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

[Tinkerer]

Gold test standard

Hi Tinkerer,



this is a nice yellow specimen. Such plain "nuggets" can be detected much easier. They have a bigger coil coupling area (target is a coil). Compact natural gold is quite difficult to detect. I have not optimized my configuration yet but investigating further. Some more centimeters are sure possible.

I need such a nugget for tests! I will go to the bank and ask for this type of yellow.

Aziz

The Alaska nugget is a more realistic testing target, but since there are no 2 nuggets the same, it could be better to use an universal standard for testing.

US and Euro coins are good standards. For gold, I can not think of a more precise standard than the 1 grammer depicted.

Tinkerer

[amtech2005]

Let's make a PC-based metal detector with usb interface !!!

Yes It is very amazing . Aziz saw and took advantage of the several qualities of a P.C. sound card..... 1. the low noise factor 2. the Frequency range of the input . 3. And most important "the input sensitivity" , being able to capture or see the smallest input signal level !! In my opinion , Aziz is not worried right now on how to make a super "High Gain" amplifier , but he is making a reciever with the lowest possible input signal detection of any metal detector designed or manufactured !!!! It reminds me of the 1970's when Home Stereo manufacturers were all in fierce competition to manufacture a F.M. tuner section with less than 1.8 u.v. "micro volt" sensitivity in stereo mode not mono . To make a Metal Detector great you have to have total discrimination at depths "equivalent" to a good detector running in the "all metal mode" . The search-coil and the reciever section are the "Heart of a metal detector". High power oscillator / transmitter circuits are not required for deep detection on VLF with discrimination . It is in the Reciever "RX Section" , and we see this with Aziz's work. I do not think Aziz's work is completed yet !!!..........This is my opinion , others please comment ..........Thank-you..............Eugene51

great work man

[Aziz]

Hi Tinkerer,

It is absolutely amazing that you can get such results with just the laptop and no external electronics.

Attached a 1gram fine gold bar. Detected at 25cm air test with the coil sitting on a pile of fired clay bricks.

Tinkerer

this is a nice yellow specimen. Such plain "nuggets" can be detected much easier. They have a bigger coil coupling area (target is a coil). Compact natural gold is quite difficult to detect. I have not optimized my configuration yet but investigating further. Some more centimeters are sure possible.

I need such a nugget for tests! I will go to the bank and ask for this type of yellow.

Aziz

[Tinkerer]

Amazing laptop MD

It is absolutely amazing that you can get such results with just the laptop and no external electronics.

Attached a 1gram fine gold bar. Detected at 25cm air test with the coil sitting on a pile of fired clay bricks.

Tinkerer

[Aziz]

Phase shift adjustment on a coil for discrimination:

It is quite difficult to build a perfect coil with desired phase lag and signal level of the RX signal referred to TX signal. The discrimination behaviour will change upon different coil balance position. Once built a coil and fixed this, it can not be changed anymore. You remind, the coil must be very tight (mechanical).

We can use a simple resistor divider network with a pot to balance the coil. To be able to do this, we have to drive the coil in differential mode (stereo-mode). The resistor divider has linear phase response and gives a good balancing possibility.

Once the coil is balanced well, it delivers a clear phase shifts between ferro-magnetic and not ferro-magnetic materials.

Aziz

[Aziz]

Gold Nugget Detection Depth:

The following gold nugget can be detected at ca. 20 cm distance (air-test). It is a quite compact gold nugget: ~10 mm length, ~7.5 mm width, ~4.5 mm thick, 3 gram from Yokun (Alaska).

Laptop detector without external amplifier. Driven in stereo-mode (differential feed). DD coil shown above.

Important behaviour found:
Even the lock-in magnitude output (signal output level) is not changed much or not, there can be significant lock-in phase change detected. Both magnitude and phase signals must be processed for a good detection and discrimination!

Aziz

[Aziz]

Does anybody knows, how the USB headphones are integrated in the operating system? I mean, look they like a typical sound-card (in the driver stage)? Will they be simulated as a sound-card?

