Voodoo Main Discussion

[surfdetector]

All my coils are used, "as-is", for parts coils. Most of them suffered from a bad cable. One has a bad pre-amp board. I have gotten rather good at lifting out the epoxy and replacing cables.

I am trying to avoid buying a brand new Fisher F75 coil, as they cost a bit, and I would feel bad about cutting the cable apart.

.

There is no need to cut the cable of the Fisher F75. I purchased a new F75 for this project and made the appropriate connections at the detector without cutting anything.

As George said regarding the Whites coil, a 470nF parallel capacitor on the RX should work for you.

Damping the TX on the Voodoo is no different from any PI, its the same procedure. If you need more info look at page 2 of the book. I also did a couple of Damping videos on my Youtube channel which might be helpful to you so you can see what the waveforms look like.

[dingbatca]

All my coils are used, "as-is", for parts coils. Most of them suffered from a bad cable. One has a bad pre-amp board. I have gotten rather good at lifting out the epoxy and replacing cables.

My coils:
Whites DXF. This is a well know coil, full specs back in post #1400.
HotHead. Unknow detector. Pins 1-3, RX, 472.3K. Pins 1-4, TX, 4.8ohms.
Detech S.E.F. Unknow detector. New wire. Pins 1-2, TX, 1.1ohms, 700uH. Pins 3-4, 5.9ohms, 580uH. Has Litz wire on the TX loop.
Coiltech. Unknow detector. Had to remove failed pre-amp. Pins 1-2, TX, 2.6ohms. Pins 3-4, RX, 5.9 ohms

I have two variables currently. The Voodoo boards build quality. I already found and replaced one counterfeit part. I was a good boy and built it step by step and followed the instructions. But there is only so much I can do with out a proper coil. At the same time I have a bunch of unknow coils. This makes it REALLY hard to track down a problem. I am trying to avoid buying a brand new Fisher F75 coil, as they cost a bit, and I would feel bad about cutting the cable apart.

The Whites DFX coil is perhaps the most well known of the lot. I think the Detech S.E.F is the best quality of my collection. I believe both of them are working as expected. Buying the F75 a just copying the configs from the book would work, but I would really like to understand all the tuning and setup. Yes, the book does cover this, to a point.

The "Voodoo search coil setup" walk through, I hope would be able to help many people, and help teach those of us who have only built a few simple detectors some new tricks.

[surfdetector]

I keep seeing coil questions come up in this thread.


I am stuck and could use the wisdom of the group. I have 4 different commercial coils, but I am failing at getting even one of them to work well.

What are the 4 commercial coils that you are using and what are the TX and RX inductance measurements of each?

[dingbatca]

I keep seeing coil questions come up in this thread.


I am stuck and could use the wisdom of the group. I have 4 different commercial coils, but I am failing at getting even one of them to work well. I can tell the difference between an aluminum plate and a sledge hammer, but that's the best I got. Would some one be willing to write up a "Voodoo search coil setup" walk through?


Something like:
1) How, on the Voodoo, to adjust for critical damping of the TX loop
2) RX capacitor tuning
3) Pi Main Sample delay tuning
4) DISC delay tuning
5) Tuning R30 and R22
*) Examples of known failure states, like reversing the polarity of the RX coil or TX coil. Missing, or wrong damping resistor. Missing, or wrong RX capacitor....


Basically start off with an unknow coil and get the Voodoo all tuned for said coil. I realize this is a big ask, but I think a lot of people would benefit from it.

[waltr]

my signal looks like is doing opposite, what i do wrong?

My guess is the Coil Null is not correct or you have the polarity of the RX coil backwards.
Try swapping the RX coil leads before trying to readjust the Null.

[ionut_mtb]

Here is the sample with and without the 100n cap. Perfect sample for non ferrous detection is on the declining slope of the oscillations. The net signal will decrease in amplitude for non ferrous and phase shift right plus increase for iron... same for the ground signal so it seems best to sample on the declining slope between 90 - 180 degrees.

my signal looks like is doing opposite, what i do wrong?

[waltr]

Genuine 78L05 tested and installed. Everything is working as expected.

Good news and bad news that you got a 78L05 from a bad maker.

[dingbatca]

Genuine 78L05 tested and installed. Everything is working as expected.

Does any one have some sensitivity numbers? Such as the air/bench test of a US quarter from X distance away, or something like that?

[billr]

Great job . It's so satisfying coming through the other side after a struggle.

[dingbatca]

I will always do my best to report back, so others may learn from my failures.

I moved an 8lb sledge hammer over the coil and it detected iron.
I moved an aluminum plate over the coil and it detected non-iron.

I am ECSTATIC! Now I have a ton of work to get a proper coil up and running. But the circuit basics are up and running. I am thrilled.

[Qiaozhi]

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.....

That's good news, and thanks for coming back with the results. Quite often people just say that they've fixed the problem and leave everyone guessing what was wrong.
So the 78L05s you have are not up to scratch. They should be able to supply 100mA.

[waltr]

Great news you got the +5 back. Yep, been there with bad parts.

[billr]

Yup. I can relate. Really chaps my butt going round in circles only to discover a defective part was the culprit.
Glad it's all good now.

[dingbatca]

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.....

[MartinB]

I did build this board in stages, and tested as I went. Lets just go back to the basics.

TP2 & TP3 wave form. Per page 31 of the book, I should see a drop of about 20us, but I am getting closer to 224us? Pic attached.

Thanks for sharing. It is always good to learn.