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

[Aziz]

Single/Dual-Frequency VLF Detector Controller (preliminary circuit)

Hi guys,

I am willing to make the following single/dual-frequency VLF detector controller circuit (see below). The mosfet-driver (MCP1407) should give a little bit more power compared to the line-out of the sound card. If this isn't enough, one can plug additional driver-stages at the output of the mosfet-driver of course.

I just have modified one of the previously published detector controller version. So it's looking quite similar to it. I have added a 8.5V LDO voltage regulator and the circuit is intended for low-power/low-voltage operation (9-10 V input).

If this circuit works well, I'll remove the H-bridge part of the previous detector controller and add some improvements to it.

Cheers,
Aziz
PS: Sorry for the small circuit picture. I have a low resolution display (1024x768 ) .

[tjmlena]

24b/196k

Hello Aziz,

do you think this module will help you to your project :

http://www.ebay.es/itm/CS8416-AK4393-5532-24BIT-192K-2496-DAC-Kit-/150637584659?pt=UK_AudioTVElectronics_HomeAudioHiFi_Amplifiers&hash=item2312b32113#ht_1044wt_1125

[Aziz]

I once done it with series inductances + an additional common inductance for tuning to a MW frequency, thus providing also some degree of impedance transform. Worked fine throughout MW band.

I'll be closely following your progress here.

Anyway, in case you missed it, I asked you about concentric coils idea here: http://www.geotech1.com/forums/showthread.php?t=14121

Yep, I've missed it. See the answer there.
Aziz

[Davor]

I once done it with series inductances + an additional common inductance for tuning to a MW frequency, thus providing also some degree of impedance transform. Worked fine throughout MW band.

I'll be closely following your progress here.

Anyway, in case you missed it, I asked you about concentric coils idea here: http://www.geotech1.com/forums/showthread.php?t=14121

[Aziz]

Hi guys,

I'm quite lazy. But I have a very interesting observation to tell you.
It is relating to single/dual-frequency VLF and lock-in amplifier.

If you tap the TX coil voltage (simple voltage divider to reduce the high voltage) and feed it to the sound card as a TX reference (one channel RX signal the other TX reference) and if you demodulate the reference clocks for the lock-in amp sin(w*t)/cos(w*t) or sin(w1*t)/cos(w1*t) and sin(w2*t)/cos(w2*t) out of the TX reference, you get a very very clean and quiet demodulated RX signal.

It eliminates a huge portion of TX "switching noise". I'm indeed very impressed about it.

BTW, I'm still experimenting with the passive detector controller. The passive detector controller experiments showing clearly, that:
- amplifying the RX signal is required (20 - 30 dB gain should be enough).
- more bang to the TX coil (more TX power) is required.
- using the TX reference for eliminating further noise is of huge benefit.

I have prepared a new circuit for the single/dual-frequency prototype version. It will be a simplified low-power circuit of course (I'm using a mosfet-driver to drive the TX coil).

I'm following the dual-frequency VLF idea further as I see a huge potential.

Cheers,
Aziz

PS: You can leave the mixer inductor in the earlier posting. It works with capacitive mixing as well.

[Aziz]

Ok guys,

I'll plug a small bi-polar driver stage to the output of the detector controller soon. Let's see, whether it gets stable in the dual-frequency transmitter. It will take some time due to getting fine weather and week-end time.

Cheers,
Aziz

[Aziz]

Aziz, how this complies with K.I.S.S.-principle?

*LOL*

Very good and demonstrating it well. The boot-strapping in H-bridge driver is more complex & critical so it makes sense to make it much more simple.

Back to the roots for the dual-frequency h-bridge driver. I just want more bang to the TX coil to see it's potential. I'm curious.


Cheers,
Aziz

[WM6]

I'll try (yes, I'm a "trial & error"-"engineer" )

Cheers,
Aziz

Aziz, how this complies with K.I.S.S.-principle?

[Aziz]

Bad News.

Hi all,

I have connected the passive dual-frequency transmitter to the latest detector controller in the h-bridge mode (both mosfet's connected). Unfortunatelly, the low frequency switching isn't stable and producing therefore noise (you can even hear it in the detector controller). The high frequency switching is stable. Also when I set the low frequency switching off the resonance frequency, it's getting more stable. In the chirp switching mode, one can exactly see, when it gets unstable.

Well, the topology of the mosfet-driver with the bootstrap-circuit is critical for dual-frequency transmitter. Fortunately, this is the only configuration, that makes switching unstable. Other coil configurations work well.

I'll try (yes, I'm a "trial & error"-"engineer" ) a different h-bridge topology to see, if the switching is more stable this time.

Cheers,
Aziz

[Aziz]

Versatile Passive Dual-Frequency Transmitter

Hi guys,

did you see the clever (inductive) mixing of the TX driving sources? It's quite versatile:
- driving with dual channel sources (each frequency on its own channel, total TX power is summed together)
- driving with mono channel source (both inputs connected to the same source, the mixer inductor eleminates (almost) itself)

The active booster module can be either a single channel (simple & low cost) or a dual channel (stereo, doubling the TX power). The passive transmitter module need no changes...


Aziz

PS: Do not forget, that the transmitter is also able to transmit a chirp waveform!!!!!

[Aziz]

BTW guys,

the universal and "world's best ground balance & discrimination" scheme (algorithm) works with any coil current wave forms and hence with dual-frequency VLF systems as well.
That's the reason, why it makes sense to develop a very simplified dual-frequency VLF detector controller.

Aziz

[Aziz]

Looks great -- can you show some kind of scope trace that shows the current in the search coil?

-SB

Hi SB,

why don't you play with the LTspice file? For your and others convenience, I have modelled the sound card driver and you can see all the details (see below LTspice file). BTW, I do measure (almost) the same node voltages in real life.

If you change the power source to the 12 battery source and replace the inputs by Drv1 and Drv2, you can push more current to the mixer. The mixer can also be digitally sourced (simplifies the design and improves power efficiency rather than using a linear power amplifier with sine wave source).

BTW, I'm toying with the idea to make a simple dual-frequency VLF detector controller. Because it makes sense. Very much sense.

Cheers,
Aziz

PS: I'm trying different dual-frequency topologies.....

PPS: My real RX coil hasn't 6 mH! I've forgotton to change this. Anyway.

[simonbaker]

see pic..

Looks great -- can you show some kind of scope trace that shows the current in the search coil?

-SB

[Aziz]

The Dual-Frequency VLF Transmitter

see pic..

[Aziz]

Passive Dual-Frequency VLF

*LOL*

The passive dual-frequency VLF detector is really working nice!
(simultaneous 10.5 kHz & 32.3 kHz, yup!, no battery required ).

I'm surprised about the performance of it despite the fact of lossy components (lossy inductor cores, inductors, capacitors) and very low transmit power (using line-out stereo channels, 1 V (rms), approx. 30 Ohms impedance).

I need a passive RX amplifier:
- either more RX coil windings or
- dual frequency resonant filter (passing & amplifying of the two TX frequencies).

The latter one is complex to tune to the TX frequencies (adding more complexity). I wonder, whether high Q inductors & capacitors would give more improvements.

It's easier to plug an amplifier instead and use the wide band RX coil. BTW, I should plug a small amplifier to the TX side as well. Interesting application and really worth to follow it due to it's simplicity.

Cheers,
Aziz