Voodoo Main Discussion

[yatahaze323]

Thanks Surfdetector for confirming the 470r I'll start with that.

Carl, thanks for the clarification I just got the coil in today and luckily I had some connectors that fit the plug perfect.

If anyone else decides to go with a f75 coil you'll need a 16mm microphone connector, these are the ones I had for a previous project but are not in stock anymore: https://www.amazon.com/gp/product/B015K3Z5AG/

[MartinB]

No, they are microphone-style connectors.

I am not familiar with Fisher coils, but just found this in a local store. Is it the same as a F75 coil and is it suitable for the Voodoo? What connections can expect in the plug?
FISHER 11" X 7" BIAXIAL COIL FOR ALL F5 MODELS. PLEASE NOTE THAT IT HAS A DIN MALE 6 PIN PLUG CONNECTOR
Thanks

[reiv]

Hi, after reading 63 pages, one question:

Is there any just one short movie showing how it performes on the field/different targets etc. (excluding educations movies Surf Detector) ?

Regards

[Carl-NC]

Ah those must be switchcraft connectors, I bought a few 5 pin connectors when I was building my tgsl's.

Thanks again for your help!

No, they are microphone-style connectors.

[surfdetector]

Hi George,

I decided to order a f75 11" coil like you used in the book.

What damping resistor did you use for this coil?

I used the F75 coil for this project and used a 470 ohm damping resistor

What brand/model was the socket that you made for the adapter?

Thanks!

I used the F75 coil for the Voodoo project and used a 470 ohm damping resistor.

[yatahaze323]

Ah those must be switchcraft connectors, I bought a few 5 pin connectors when I was building my tgsl's.

Thanks again for your help!

[Qiaozhi]

Hi George,

I decided to order a f75 11" coil like you used in the book.

What damping resistor did you use for this coil?

What brand/model was the socket that you made for the adapter?

Thanks!

I repurposed the adapter for another project so am unable to check, but I think it was 470R. It would be advisable though to check for critical damping using an oscilloscope.
The socket is a standard 5-pin connector used by many metal detector manufacturers such as Whites, Minelab, and Fisher.
If you have trouble finding the correct one, let me know and I'll try to locate the part number.

[yatahaze323]

Hi George,

I decided to order a f75 11" coil like you used in the book.

What damping resistor did you use for this coil?

What brand/model was the socket that you made for the adapter?

Thanks!

[Carl-NC]

English please, thanks.

https://www.geotech1.com/forums/showthread.php?10526-Basic-Rules-of-the-Forums

[Latino]

bom dia a todos !

Bom dia a todos, interesantissimo esse projeto, espero ver um video deste em acao quando estiver pronto.

[Qiaozhi]

The Voodoo schematics are not available on the forum. You can purchase the book from here ->

https://www.amazon.com/Voodoo-Project-INDUCTION-DETECTOR-REJECTION/dp/B08KH97NWG/ref=sr_1_1?dchild=1&keywords=the+voodoo+project&qid=1602613396&s=books&sr=1-1

It is also available from Amazon websites worldwide, and from booksellers.

[Hamza Aamer1]

Voodoo Schematic available on forum?

[Olly]

The difference between Voodoo and a standard PI is that the filter channels are double-differentiating, which has the side-effect of making the detector more susceptible to noise. Hence values were chosen for the R and C values in the sample gates to reduce noise entering the filter stages. With 22k and 470nF, the low-pass cut-off frequency is 15.4Hz. I did experiment with different values, but found that these gave the best results.
However, please feel free to experiment.

Thanks for explaining, I'll have a little play in the lab

[Qiaozhi]

Hi George,

Looking at the synchronous demodulator RC values of 22K and 470nF, aren't you severely limiting the gain?
Correct me if I'm not understanding this correctly but with a smallish coil (say 8 inches diameter) we can expect the target to be under the coil for around 100mSec or so.
With a PI sampling pulse width of 60uSec and the time constant of 22K/470nF at around 10mSec, this would need approximately 167 samples (or 167mSec with 1mSec pulse rate) to fully transfer the preamp output voltage to the 470nF capacitor. Wouldn't it be better to do this in at least half the time the target is under the coil (or around 50mSec) by changing the resistor value to 6K8 or thereabouts?

I may have this totally wrong or there may be other factors at play that I'm not seeing, please set me straight if that is so.

Totally loving the Voodoo project but still trying to understand it fully, my book is getting quite dog-eared in the process.

Kind regards,
Olly

The difference between Voodoo and a standard PI is that the filter channels are double-differentiating, which has the side-effect of making the detector more susceptible to noise. Hence values were chosen for the R and C values in the sample gates to reduce noise entering the filter stages. With 22k and 470nF, the low-pass cut-off frequency is 15.4Hz. I did experiment with different values, but found that these gave the best results.
However, please feel free to experiment.

[Olly]

Synchronous Demodulator RC values

Hi George,

Looking at the synchronous demodulator RC values of 22K and 470nF, aren't you severely limiting the gain?
Correct me if I'm not understanding this correctly but with a smallish coil (say 8 inches diameter) we can expect the target to be under the coil for around 100mSec or so.
With a PI sampling pulse width of 60uSec and the time constant of 22K/470nF at around 10mSec, this would need approximately 167 samples (or 167mSec with 1mSec pulse rate) to fully transfer the preamp output voltage to the 470nF capacitor. Wouldn't it be better to do this in at least half the time the target is under the coil (or around 50mSec) by changing the resistor value to 6K8 or thereabouts?

I may have this totally wrong or there may be other factors at play that I'm not seeing, please set me straight if that is so.

Totally loving the Voodoo project but still trying to understand it fully, my book is getting quite dog-eared in the process.

Kind regards,
Olly