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

[Aziz]

Hi Simonbaker,

What are the principles you are using for detection? Amplitude and/or phase, etc?

-SB

Amplitude and Phase (Frequency shifts)

Probably on three frequencies around the resonance frequency:
Fr-BW/2, Fr, Fr+BW/2,
where Fr = resonance frequency and BW = bandwidth (few 100 Hz).

But not decided yet. I will see this, if I made a field test.

BTW, I can increase the coil voltage up to +/-3V (6Vpp) until it gets clipped by the sound card. Using differential mode, I can process up to +/- 6V (12 Vpp) coil voltage. It is interesting, that very small changes in the amplitude and phase shift are sufficient enough to make this detector with mono coil possible. The current hardware consumes only 4.25 mA at 9V. So a 9V battery block can last for a long period.

The new coil is finished but not shielded yet. I have taken 5 strands of magnet wire, each 0.3 mm diameter. So the finished coil has ca. 20,6 cm mean diameter, 12 mm bundle thickness, 2x37 turns (74 turns total), 2.2 mH inductivity and a resistance of 1.8 Ohm total. It has a center tap and is operated in fully differential mode (differential stimulation, differential sensing). The difference is made digitally in the software of course.

Aziz

[simonbaker]

Hi all,

Regarding the VLF 2.0:

I have solved some of the generic problems listed before. These are:
- sensitivity to ferrite and magnetite:
These materials cause a rising coil voltage. The metals a falling coil voltage (eddy current losses). Only the heavy mineralized ground may affect the operation, if the coil height varies during coil sweep.

- earth magnetic field and magnets:
The inherent high pass filter does not pass the slow signal change to the sound card. And the very sharp digital filter does not see the slow signal change at all (coil sweep is very slow compared to the resonance frequency). I have tested a strong ceramic magnet. Interestingly, it does not affect the detector significantly.

I have wired my own Litz wire to make a high Q-factor coil (low resistance, high inductance). For better target response, the stimulation impedance must be high and the Q-factor of the resonant circuit high too. So the losses of the resonant system must be minimized. Only tiny stimulation current is necessary to achieve a good sensitivity. So the stimulation can be driven directly from the sound card output lines.

The principle is a mixture of off-resonance and VLF system. The coil voltage(s) should lie between 80% .. 90% of the input dynamic range of the sound card without clipping the input voltage. The internal amplifier of the sound card should not be used and should be attenuated as much as possible to increase the coil voltage.

I will finish the new coil soon and make some more optimisations. After making all this, the final schematics + SPICE simulation files will be published here.

Stay tuned.

Aziz

What are the principles you are using for detection? Amplitude and/or phase, etc?

-SB

[Aziz]

Hi all,

Regarding the VLF 2.0:

I have solved some of the generic problems listed before. These are:
- sensitivity to ferrite and magnetite:
These materials cause a rising coil voltage. The metals a falling coil voltage (eddy current losses). Only the heavy mineralized ground may affect the operation, if the coil height varies during coil sweep.

- earth magnetic field and magnets:
The inherent high pass filter does not pass the slow signal change to the sound card. And the very sharp digital filter does not see the slow signal change at all (coil sweep is very slow compared to the resonance frequency). I have tested a strong ceramic magnet. Interestingly, it does not affect the detector significantly.

I have wired my own Litz wire to make a high Q-factor coil (low resistance, high inductance). For better target response, the stimulation impedance must be high and the Q-factor of the resonant circuit high too. So the losses of the resonant system must be minimized. Only tiny stimulation current is necessary to achieve a good sensitivity. So the stimulation can be driven directly from the sound card output lines.

The principle is a mixture of off-resonance and VLF system. The coil voltage(s) should lie between 80% .. 90% of the input dynamic range of the sound card without clipping the input voltage. The internal amplifier of the sound card should not be used and should be attenuated as much as possible to increase the coil voltage.

I will finish the new coil soon and make some more optimisations. After making all this, the final schematics + SPICE simulation files will be published here.

