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Simple pulse induction metal detector

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radhoo

Sat Jan 12 2013, 04:36PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

teravolt wrote ...

I've seen pictures of metal detectors wiht big loops over a meter for deeper stuff like meteorites. Do you think your circuit could do that

Increasing the coil diameter will increase the detection range, but put in practice this won't help you, as you are loosing on the pin-pointing precision side. Digging cubic meters of earth just because your detector is so sensitive, is not much fun.

@Reaching: At what distance does it detect a 1 EURO coin?

Reaching

Sat Jan 12 2013, 04:43PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

a 1euro coin is detected at a distance of around 10cm in free air.. Maybe theres a way to get this thing even more accurate

Inducktion

Sat Jan 12 2013, 05:01PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Wouldn't increasing the number of turns reduce the current required, kinda like how relays work?

If you get enough turns I'd think you could get the current draw down pretty low. This is pretty cool though, I may want to try and build one myself too!

Reaching

Sat Jan 12 2013, 07:30PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

The current will be lower with more turns, yes. But with lower current, the eddy currents induced in metal objects are lower and so on the back em pulse. but maybe a coil with more turns is more sensitive too..

But, another thing.

I looked at the web today and found a nice 2 coil system which is really interesting.. i just build a quick test setup today with 2 coils and got nice results, just with a frequency generator and an oscilloscope added to the receive coil.

I added a picture^^ the red coil is the transmit coil which is sending out an ac signal, in my case this was a squarewave signal with 50% duty at 6,4khz and 12v amplitude. the 2blue coils in the middle are actually just one coil, but overlapped in the middle to form a 8 . so one part of the coil is getting positive in the ac while the other side is getting more negative. so the ac of the transmit coil is completely eliminated^^ if you bring some metal close to the coils, you will get an increase in voltage or you will lower it. another nice thing to improve efficiency is to form a parralel resonant circuit with the recieve coil and tune it to resonance with the frequency of the transmit coil.. you will get a nice sinusodial waveform out of the receive coil which will significantly increasy in amplitude by any metal object..

so, maybe thats the way to go^^

Inducktion

Sun Jan 13 2013, 04:05AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Another idea; would this also potentially work as a sort of "speed" detector, by comparing phase, and then you could calibrate it to output a certain voltage = mph.

Sulaiman

Sun Jan 13 2013, 08:55AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

By slightly adjusting the area of the left or right half of the figure-of-eight coil
you should be able to almost completely null-out the coupling
between the transmit and receive coils.

A simpler alternative is

One coil is transmit, the other receive.
By adjusting the coil overlap a null is achieved.
The two coils do not need to be the same size.
Shielding around the coil windings will reduce capacitive sensitivity to soil etc.

P.S. in the '70s my work colleagues designed a metal detector to find lost (test) torpedoes at the bottom of Scottish sea lochs along similar principles,
I expect technology has improved since then.

radhoo

Sun Jan 13 2013, 10:52AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

4HV is a great place for high voltage related various results, and our forum is hosting some great projects already.

In a similar matter, there are other online resources for other fields of science. When it comes to metal detectors, geotech is one of best:

In regards to Sulaiman's note on ever-changing technology, today detectors come in various flavors. DIY enthusiasts seem to oscillate between two topologies: discriminating (where the detector can indicate the type of detected metal: ferrous or not) and non-discriminating. Top constructions in the first category include the VLF detectors, while the top for non-discriminating is the PI (pulse induction) detector.

Reaching built a PI detector, which is a good approach since it offers great range, simple construction, but has some drawbacks like poor results on non-ferrous metals, especially on gold or silver.

I've seen a great PI design, that works very good: with a 20cm diameter coil (30 turns, 0.3mm CuEm) will detect a 1 EUR coin from 25 to 30 cm according to its fine tunning. The circuit, also posted on geotech, can be seen below:

1358074324 1938 FT148849 Polish Detector 1

1358074324 1938 FT148849 Polish Detector 2

1358074324 1938 FT148849 Polish Detector 3

Ash Small

Sun Jan 13 2013, 12:10PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

radhoo wrote ...

In RO, it's illegal to build, hold or use a metal detector

, unless one gets a permit, but it's impossible to get a permit for a home-built device.

How do they define 'metal detector'?....Your wireless power transmitters use virtually the same technology, and Theramins also work along similar lines.

Reaching

Sun Jan 13 2013, 04:37PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

mhh, wireless power transmitters and a theremin are usually used inside^^ a "metal detector" in this case is a battery powered circuit for outdoor use to find metals such as coins jewelry old military stuff etc..

there are many countrys such as romania or scotland where ou cant simply build a detector yourself and go for gold and this stuff.. you can use some metal detectors as a theremin though^^

radhoo

Sun Jan 13 2013, 07:43PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

@Ash Small, I searched for the definition, but couldn't find it so far. The law itself has a few issues like: it forbids you to have a detector without license, but then it writes that in order to get a license, you need to go to a police station with a few documents but also with the invoice from your purchased detector... obviously this is crazy but the last thing you want is to be at the mercy of some police man that interprets it in its own way. There is also jail penalty in some cases.

@Reaching: this problem is only for DIY devices. It appears it is not so difficult to get a license for a commercial detector, like this guy did:

In some other countries you are allowed to use a detector, but you have to bring in any findings to a museum.

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