I did not see such a device yet.

Aziz

[Aziz]

Hi Joe,

Aziz,

You have to use some sort of headphones.
Then why not use USB headphones?

Regards,


joe

Thank you very much. I know now what you mean. Yes, this could be an alternative to the audio detection signalling instead of USB sound-card.

I have obviously overslept this technology (USB headphones). I hope the USB headphones simulate a sound-card in the driver stage.

Aziz

[joecoin]

Aziz,

You have to use some sort of headphones.
Then why not use USB headphones?

Regards,


joe

[Aziz]

Hi Joe,

Aziz,

"In case of the earphone output is not delivering much current, one speaker channel (left or right) could be deattached and be taken out for the coil via 3.5 mm plug."

Why not just use the +5V from a USB port?

because I won't use any external electronics. We don't need it yet. If we should need one, we could use it from +12 V external power supply. We don't use the digital USB interface. We are just using sound-card interface (line/earphone output, line-in/mic-in input).

In case of more search depth, an external amplifier module must be plugged between the search coil and laptop. This would give much more performance later (already intented).

But I want to make things much easier as possible:
- no modifications to laptop
- no external electronics
- just a search coil and the state of the art detector software

And this is realizeable and will probably outperform existing VLF detectors and can reach to common PI performance.


Aziz

[joecoin]

Aziz,

"In case of the earphone output is not delivering much current, one speaker channel (left or right) could be deattached and be taken out for the coil via 3.5 mm plug."

Why not just use the +5V from a USB port?

[Aziz]

It might be possible to modify the Eee PC 901:

In case of the earphone output is not delivering much current, one speaker channel (left or right) could be deattached and be taken out for the coil via 3.5 mm plug. This could deliver much current for the transmit coil. The other speaker could then be used as an audio signal output.
In this case, we wouldn't need an extra USB sound card!

I will check this possible option. But any modifications on Eee PC 901 will expire the guarantee. Anyway, it is damn cheap and we can count on better versions in the coming years.

Aziz

[Aziz]

Again the Asus Eee PC 901 as a possible field testing reference device:

I have found the built-in CODEC chip today: ALC269
ftp://61.56.86.122/pc/audio/ALC269_DataSheet_1.1.pdf

According to the data sheet, the CODEC is capable of:
- full-duplex
- input ADC: 98 dB dynamic range, 24-Bit up to 96 kHz SR
- output DAC: 98 dB dynamic range, 24-Bit up to 192 kHz sampling rate (SR)
- stereo microphone input with programmable +10/+20/+30 dB boost
- higher output voltage (very good)
- built-in class D amplifier with 1 Ohm impedance delivering up to 0.6 A (!!!)

I hope, the microphone input has not much noise and we can amplify enough to use the whole dynamic range of ADC. It is also interesting whether the output (earphone) is using the class D amplifier (it would be very nice if the case is).

BTW, the Eee PC 901 has a +12 V DC input plug! We can extend the operating duration with an external 12 V accu pack!!! The built-in 6600 mAh accu can be operated up to 4.8 h (full speed). I will optimize my code to extend this up to 6 or more hours!

I will get an Eee PC 901 next week and will see the performance!


Aziz

[Aziz]

About Discrimination:

Despite of phase anomalies on DD coils, laptop detector gives a better and more accurate discrimination opportunities. To make same phase shift behaviour, one must do the right coil balance. I will investigate some balancing positions more in detail later (minimum RX response=exact balancing position, positive RX response=RX/TX wider from exact balancing position, negative RX response=RX/TX narrower from exact balancing position).
The detection signal will then be dependent either of measured voltage and phase on the coil. The phase shift for lock-in amplifier must be adjusted to give 0 degree for lock-in phase output.
Different metal types and shapes gives different voltage and phase changes. Iron (ferro-magnetic materials) gives the much more phase shifts.

Laptop VLF MD can discriminate!

Aziz