Stay tuned.

Aziz

[simonbaker]

Hi all,

is there any interest of a new VLF version? This version is much more robust in coil design as it uses a true mono coil in single ended or differential ended design. So there is no problem with induction balance DD coil anymore (I still hate them ). No more adjustments necessary. Very simple design.

A small j-fet buffer amplifier module is necessary to match the impedance between the coil voltage and sound card input. The circuit is very simple and trivial and its power consumption is max. 10mA at 12V. It can be operated with a small 9V battery block.


Principle of operation:
A LC resonant circuit (coil and parallel resonant capacitor) is stimulated through the sound output lines. The stimulation impedance is quite high and any target influenced effects affect the coil voltage and its frequency (phase) slightly. The coil voltage is buffered and fed into the sound card. The detector software measures the small changes. The sensitivity is quite amazing for its simplicity.

Negative effects:
- earth magnetic field sensitive (tilting the coil will give false signals)
- sensitive the magnetic minerals (ferrite, magnetite, ..)
- ground may give false signals due to mineralisation change (changing the relative permeability of the coil, conductivity changes of ground, ...)
- mechanical stability of the coil is critical (may not change its geometry)

It is possible to make a discrimination.

I did not test the new version in the field yet and will do this as soon as the weather allows (we have snow here). I also have to build a new coil for this version.

If there is some interest, I will publish more details. Just need more time.

Aziz

PS: PI project is still freezed until I have the new parts.

I have always been interested in the possibilities of single coil VLF designs. The challenge is picking small signals out from the large TX signal; finding some way to "null" that TX signal electronically. I think it is a worthwhile design to pursue, to see how good you can make one.

Regards,

-SB

[Aziz]

VLF 2.0 (reloaded)

Hi all,

is there any interest of a new VLF version? This version is much more robust in coil design as it uses a true mono coil in single ended or differential ended design. So there is no problem with induction balance DD coil anymore (I still hate them ). No more adjustments necessary. Very simple design.

A small j-fet buffer amplifier module is necessary to match the impedance between the coil voltage and sound card input. The circuit is very simple and trivial and its power consumption is max. 10mA at 12V. It can be operated with a small 9V battery block.


Principle of operation:
A LC resonant circuit (coil and parallel resonant capacitor) is stimulated through the sound output lines. The stimulation impedance is quite high and any target influenced effects affect the coil voltage and its frequency (phase) slightly. The coil voltage is buffered and fed into the sound card. The detector software measures the small changes. The sensitivity is quite amazing for its simplicity.

Negative effects:
- earth magnetic field sensitive (tilting the coil will give false signals)
- sensitive the magnetic minerals (ferrite, magnetite, ..)
- ground may give false signals due to mineralisation change (changing the relative permeability of the coil, conductivity changes of ground, ...)
- mechanical stability of the coil is critical (may not change its geometry)

It is possible to make a discrimination.

I did not test the new version in the field yet and will do this as soon as the weather allows (we have snow here). I also have to build a new coil for this version.

If there is some interest, I will publish more details. Just need more time.

Aziz

PS: PI project is still freezed until I have the new parts.

[WM6]

The advantages will lead to a high performance P.I. detector/instrument. And the disadvantages can be kept minimal. I would not invest so much in such an instrument, if I were not convinced enough. So the development will still continue until I have a running prototype. Due to the limitted budget, its going slowly at the moment. But it doesn't matter much - I am still young and highly motivated.

Aziz

Ok, nice to read this, but ,please, keep your outer control box as simple as possible - mean suitable for the manufacture of our poor hobby labs.

[Aziz]

I am sorry Aziz you start fighting with Bill Monopolist. No hope to win. Are you use electric heating?

I find portable unpractical on some way. Did you changed your mind about Win-Mobile MD project?

Hi WM6,

no my PC is running too long and we had a price increase last year(electricity). I will pay 45,- EUR monthly now. I am also looking TV on my PC, so it is running anyway longer.

