Let's made a PC-base metal detector with usb interface !!!

[Midas]

Ok, here is the myth of the "noisy" Al-strip coil:

Where does the coil sounding comes from?

I have built an another TX coil with loudspeaker cable this time. Bound the coil bundle very very tight so it can't resonate anymore. Connected it to the detector controller: the beast is still sounding .
Hey, my active PC loadspeaker system has been picking up the TX signal. After switching off the device, the sounding of the TX coil has been disappeared.

Connected back the Al-strip coil: the beast is still sound.
Ok, the Al-strip TX coil has a big styrofoam membrane (coil former), which resonates and is giving some sound (forming a small loudspeaker). But it's not much loud and acceptible. One can drive the TX coil with high frequency and its becoming very quiet then.

Where does the increased noise comes from?

Well, I have forgotton, that I didn't change the pre-amp gains to the same value. One channel had 5 times more gain.
Sure, the coil becomes more noisy on the channel with more gain.

Funny experience & results:
- Al-strip coil doesn't cause more noise due to electromotive force (at least not measureable). I was on the wrong way.
- Al-strip coil is generating more eddy currents and is causing some sounding therefore. The sounding was amplified by the coil former (styrofoam).

Ok, I did re-invent the loudspeaker system.

Cheers,
Aziz

So tell me Aziz while you were listening for coil noise did you start to feel any reduction in depression, migraines or auditory hallucinations ?

http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation

Midas

[Aziz]

*LOL*

Mr. Murphy's Law has faked my results!
"Anything that can go wrong will go wrong".

More later.
Aziz

Ok, here is the myth of the "noisy" Al-strip coil:

Where does the coil sounding comes from?

I have built an another TX coil with loudspeaker cable this time. Bound the coil bundle very very tight so it can't resonate anymore. Connected it to the detector controller: the beast is still sounding .
Hey, my active PC loadspeaker system has been picking up the TX signal. After switching off the device, the sounding of the TX coil has been disappeared.

Connected back the Al-strip coil: the beast is still sound.
Ok, the Al-strip TX coil has a big styrofoam membrane (coil former), which resonates and is giving some sound (forming a small loudspeaker). But it's not much loud and acceptible. One can drive the TX coil with high frequency and its becoming very quiet then.

Where does the increased noise comes from?

Well, I have forgotton, that I didn't change the pre-amp gains to the same value. One channel had 5 times more gain.
Sure, the coil becomes more noisy on the channel with more gain.

Funny experience & results:
- Al-strip coil doesn't cause more noise due to electromotive force (at least not measureable). I was on the wrong way.
- Al-strip coil is generating more eddy currents and is causing some sounding therefore. The sounding was amplified by the coil former (styrofoam).

Ok, I did re-invent the loudspeaker system.

Cheers,
Aziz

[Aziz]

Murphy's Law

*LOL*

Mr. Murphy's Law has faked my results!
"Anything that can go wrong will go wrong".

More later.
Aziz

[Aziz]

TX Coil Sounding Issue

Hi all,

you remember my Al-strip sounding coil? (Bzzzzzzzzzzzzz, it will drive you mad ).
And the electromotive force causing this sounding?

I have observed more noise. This is due to inherent stochastic mechanical (vibration) process of the AL foil filaments in the TX wire strip (not tightly bound and there is air between the foil filaments, particularly due to bending).

If you drive your TX coil with more current (more current -> more electromotive force -> more mechanical vibration), the TX wire and TX coil bundle have to be fixed very very very tight! Otherwise, it will cause more noise in the IB coil systems.

Now trying a cheap loudspeaker cable to prove it (expecting less noise of course).

Aziz

[Aziz]

PCB V1.3

Damn it! The labels Rout and Lout must be exchanged of course!
Ok, I have to look better next time.
Aziz

[Aziz]

*** Latest PCB-Layout, Schematics and Eagle Files ***

Hi all,

this is a hot candidate for PCB fabrication. I hope, I didn't make any bugs.
Same open-source restrictions made before apply here further of course.
Be honest. Be fair. And don't be (much) greedy!

Cheers,
Aziz

PS: This is detector controller version 1.3

[Aziz]

You could also use 2 capacitors "back to back". The voltage needs to be double.

Tinkerer

Yep, also a good idea.

But the standard Al-electrolytic capacitor in the circuit isn't critical at all. We can take the cheap and small Al-electrolytic capacitors, which will fit well into the new PCB.
At high frequency EMI induction, there won't be high reverse voltage (only offset error voltage of the op-amp). Other detectors EMI induction isn't critical therefore. It's only critical below the cut-off frequency fc of the high-pass filter.

Well, in the 12 V battery configuration (+/-6 V supply), I measure with very low frequency EMI (10 .. 100 Hz) only +/-0.2V at the capacitor (that's max. peak only). This isn't critical.

