Anyone Built a DIY metal detector?

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Interesting you are using Arduino for this project. I use them for robotics and a homebuilt Segway.

SteelPat said:
Marauder said:
Thanks for the info guys, I'm pretty set of the Surf PI 1.2, but can't get silverdogs website to work. Stack of SQL errors.

Electronicly speaking ground balance doesn't seem that hard to do with PI, assuming that it doesn't vary greatly over a given area, it should be easy enough to tune out, before the comparators. I'd also like to note here that if i'm talking out my arse feel free to point out.

My Plan is to buy two of the kits and set them up so that the electronics board is interchangeable to that i can mod one board and compare it with the original design. At the moment I'm thinking of adding ground balance(manual) and replacing the components with high precision pieces, to see if it makes a difference.

Also I'll just jump on the Discrimination on PI band wagon and say that i think it would be possible. As far as i understand the PI detector induces current in the target, which creates a magnetic field from the target, which in turn creates a current in the detector coil, which is observable whilst the pulse is inactive. Each type of target should create a different field, depending on the target composition, a different frequency probably, and the amplitude would be determined by size. With a micro controller you could compare these against known types(iron) and reject the signal(not relay the signal to the rest of the detector) ie discrimination. The only problem would being able to do the comparisons at speed. There are specific high speed comparators that are capable of this, but I'm sure its not easy to do in practice.
Click to expand...

Electronic ground balance isnt hard in theory - it's the electronics that will let you down. Basic idea is to get a sample with no target present - this is your ground base signal. You then subtract that from the mixed target response (this will contain target response and ground response). Then all that should be left is target. This is good in theory but in practice the ground signal will vary even moving a few cm away. Also you have EF to contend with. This is the signal that the earth itself creates. This too needs to be removed from mixed target response. Then there is EMI - another signal that needs to be removed.

I will post some more tech info on timings for basic GB/EF that I implemented on the MPP (via a ChipKIt Uno board that I wired to basic circuit) that will show you the basics of that. These timings work but the electronics let the thing down a little as they are a bit noisy and not fast enough.

As for the discrimination: With a PI you are not looking at the current created as such - what you are looking at is the decay time after the pulse has been turned off. Yes you are correct, the decay time does change for different materials and the basic idea of comparing it to known decay times will work - BUT - not with the way analogue receive circuits work. And unfortunately thats how most, if not all, kits are done (this goes for a most commercial pi machines too). There is a way around this which I have yet to implement. By using a ADC and taking 1uS (or less) samples straight after pulse turnoff (after any significant EMF if no snubber is in circuit) you can create a sample wave shape. After all EMF/EF and GB have been removed the resultant wave slope could be compared against known materials. This should work but as I said I have yet to implement it. I do have a ARM Cortex4 discovery board all ready for this project - just need time to do it.

Here is my modded MPP:
https://www.prospectingaustralia.com/forum/img/member-images/4467/1444701561_mpp_mod.jpg

And this is a design of my own thats also controlled by a Chipkit Uno:
https://www.prospectingaustralia.com/forum/img/member-images/4467/1444701595_ccpi.jpg

If you want more technical info ie timings/arduino code etc feel free to ask.
Click to expand...
 
Did my first one a BFO in 1979, latest PI projects are a Mini pulse Rev.D and a Crossbow Classic Rev C. Mar 2015.
 
Well now i got the coils on the way, i've got to order the electronics. 2 X surf PI1.2 longboards. and I'll grab an extra PCB. $155 AUD including shipping.

SilverDogs website is back up and running again so should be able to order in the next couple of days!
 
The thing that got me was the poly caps. Never seen/used one before and i couldn't find out which way they went! And i taught Electronics! Back in my day capacitors had a positive and a negative leg...

Time to break out the soldering iron!
 
Well i spent a couple of hours on sunday soldering up the board. Only accidentally switching the 220K resistors with the 22k resistors, fortunately there were three of one type and 2 of the other, so i ran out and noticed. Unsoldering things suck.

Anyway the boards built, but I'm waiting on a set of 5 pin connectors(ex china) and I've got to find my self a good cordless drill to pinch the battery/charger, and an audio jack with a switch.

Will post some pictures later
 
Well this escalated quickly hahaha, good reading though all the Q&A's sound like our goverment asking stupid questions get a stupid answer!!!! :D
 
1447890128_imag1151.jpg

1447890149_imag1150.jpg


That the board all soldered up.

Thats the power supply before picture.... thinking of using the handle of the drill as part of the detectors handle. will need an arm cuff though.
 
Hey mate, I managed to find quite a few handles in old hammer drill boxes as no one really keeps them. If you have any or know someone with many drills, chances are you will have a nice soft handle. You may even b able to get just the handle at bunnings to save ripping up your drill

Board is looking nice n neat! Great work mate :) :)
 
Was going to use the drill casing to mount the board and hold the battery, good thing is that the battery slides out and plugs into an external charger.

I bought the drill to use where it was a pain to run the extension cord, but it was never strong enough to do what i asked it to, So it just sits in the cupboard, was actually really lucky that the battery is still ok, I haven't charged it up in over a year, and it was flat when i got it out last night.

Held charge over night, so will charge it up again tonight.

So the plan is to strip the guts out of the drill, mount the control board inside, while maintaining the battery holder and integrate the drill body and handle into the design of the shaft. using the drill handle as the hand hold below the cuff. Might be hard to envisage, but I think its going to work. Biggest issue is getting the shaft to mount strongly enough to the drill body.
 
Sounds like it'll work mate, if the batt isn't charging do you know how to replace cells to fix that?

I see the issue with drill and shaft, I'd probably use my soldering iron as a plastic cutter and leave myself enough plastic to attach some small alloy 'frame' to or at least mabey screw or rivet for the time being.

You can melt hdpe plastic ( wheely bin for instance ) and cut n bend up some mounting point.. even alloy?
 
I'm thinking some PVC pipe that i've got thats about the same diameter as the drill, and lots of tech screws...will give it a crack this weekend and have a rumage through the junk tonight to see what i can use.

also Its very likely to end up mission brown. Just a heads up to the fashion police.
 

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