Regarding portable unpractical:
Sure, its has some disadvantages:
- extra weight, extra battery, higher battery capacity required
- backpack carrying

But there are some delicate advantages:
- flexibility, state of the art processing methods,
- user programable, unlimitted search modes, extendable,
- and more

The advantages will lead to a high performance P.I. detector/instrument. And the disadvantages can be kept minimal. I would not invest so much in such an instrument, if I were not convinced enough. So the development will still continue until I have a running prototype. Due to the limitted budget, its going slowly at the moment. But it doesn't matter much - I am still young and highly motivated.

Aziz

[WM6]

I am sorry Aziz you start fighting with Bill Monopolist. No hope to win. Are you use electric heating?

I find portable unpractical on some way. Did you changed your mind about Win-Mobile MD project?

[Aziz]

Project Delay

Hello friends,

sorry again, but the current project will delay further. Reason:
- I've got the bill for the electricity today (no more money left for the new parts)
- got a bill from the perverted and degenerated german justice (I will not pay this unlawful demand of course but I might be arrested for a while due to criminal german government).


Project related:

The new improved LDO design is finished and I will wire up this in the coming days.

I am also working on the efficient, inductor-free step-down converter. The first throw is running (fixed frequency, PWM control with synchronisation).

Fortunately, the schematic design cost me nothing. So I will work on this in the mean time.
Aziz

[Aziz]

Hi WM6,

Hi Aziz

Nice to read about progress.

You do need separate power supply (battery) for laptop and MD stage or not?

Best wishes to your project.

it depends on the situation. Laptop has its own battery and the PI controller would need an extra battery. But if you have a big high voltage battery pack (>14V), then both devices could be driven with single battery. The battery power conversion must be realized efficient then. A linear regulator is not efficient. A SMPS with inductor is causing heavy noises. Although, the laptop has SMPS's built-in and will disturb the operation anyway.

A buck/boost SMPS could probably be used for this application. It must be synchronized to the timings at least.

I have to try it out, whether it is ok or not.

Just working on the efficient laptop power supply at the moment. It should be able to run even with solar panels and charge the laptop battery.

Aziz

[WM6]

Hi Aziz

Nice to read about progress.

You do need separate power supply (battery) for laptop and MD stage or not?

Best wishes to your project.

[Aziz]

Working on LDO Improvement

Hi all,

I am just working on LDO improvement. The new version will be:
- very stable (better compensated, immune to oscillations)
- automatic soft start (inherent feature)
- fast regulating
- no need for rail-to-rail op-amp (a common one will work too)
- will allow higher input voltages (up to 40V, depending on some transistors and mosfet)
- high side p-ch mosfet gate driving will be protected (will not exceed max. gate voltage)
- will allow still high current loads

I have to test the circuit extensively until I am going to publish a schematic here. I just want to say: "forget the previously published LDO circuit".

Aziz

[Aziz]

Power Saving Techniques

Hi all,

I am thinking of investing some time in power saving techniques. As you have seen some power consumption figures, this topic seems to be important part too. The Netbook will consume 10-15 W. Of course, the software will not fully occupy the CPU and I will achieve below the 15W peak power (maybe 10-12 W including the usb sound card).

The PI controller will consume much less power of course depending on the user program code (adjustable power consumption).

I did look for a PWM controller chip spice model but I didn't find one yet. Particularly the TL494/TL594 would be fine to see its operating behaviour. Unfortunately, there is no spice model.

So I have to design a discrete, inductor-free, high power step-down converter. I am also planning to support a wide input voltage range for the PI controller (10 - 24V). It is a non-trivial challenge to achieve this without loosing much battery power. The power supply part will be changed heavily to meet this demand.

Maybe a small MCU (ATtiny chip) could do this task efficiently with less part count.

Aziz

[slimabidi]

Hi Aziz,

Great work there. It looks fantastic!

[WM6]

Great Work Aziz.