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

[Aziz]

I forgot the main reason for two stages and Boxcar integrator:
Saving battery power!

This is becaming an important issue to laptop MD and therefore reducing the computation power and saves lots of miliamperes!

[simonbaker]

Thanks Aziz.

I think I would need to see your actual equations or algorithm to fully understand, maybe someday .

I will follow your progress. Cheers.

-SB

[Aziz]

Hi guys,

I have continued this project again for preparing some new ideas and breaking new detection records..
But lots of software changes to implement..
I will let you know about the results.

Aziz

[Aziz]

Very good news:
1.
I experimented with simple line-out impedances (200 ohms on my sound-card measured) till now. I have found the right software adjustment of my sound-card to change this impedance to ear-phone output. The impedance now falls down to ~20 Ohms!!! I have now without an external amplifier more power (current) for the transmit coil.

2.
I will put both left and right input channels together to acquire same signal on the rx coil. The signal-to-noise ratio will be of 1.41 times better, when I take the average of the left and right channel ( signal = 0.5*(left+right) )

So, I will make a new measurement experiment on a very simple laptop MD architecture without an external circuitry (only search coil connected to laptop).

Now, I can break new records.

Aziz

[Aziz]

More good news:

3.
Taking a very clever feed circuit, the coil current can be increased despite of the low current feed from the sound-card. This can be achieved by getting the LC circuit totaly in resonant so the sound-card delivers power only for the coil/capacitor/cable losses. My sound-card delivers only 4 Vpp and with a clever feed, the coil voltage can go upto 40-50 Vpp!

This is the most good news I can tell. To design the feed circuit proper, I have to measure some coil parasitic parameters (Rpar, Cpar).


This exploration omits the need for power amplifier for the TX coil.

Aziz

[Sean_Goddard]

I have been SAYING this was the way to go for the last 15 years and no one listened..Why do I bother??

WELL DONE AZIZ, you are truly a genius BTW Would a Pentium 3 1GHz laptop do you? I have one but there are plenty on Ebay cheap if you look.

I was planning to give this machine to my sister, but you can have it if it will do what you require. Needs a new CD rom drive. The one in there is KAPUT.

Let me know here or PM me with your address. BTW you will have to pay the shipping, but that's the ONLY thing you pay for. Laptop is free.

Sound card in it is not good though, so it may not be any use. What about a USB sound card??

[Aziz]

Hi Sean,

thank you for your offer. I will have soon a brand new one sponsered by my parents with high fidelity sound-card built-in with 4-6 hours operating time.
1 GHz will be enough for the laptop MD. Even lower to save battery time.

Enclosed the AC signal analysis (frequency response on the coil). There is an excellent frequency response to minimize the 50/60 Hz, 100/120 Hz line injected ripple noise. Also the 1/f noise will be removed excellent.

Now it makes sense, to develop the laptop MD further.

Aziz

[simonbaker]

More good news:

3.
Taking a very clever feed circuit, the coil current can be increased despite of the low current feed from the sound-card. This can be achieved by getting the LC circuit totaly in resonant so the sound-card delivers power only for the coil/capacitor/cable losses. My sound-card delivers only 4 Vpp and with a clever feed, the coil voltage can go upto 40-50 Vpp!

This is the most good news I can tell. To design the feed circuit proper, I have to measure some coil parasitic parameters (Rpar, Cpar).


This exploration omits the need for power amplifier for the TX coil.

Aziz

Are you saying you found a way to increase the Q of your coil resonance with a clever feed? How is it different from a normal coil / feed? Are not all coils designed to resonate with as little power as possible?

Here is an experiment -- put coil on mineralized ground -- how does it affect the voltage (Q)?

I think your research is very interesting to help us design MDs. Thank you as always.

[Aziz]

Are you saying you found a way to increase the Q of your coil resonance with a clever feed? How is it different from a normal coil / feed? Are not all coils designed to resonate with as little power as possible?

Here is an experiment -- put coil on mineralized ground -- how does it affect the voltage (Q)?

I think your research is very interesting to help us design MDs. Thank you as always.

Yes, that's true. I have increased the Q of the resonant circuit much. This is a typical narrow band filter. And this is predestined to lock-in amplifier to react to phase and frequency changes.
It makes sense to use HF litz wire for the search coil to minimize the coil losses (higher Rpar).
I will make more simulations on multi-pole resonant filters.
At the end of the week, I will see the difference to my previous experiments.
It's getting very interesting.

Aziz

[Aziz]

Hi guys,

here are some interesting spice simulation results and the example feed circuits. It compares the normal series LC feed with the proposed one (I don't know how it is called). Both circuits take the same signal source (earphone output, 20 ohms impedance, ±2V = 4 Vpp) and the coils are exactly same with same parasitric characteristics. C1 of the first circuit is matched to have the same resonant frequency of 10.63 kHz. See the coil parameters on the attached picture.

The proposed circuit will load the signal source much lower than the first one despite of more voltage and current on the coil.
The current limiting resistor R1, R2 is necessary due to zero impedance of the resonant circuit.

Other combinations of the capacitor values could have better frequency response. If you like, then try to find a better one.

Enjoy,

Aziz

[simonbaker]

Enjoy is correct! This is very interesting.

I have been thinking about coil resonance, maximizing current, and saving energy for a while.

Here is a question/comment: Correct me if I'm wrong: I understand why the current limiting resistor in the series resonant circuit, because at resonance the impedance of the LC goes to zero. Theoretically we can use a very high limiting resistance and still have a large current in the coil because it exchanges current and energy with the capacitor (I think) without going through the resistor. If we make the current limiting resistor low, we just waste energy in the resistor, there is probably a good trade-off value we want to find.

But I don't think we need any limiting resistor with the parallel resonant circuit because the impedance of the LC goes to infinity - the current in the coil is only limited by the Q of the LC. I guess if the Q is very high, the current limiting resistor doesn't play much part anyway, but better to take it away completely, no?

Just some thoughts without calculations. I'll try to do some calculations and correct my thoughts if I understand it differently.

[simonbaker]

I am adding this new comment: my above comments probably may not apply to your circuit because you chose capacitor values that make it behave more like a "series resonant" circuit than a "parallel resonant" circuit.

So my new question is: suppose you simply substituted a parallel configuration for the series configuration without the extra feed capacitors in your very first diagram, and also remove the current limiting resistor. You would need to assume some Q for the LC circuit. In other words, what is the difference between a basic series LC configuration and a parallel LC configuration for use in a metal detector?

[Aziz]

Hi,

you all are welcome to simulate the feeder circuit. Attached file is the source code for LTspice. Remove the ".txt" extension and load into LTspice.

Enjoy,

Aziz

[Mrand]

Maybe I will give the new impulse in the discussion reminding the TX circuit Troy X5.
Mrand

[Aziz]

Hi,

a parallel LC feeder shown in the attachement below has similar characteristics with less components (two capacitors). So this can be seen as an improvement of the proposed circuit.
The capacitors C5/C6 and the inductor L3 defines the resonant frequency. Has anybody interest to write the complex equations for this?

Aziz