Arduino Nano VLF Metal Detector Project - Main Discussion

[Goaty]

Another minor "Oops" on P 135 of Parts List-

SENS external pot (1k) Connects to J5 (J3 is the coil)

AM/DISC switch Connects to J7 and J12 (Not J9 which is the Speaker/Headphones output)
2-pin plug Add J12 8

PS Hope I got this re-entered correctly as I had to re-type it after accidentally powering down the UPS with my foot and losing everything

[Qiaozhi]

Another minor "Oops" on P 135 of Parts List-

SENS external pot (1k) Connects to J5 (J3 is the coil)

AM/DISC switch Connects to J7 and J12 (Not J9 which is the Speaker/Headphones output)
2-pin plug Add J12 8

PS Hope I got this re-entered correctly as I had to re-type it after accidentally powering down the UPS with my foot and losing everything

Thanks. Errata updated.

[Olly]

Hi George,

The depth results you achieve with this design are particularly impressive (Especially with the Fisher F75 coil). This led me to wondering whether a forced TX signal has an inherent sensitivity advantage over an Oscillator based TX.

The coil inductance will change (however slightly) as it passes over a target, and with an oscillator based TX, this in turn would alter the TX frequency and possibly its phase as well (also very slightly). In turn, it is possible then, that the perceived phase shift between the TX and RX signal may be less than the case where the TX signal is forced and there is no alteration of the TX frequency and phase as the coil passes over a target.

Any thoughts?

[Carl-NC]

I doubt this will matter much. You are right that a target will slightly alter the TX frequency of a free-running oscillator but probably not enough to make a difference. And the driven coil has a similar issue; a target will still alter the inductance, but since the frequency is fixed it will instead distort the TX waveform. Again, probably not enough to make a difference.

[Qiaozhi]

Hi George,

The depth results you achieve with this design are particularly impressive (Especially with the Fisher F75 coil). This led me to wondering whether a forced TX signal has an inherent sensitivity advantage over an Oscillator based TX.

The coil inductance will change (however slightly) as it passes over a target, and with an oscillator based TX, this in turn would alter the TX frequency and possibly its phase as well (also very slightly). In turn, it is possible then, that the perceived phase shift between the TX and RX signal may be less than the case where the TX signal is forced and there is no alteration of the TX frequency and phase as the coil passes over a target.

Any thoughts?

As Carl said ...

[Olly]

Thanks guys, just a thought... ;-)

[BigDutch]

Hi

Just purchased (and awaiting) both the Arduino PI and VLF books from George.
On a less technical note (and apologies if this is detailed in the books) but have the enclosures (VLF display) and choice of shaft also been discussed?
I can imagine the PI being a bit easier (no display) with everything being done a panel and box site like Schaeffer.
I was thinking more of the VLF LCD display.

Apologies if this is all in the books but I am just getting a bit excited!

Best regards
Richard

[Qiaozhi]

Hi

Just purchased (and awaiting) both the Arduino PI and VLF books from George.
On a less technical note (and apologies if this is detailed in the books) but have the enclosures (VLF display) and choice of shaft also been discussed?
I can imagine the PI being a bit easier (no display) with everything being done a panel and box site like Schaeffer.
I was thinking more of the VLF LCD display.

Apologies if this is all in the books but I am just getting a bit excited!

Best regards
Richard

There is some brief information on pages 111/112 in the VLF book, but the mechanical construction is mainly left to your imagination.
Chapter 4 of the PI book has more detailed information on the enclosure.
The Voodoo project has Chapter 4 dedicated to the mechanical construction.

[Ronin0000]

Hi I love the project and I am currently sodering the circuit board. I was wondering if there were any LTS Spice simulation files so I could try my own experiments without having to physiclly build them or create the simulation from scratch.

Thanks, Ronin

[Qiaozhi]

Hi I love the project and I am currently sodering the circuit board. I was wondering if there were any LTS Spice simulation files so I could try my own experiments without having to physiclly build them or create the simulation from scratch.

Thanks, Ronin

I'm away from home at the moment, but can check for you at the end of the week.

[Qiaozhi]

Here are the simulations files, plus a spreadsheet to calculate the R-2R ladder values.

[Ronin0000]

I'm having trouble getting the voltage converters on my circuit board to work. I've built the converters exactly as shown in the diagram, but I'm unable to get the negative voltages to function properly. I've double-checked that all components are functioning, correctly placed, and are the right ones, but I still can't pinpoint the issue.

I tested the Arduino Nano's voltage output, which reads 4.72 volts—close enough to 5V. All the negative voltages seem non-functional, so I suspect the LT1054 chips may not be working. The 5V and 12V outputs are fine, but the negative voltages are not.

I verified the code is sending a sync pulse to the chips, and I've measured the input voltage (VCC) on both LT1054 chips, which shows 12V. However, the Vref is reading 1.7 volts, and the Vout shows a very small voltage that seems incorrect. None of the negative voltage test points register significant voltages.

Any suggestions or ideas would be greatly appreciated.

Thanks, Ronin

[Gunghouk]

Hi Ronin, I've found issues with the LT1054 similar to yours but on the Voodoo, ie no negative supplies. Try lifting pin 7 of the IC out of its socket and see if theres any change .I used that temporary solution for a while until I placed a 100pF cap between the collector of the sync transistor and pin 7, the same as on the Minipulse Plus, but you'll need to do it for both LT1054.
Just to point out that both circuit variants produce the -12v output in LTspice with 1kHz sync, though with the 100pf cap the output settles faster. Go figure.
Good luck

[Qiaozhi]

Since both -12V supplies are not working, then I suppose it's possible that U2 and U3 are faulty/fake.

However, I would try this first:
1) Remove U2 from its IC socket. You will need to leave U3 in circuit to generate VREF.
2) Check the SYNC pulse at the collector of Q1 (2N3904) is correct using an oscilloscope. (I note you've already checked that the Nano is generating the SYNC.)
3) If SYNC pulse looks OK, then try replacing U2 with an LT1054 from another supplier.

My suspicion is that Q1 is faulty.

[Ronin0000]

Here is an update:
I have recived new LT1054 chips from a diffrent supplier, but unfortuantly that did not solve the problem.
I have wired the circuit standalone on a breadboard to see if my pcb was messed up, but that also did not solve the problem and the results are the same.
All of the components have been replaced at least once, and the sync pulse appears to be working when I check it through a oscilloscope.

I have tried Gunghouk's suggestion of temporarily removing the sync pulse, but that also did not change anything.

I am a little bit stumped at the moment.

Any tips would be appreciated.

Thanks