At normal operation, the low frequency isn't there as the detector would operate at much higher frequency.

Cheers,
Aziz

PS: The PCB layout is optimized further... wait until I publish the final version. Be patient please.

[Tinkerer]

BTW,

you can test an AL-electrolytic capacitor for the circuit above before you solder it to the circuit. Just use a battery cell (1.2 or 1.5 V) and connect the capacitor in reverse polarity. Measure the current flow. It must not exceed a few µA.

I have put a test capacitor yesterday and the current flow is less than 1 µA. And it's still alive.

(If this is my last response, you know that the small electrolytic capacitor has been exploded at killed me. )

Cheers,
Aziz

You could also use 2 capacitors "back to back". The voltage needs to be double.

Tinkerer

[Aziz]

Preliminary PCB Layout of Version 1.3

And this is the current PCB layout. But it is not finished and needs some changes as well.
Aziz

[Aziz]

BTW,

you can test an AL-electrolytic capacitor for the circuit above before you solder it to the circuit. Just use a battery cell (1.2 or 1.5 V) and connect the capacitor in reverse polarity. Measure the current flow. It must not exceed a few µA.

I have put a test capacitor yesterday and the current flow is less than 1 µA. And it's still alive.

(If this is my last response, you know that the small electrolytic capacitor has been exploded at killed me. )

Cheers,
Aziz

[Aziz]

AL-Strip TX Coil And The New AC Pre-Amplifier

Hi all,

I have connected the new self-made AL-strip TX coil to the detector controller yesterday. Oh boys!, it's "singing" due to high current flows. *LOL*
This is due to the loose coupled stripes in the folded AL-strip itself, which are swinging due to electromotive force even the whole TX coil bundle is fixed tightly.

And it produces high self-made eddy currents so I can't reduce the residual voltage in the induction balanced receive coil completely. That's the reason, why an input compensation is important and is therefore required for such coils or situations.

Apart from the disadvantages, it works pretty good and is achieving great performance due to it's size. I didn't try the "Top Hat Anti-Interference Coil" so far but I'm working on it.

-------------

Below is the standard non-inverting AC amplifier configuration, which works very well. Notice, that the DC blocking capacitor C23 should be a bipolar AL-electrolytic capacitor, which should withstand reverse voltages of up to at least +/-1.5V. Well, the poor man's solution is to use the standard AL-electrolytic capacitor, which works in the high voltage clampling circuit only (reverse voltage limitted to +/-0.7V and the AL-electrolytic caps can do with +/-1.5V reverse voltage).

Attention! Don't use Tantal types, they don't withstand even small amount of reverse voltage. Well, the PCB space is limitted and I have to put the small AL-electrolytic caps.

But we should know, that the reverse voltage causes a faster aging of the AL-electrolytic capacitor. We could also use the high capacitance MKS-2 caps (1 .. 2.2 µF) but we have to increase the RG2 to achieve the low cut-off frequency fc of the high-pass filter. In the example below fc is set to 72 Hz.

I'll try a loudspeaker coil wire for the next TX coil.

Cheers for now,
Aziz

[Aziz]

For a reason:

http://www.kunstnet.de/thema/18718-terpentin-und-andere-schdlinge

Careful with organic solvents!

Thanks WM6,

but I know this stuff from art painting (oil).

BTW, orange oil (limonene) is a good solvent too. And it's smelling fresh.

https://en.wikipedia.org/wiki/Orange_oil

https://en.wikipedia.org/wiki/Limonene

But expensive.

Cheers,
Aziz

[WM6]

Unfortunately, Europe has banned the turpentine from the market.

For a reason:

http://www.kunstnet.de/thema/18718-terpentin-und-andere-schdlinge

Careful with organic solvents!

[wallker]

Hi Marko,

yep, that's for sure a good styrofoam solvent.
It's only a question of drying time.

Best solvents for styrofoam:
- balsam turpentine
- nitro thinner

Note, some substitues for turpentine are bad solvents. Even acetone doesn't solve styrofoam good (for painting useless, until you wait for a long time for dissolving duration). Unfortunately, Europe has banned the turpentine from the market.

Cheers,
Aziz

Thanks for the tip Aziz, i have some turpentine and i will try it
Regards Marko.

[Aziz]

Hi all,

ok, the 10 Ohm input resistor to the high voltage clamping 1N4148 diodes seem to work without damaging the receiver amplifier op-amp (yet). I have put the RX coil to the maximum induction coupling and there is no problem at all. Note, that the clamping diodes must withstand the external EMI (other detectors, etc.). In induction balanced coil configuration, the own generated high TX voltage won't be seen.

To be save for operation, just increase the input resistor to a higher value (e.g. 47 - 100 Ohm, which causes more noise of course).

What's next?
Input compensation, new circuit and PCB with incorporated non-inverting AC pre-amplifier.

Cheers,
Aziz