Rookie question on which design to chose

[gjw]

Hello,

I'm new to metal detection, but familiar with embedded systems / MCUs. I read heaps of threads here but am still not sure - so I was hoping for some guidance from the metal detection gurus for a tinker project. What kind of detection circuit / approach do I best use for the following requirements:

- must safely recognise gold only / should not give positive for other metals if possible
- should recognise very small gold pieces
- distance of object max 10-12cm from coil required. So depth/range isn't an issue.

Ideal would be a design that does as much as possible in software/MCU. I did find many MCU based projects (Arduino, etc), but most of them focus on depth, as far as i can see.

Any pointers much appreciated - thanks much in advance!

Best regards,
gj

[Carl-NC]

- must safely recognise gold only / should not give positive for other metals if possible

Not possible no matter the design.

- should recognise very small gold pieces

Define "very small." 1/2g and up favors PI, smaller favors VLF. Unless the soil is harsh.

- distance of object max 10-12cm from coil required. So depth/range isn't an issue.

Unless the gold is "very small!"

[gjw]

Carl, thanks for your answer.

Recognise gold: OK, I'll settle for whatever gives best differentiation.

Small: >1mm. Is that possible?

Distance from object is a given in the apparatus (objects will pass in distance of 1-12cm from coil). So I guess VLF is the way to go...? Any proven/recommended designs?

Thanks+regards,
gj

[golfnut]

what area are you hoping to detect. Do you have basic electronic lab kit? S

[Waikiki_Sweep]

There is some way to get target information. You can find US patent US5506506.

One Russian electronic guru even checked it and make working detector: "Chanse".

You can make that machine following his instructions: http://fandy.hut2.ru/Chance.htm

It was attempt to recognize the metal of target using PI detector with monocoil.

But if balanced RX/TX coil is used it works much more reliable as you can see it in Minelab detectors
like Explorer, Sovereign, Excalibur, CTX3030 and others FSB/BBS.

Many regular VLF (not PI) detectors will give you target ID (or VDI) to let you know what you want to dig.
But VLF machines fails is mineralised grounds and salt water.

Multifrequency VLF machines like Fisher CZ working great in salt water and giving some target IDs.

[Carl-NC]

A 1mm diameter gold sphere is 0.01g as I figure it, which is less than 0.2 grains. You can rub this target directly on the coil of even the best detectors and most won't make a peep. I'll guess that for 12cm the minimum target size is ~4-5mm.

[Waikiki_Sweep]

Probably to detect 1mm you need PI detector with 10-20kHz frequency and some small unshielded coil using litz wire.
I have read on the forum lately that Eric Foster have created some industrial PI solution to detect smallest pieces of broken needles in woven if I get it right

[gjw]

All, thanks very much for your helpful comments!

This is intended to monitor the exit of a sluice box. Carl's input on minimum grain size is a good indicator for me - i'll probably need a different solution for this to make sense.

Will have a look at the Russian design & the magic needle detector, though

Thanks+regards,
gj

[Carl-NC]

If your sluice box is passing 1mm gold, you need to look for a new sluice box! Also, monitoring the exit material is "too late" unless you save it all, or just want to know if you are passing gold. And the material is probably moving too fast for most detector designs. Anyway, take a look at the Falcon gold probe which is designed to detect sub-grain gold, though you have to be right on it. It's IB and runs at 300kHz.

[green]

Is there a material such as lead or aluminum that by adjusting weight, area, or volume that would give a similar response to gold with a PI detector?

[Waikiki_Sweep]

Practically any metal gives same response on classic PI detectors.
Ferric targets may be a little different because of their influence on magnetic field but any colored metal with lower conductance like lead or aluminum will act like gold.
If you want see some difference between metals you need VLF detector with VDI or Target ID. Or use Minelab inventions FSB/BBS kind. But anyway you will see lead and aluminum in gold range.