 2008 - 09:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

....oooops!
I meant 200mm!!!!
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vlf
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Post Number: 187
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Posted on Wednesday, 26 November, 2008 - 11:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi, I doubt the cheap and cheerful will reach the depth you need, they say 15 to 20 cm; more like mm ?.

Anyway, best to use Pulse Induction, this was an EPE project, and is there last PIC controlled version; I have built this one, and it will go to 9 inch deep.

That is, if you want to pay the price for the kit... URL below:

http://www.magenta2000.co.uk/acatalog/Treasure_Hunter_Kits.html

Regards, Brian.
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hackinblack
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Post Number: 331
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Posted on Thursday, 27 November, 2008 - 08:55 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

i dont think either the item or the price is attractive,certainly not for a KIT! they will be gathering dust on their shelves,not mine
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twintub
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Post Number: 16
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Posted on Thursday, 27 November, 2008 - 10:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Why not try the ones I suggested?
If they turn out to be no good, then at least you'll have a stud detector for your DIY jobs!
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mikeb
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Post Number: 59
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Posted on Friday, 28 November, 2008 - 12:53 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think you should think about the detection technology.

Detecting small targets using inductive changes is very tricky - plus it very difficult to get the search coil evenly moved across a tree surface.

I'm thinking out loud here -

perhaps an old airport (or school) 'walk through' scanner - they seem to detect quite small targets

perhaps an old airport Xray scanner.

some sort of array of magnetometers above which is placed a fixed known magnetic source - look for disturbance.

same idea but with a RF source - look for radiation from metal objects due to reradiation.

... might be something in there for you.

rgds

mb
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boris
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Post Number: 389
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Posted on Friday, 28 November, 2008 - 06:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

http://www.dealextreme.com/details.dx/sku.6646
Thank you for not breeding.
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heap
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Post Number: 1
Registered: 12-2008

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Posted on Wednesday, 03 December, 2008 - 09:24 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

One promising idea was originally published by Ambit International and that was the radiometer, or phase angle metal detector. It not only discriminates but in my proto-types offered 'in air' sensitivities of around 18" for a pre 1947 florin (2 shillings coin).

The original Ambit International article has now disappeared from the net but the basic principles can be found at ....

http://homepage.ntlworld.com/moonshadow/New_Folder/metal_detectors1.htm

It served me very well, because each evening when my wife took our dog for three laps around the local park, I tried out my latest version and very seldom ended up with less than 15 coins per excursion.

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