Voodoo Main Discussion

[Qiaozhi]

What I don't understand is why so many designers use positive voltage as ground for the analogic circuit, to me it looks like everything would be easier with negative...

Not all designs use positive ground for the analog circuit. Have a look at the Geotech Baracuda, and also Arduino Nano PI.
Geotech Baracuda - REV-A - Project Details
Arduino Nano Pulse Induction Metal Detector Project - Published March 2021

Note that those particular designs are using a PMOS device to switch the coil, whereas Hammerhead, Voodoo, Minipulse Plus, etc., are using an NMOS device, which means that the coil needs to be referenced to the positive rail.

[nixie]

yes, a very interesting piece of code
on the other hand supported by an essential forum
(stm32duinoforum.com) with Roger Clark, mrburnette and many many more.
A place where you can truly learn.

[MartinB]

Really? I've taken an interest in metaldetectors only recently so I may have noticed just because I wanted to better understand how they work. One day I would like to port it to arduino (stm32c103 bluepill) without changing the schematic, meaning I would just port the code you have kindly given to the bluepill and build a small costumized board. The bluepill costs only 2,50 Euros on ebay, 72mhz, adc 12bit with 3,3v and 0,6uS dual channel dma conversion! Resolution: 3,3V/4096! (check it out here: https://github.com/pingumacpenguin/STM32-O-Scope) Obviously this brings noticeables improvements, but I don't belive I would be able to project it by myself...
So now I'm trying to build Voodoo! What I don't understand is why so many designers use positive voltage as ground for the analogic circuit, to me it looks like everything would be easier with negative...

Hi Nixie
Strange that you mention the STM-O-Scope; I am just about start building one.

[nixie]

Really? I've taken an interest in metaldetectors only recently so I may have noticed just because I wanted to better understand how they work. One day I would like to port it to arduino (stm32c103 bluepill) without changing the schematic, meaning I would just port the code you have kindly given to the bluepill and build a small costumized board. The bluepill costs only 2,50 Euros on ebay, 72mhz, adc 12bit with 3,3v and 0,6uS dual channel dma conversion! Resolution: 3,3V/4096! (check it out here: https://github.com/pingumacpenguin/STM32-O-Scope) Obviously this brings noticeables improvements, but I don't belive I would be able to project it by myself...
So now I'm trying to build Voodoo! What I don't understand is why so many designers use positive voltage as ground for the analogic circuit, to me it looks like everything would be easier with negative...

[Qiaozhi]

Hello!
I finally got the book and reading through it I noticed that at page 29 figure 3-3 shows a 47uF c26 electrolytic cap when it has to sustain both a positive and negative voltage, which changes depending on the position of the potentiometer.
When the voltage is positive the cap is connected backwards! Which obviously isn't a good thing!
Seeing as the caps already present are sufficient for the two tensions (+5v and -5v) a 100nF c26 not electrolytic cap should fix the problem.
I hope this can be useful to you

regards nixie.

Did you know that the null offset adjustment is very similar to the Hammerhead design, except that Voodoo uses a two-stage preamp?
It's only taken 15 years for anyone to notice that issue.

[nixie]

Hello!
I finally got the book and reading through it I noticed that at page 29 figure 3-3 shows a 47uF c26 electrolytic cap when it has to sustain both a positive and negative voltage, which changes depending on the position of the potentiometer.
When the voltage is positive the cap is connected backwards! Which obviously isn't a good thing!
Seeing as the caps already present are sufficient for the two tensions (+5v and -5v) a 100nF c26 not electrolytic cap should fix the problem.
I hope this can be useful to you

regards nixie.

[MartinB]

My VooDoo is working.

For a coil I used an 8inch concentric I build for my HammerHead2.
This has TX & RX coils of around 400uH and has been nulled.
A 470nF film cap resonated the RX around 10kHz.

Great news waltr. Well done

[waltr]

My VooDoo is working.

For a coil I used an 8inch concentric I build for my HammerHead2.
This has TX & RX coils of around 400uH and has been nulled.
A 470nF film cap resonated the RX around 10kHz.

[dingbatca]

I used a 7805 to prove out the problem/fix. I then installed a brand name 78L05. I wish you the best of luck. It took me nearly two weeks to sort that one out!

[waltr]

Waltr. Not sure if it was the exact same problem. I was able to pull 78L05 and prove it was a counterfeit. A genuine 78L05 fixed me right up!

Ok, I thought you used a 7805 (1A version) to fix your issue.
I need to go over this again since no one else seems to have this problem.

Maybe buy some other 78L05s of a different make to try.

[dingbatca]

Waltr. Not sure if it was the exact same problem. I was able to pull 78L05 and prove it was a counterfeit. A genuine 78L05 fixed me right up!

[waltr]

The Voodoo Project does not use a ICL7660. It uses an LT1054.
The power supply voltage is too high for either an ICL7660 or ICL7660A.

Sorry, typed in the wrong part number. It has an LT1054 installed (had Nano Arduino project on my mind when typing).

[Qiaozhi]

Finally got around to working on the VooDoo.
Had all the passives & sockets soldered in and the board washed.
Without IC's except U8 (7660) and the PIC +/-5Vs came up.

The Voodoo Project does not use a ICL7660. It uses an LT1054.
The power supply voltage is too high for either an ICL7660 or ICL7660A.

[waltr]

Correction. TP15 was reading +4.96V. Now it's reading -0.8V. I swapped out the 78L05 (U7) thinking I was having a regulator issue. No luck, still reading -0.8v. Any ideas? Pics attached.

You could remove U7, and then connect an external +5V supply between TP15 and TP1. If your power supply has a current limit control, you should turn that down to prevent any magic smoke being released.

The 78L05 regulators are rated at 100mA, which is more than enough to power the Voodoo circuits. It would probably be worth testing one of the devices you have with a variable load to see how much current can be extracted before it drops out.

I got my +5V back!!!

Just wanted to give a quick update. 78L05 is now in a bread board and hooked back to the Voodoo via leads. I measured 22ma across the output (with all IC's installed). When I remove U9, or U10, it drops to 12~13ma, but I get my +5V rail back. So for fun I moved over to a known good 7805 (1A capable). I now have all the IC's installed AND +5V! current draw is ~23ma. I am now 98% convinced I got a cheap counterfeit part from Tayda. That was a pain in the.....

Finally got around to working on the VooDoo.
Had all the passives & sockets soldered in and the board washed.
Without IC's except U8 (7660) and the PIC +/-5Vs came up.

Connected a TX coil and installed the NE5532 (U3) and no output. Check Voltages, got -5V but +5V is -0.8V ???
Pulled U3 and get +5V. Replaced U7 (78L05) with a different 5V reg. No happiness.
After some troubleshooting found that the -5V comes on first and pulls the +5V regulator output negative (probably through the rail pin's protection diodes in the NE5532). This seems to 'lock up' or 'shut down' the +5V reg.
I added a 1k Ohm resistor from input to output on the +5V reg and now always powers up correctly.

dingbata,
This looks to be the same issue you had with +5V.

Next is damping the TX coil and install the 'Pulse' parts to get non-disc PI part working.

[dingbatca]

surfdetector, as always, your videos are enlightening. But I cant seem to get anything like what you are showing, unless 190us @ 22.5 ohms is acceptable. Pic's attached.