Voodoo Main Discussion

[Tibuck19]

The Voodoo Project requires 4 push buttons, but I would suggest you ask Andy directly in case he is interested.

Okie, i did not know it needed 4, then this are useless for this project, no sense hassling Andy about them.

[Qiaozhi]

Iam hearing that this metal detector vodoo is sensible for eron so what about gold and other metal. Can ennyone here anser me.

Everything is described in some detail in the book.
Voodoo is designed to reject iron and to accept all other metals.
Read post #1.

[sbalio1]

Thanks Qiaozhi good wiches.

[dingbatca]

It's a good day! I finished the board build and testing... Well, as far as I can go with out a proper coil.

Would a Whites DFX coil work? This is the classic DFX round coil. Pin out in this thread.

The chart provided on page 75 does a great job of covering a wide range of coils. Any tips on adapting this coil?

White's DFX concentric
Rx 1,2 13.8mH 36 ohms
Tx 4,5 540uH 2.7 ohms

Pin1 RX shield
Pin2 RX
Pin3 NC
Pin4 TX
Pin5 TX shield

TX Coil:
L--> 548,5uH
Q--> 1,27
C--> -46,15uF
R--> 2,72 Ohmios
Rx Coil:
L--> 13,6mH
Q--> 2,16
C--> -1,86uF
R--> 94 Ohmios

[Qiaozhi]

It's a good day! I finished the board build and testing... Well, as far as I can go with out a proper coil.

Would a Whites DFX coil work? This is the classic DFX round coil. Pin out in this thread.

The chart provided on page 75 does a great job of covering a wide range of coils. Any tips on adapting this coil?

White's DFX concentric
Rx 1,2 13.8mH 36 ohms
Tx 4,5 540uH 2.7 ohms

Pin1 RX shield
Pin2 RX
Pin3 NC
Pin4 TX
Pin5 TX shield

TX Coil:
L--> 548,5uH
Q--> 1,27
C--> -46,15uF
R--> 2,72 Ohmios
Rx Coil:
L--> 13,6mH
Q--> 2,16
C--> -1,86uF
R--> 94 Ohmios

Looking at the specifications I would say "yes".
The TX inductance is in the accepted range. If the coil is well balanced (and I expect it will be) then there should be no ringing at the PI preamp output due to the RX loop. A parallel capacitor across the RX loop of 470nF will give an frequency of 10kHz.

[dingbatca]

DFX coil attached. But TP5 has me stumped. I feel like I am missing something critical, but for the life of me I cant seem to find it. Pic's attached.

[Qiaozhi]

DFX coil attached. But TP5 has me stumped. I feel like I am missing something critical, but for the life of me I cant seem to find it. Pic's attached.

TP5 is the output of the first stage of the PI preamp, so there's some very basic fault here. Are you sure you have a +5V supply?

[dingbatca]

Arg! TP15 is reading -0.8v... It was reading -4.96v when I built the circuit. Looks like I blew something out. :-(

[dingbatca]

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.

[Qiaozhi]

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.

Check the input pin of U7 (measure at the cathode of D6). I suspect the voltage there is too low.
Then check the voltage on pin2 of U8.
U8 might be kaput or there is no sync pulse on pin 7. If sync pulse is missing, check on TP9.

[dingbatca]

Cathode side of D6 is at 9.51v. The data sheet for the 78L05 say minimum voltage should be 7V, so I think that's good.
Pin 2 of U8 is at -2.54V. Swapped out U8, no change.
Sync pulse on TP9, and U8-pin7 both look kinda correct. Its not the exact same as the book. Scope pics attached.

[Qiaozhi]

Cathode side of D6 is at 9.51v. The data sheet for the 78L05 say minimum voltage should be 7V, so I think that's good.
Pin 2 of U8 is at -2.54V. Swapped out U8, no change.
Sync pulse on TP9, and U8-pin7 both look kinda correct. Its not the exact same as the book. Scope pics attached.

Looks like something is loading the +5V line.
Pull out U1 (TL071), U9 (TL072), U10 (Tl072), and U5 (4066).
Then test if the +5V comes back.

Just noticed that your jumpers are also reversed on PL7 and PL8.
Check post #1 - Errata #1.

[dingbatca]

Jumpers fixed. Thanks for the note. I should go back and re-read post #1. That was a long time ago. ;-)

Pulled most the IC's and systematically put them back in. Looks like U9 is the problem child. Swapped out a new TL072 and the problem remains. I check the soldering and it all seems clean. I am guessing I have a component level issues. Any advice on debugging it? I will deep dive the chip data sheet and DISC schematic tonight.

[dingbatca]

Very interesting!

If I remove only:
U3 (PI dual preamp), I get +5V
or
U9 (DISC channel filter), I get +5V
or
U10 (PI channel filter), I get +5V

So what every the drain is, it's down stream of U9/U10. I am eyeing R34 or R28, I think. Hard to read Fig 3-12.

[dingbatca]

Sorry for filling up the thread with my real time debugging.

I removed U9 and U10. Everything checked out, as expected.
TP20 to Pin 2 of the PIC, 10K
TP17 to Pin 3 of the PIC, 10K
TP20 to TP14 (-5V), 20K
TP20 to TP17 (-5V), 20K

Checked R58, and R63, 1M
Checked R52, and R55, 2.2M

Any other points to check for debugging?