Voodoo Main Discussion

[dfbowers]

Thinking about that some more ... I've probably got that wrong. With the meter connected to 0V the non-ferrous would swing right and then left, but the ferrous targets should do nothing. This is (of course) assuming the meter was adjusted so that the needle is in the middle.

Maybe you could simply trigger the audio from the meter circuit.

With the way that I have it set up, depending on where I do a 90 degrees sample, it will deflect positive for ferrous and nothing for non ferrous or vice versa. I like the idea of a centered meter with opposite directions. It's pretty apparent on the o'scope looking at the samplee what it will do I thought about just doing an audio circuit blanking to save on components but you can get the idea with a meter. Plenty of room for improvement though.

[dfbowers]

If I understand your schematic (and correct me if I'm wrong) the audio is only driven from the PI channel, but the signal prior to the SAT circuit is sent to a differentiator located on an add-on board. The DISC channel is also single-differentiated, and the output is wire-ORed so that the meter only responds to non-ferrous targets. This is analogous to the signals in pane 2 of Table 7-4 on page 81. If the lower connection on the meter was connected to 0V, I presume it would swing right for non-ferrous, and left for ferrous? The only problem would be non-ferrous targets would first swing to the right followed by a swing to the left. Ferrous targets would do the reverse. So I suppose that's why you've connected the meter to -5V?

This means that the detector will beep for all targets, and reliance is on the meter to indicate the target type. Have you tried using the meter signal to disable the audio for ferrous targets?

I have experimented with a single-differentiating topology on a previous version, but separation of targets was poor. In my test garden (which is littered with ferrous trash) it was impossible to identify the location of the buried coins.

I would be interested in what you discover when you take the detector outside.
Good work by the way.

Yes, I think you have everything right regarding what you described. I look at the LM393s configured that way as sort of like an "AND" gate. The PI (ground balance channel) has to be active for anything to happen to the meter regarding the disc channel. Unless you have clean ground it might be a nightmare to try to use.

[6666]

Yes, exactly. I used the area under the clevis tabs to make the fine tuning loop. Then several pours of potting compound until I got it close. It was a whole new challenge working around the open center coil! It's much easier with a solid center.

Thanks Don, this vlf conc coil stuff is a new experience for me, I've been studying some of your TGSL coil making methods as well, seems the solid foam method may be the way to go for simplicity, planning to 3d print the coil formers and embed in the foam.

[Altair]

Here is what I have been playing around with. I can disc iron or non ferrous depending on where the sample it taken. The ground signal seems to have no overall effect. Since that original PI audio is left uncorrupted.. no double beep. All that is needed is to look at meter deflection. I can disc through two layers of brick and which is probably a stronger ground signal than my actual ground. It?s still on a breadboard but I can move my coil off the bench to a bed of bricks to test. If nothing else it had been a fun experiment!

Don, I would have expected a capacitor after the CD4066B to hold the sampled voltage; it's really missing?

[ivconic]

I put all in one schematic for easier insight.
Don can you please check this and see if i did it right?
Also i think this deserves own topic, to avoid to mix in here and make confusion.
I would start new topic but it's your work so is better you to start it, i think.
If there are no mistakes; i can start drawing pcb.

[Qiaozhi]

Yes, I think you have everything right regarding what you described. I look at the LM393s configured that way as sort of like an "AND" gate. The PI (ground balance channel) has to be active for anything to happen to the meter regarding the disc channel. Unless you have clean ground it might be a nightmare to try to use.

I meant to say wire-ANDed, but for some reason wrote wire-ORed.

Yes, if the PI channel reacts to the ground then it will be a nightmare to use. But that would also be the case for a standard PI without GB. When I designed Voodoo the intention was to be able to use it inland where the ground contained mild to moderate mineralization. For heavily mineralized ground you would certainly need to add a GB sample to the PI channel. Alternatively, adding a GB sample to the DISC channel would result in the same problems you get with a VLF. The advantage with hybrid mode is that there is no requirement to adjust a ground balance control for use in areas with normal soil.

In a VLF design the problem with heavily mineralised non-homogeneous ground arises in the GB channel, because the selected GB sample point cannot handle the varying ground conditions. The same applies to the GB sample method used in PI detectors, because the target hole created by the GB sample also cannot handle the varying ground conditions. On the other hand, if the ground mineralization is homogeneous then either method would work.

However, consider this:
In a hybrid configuration (such as the one used by Voodoo) if the PI channel reacts to the ground it will give a response as if the ground mineralization was a real target. At the same time the DISC channel would see the signal as being ferrous, and would prevent the user from hearing an audio response.
My conclusion is that a lot would depend on the severity of the ground mineraliszation compared to the response from the target. If your ground conditions are tricky, then maybe you would be able test how effective the hybrid approach is in that situation. In areas where there are desirable targets hiding amongst ferrous trash (which is often the case in the UK) hybrid mode is able to separate the good from the bad, and this was its main design goal.

I realise your variant is beeping at every target, and using the meter to signal whether the target is non-ferrous. In this particular case I can see why noisy ground would make it difficult to use in practice.

[Qiaozhi]

I put all in one schematic for easier insight.
Don can you please check this and see if i did it right?
Also i think this deserves own topic, to avoid to mix in here and make confusion.
I would start new topic but it's your work so is better you to start it, i think.
If there are no mistakes; i can start drawing pcb.

[ATTACH]54127[/ATTACH]

I don't think there is any need to start a new topic as this is all related.
Good to see lots of people getting involved.

[Altair]

I put all in one schematic for easier insight.
Don can you please check this and see if i did it right?
Also i think this deserves own topic, to avoid to mix in here and make confusion.
I would start new topic but it's your work so is better you to start it, i think.
If there are no mistakes; i can start drawing pcb.

[ATTACH]54127[/ATTACH]

Ivconic, don't forget to reference unused gates, e.g.: pin 5,9,11 of 40106 to 0V.

[dfbowers]

Ivconic, don't forget to reference unused gates, e.g.: pin 5,9,11 of 40106 to 0V.

Wow .. impressive. You are truely a wiz at that! I will check it over carefully when I get off work later today.
a few notes.. the reference pot around the LM939s can be improved. A few microvolts over ground is all that is needed. Pots can be noisy and I have had better success with a pair ok slightly mismatched resistors.
It doesn’t need to be that PI circuit. It’s just what I had laying around that worked.
There are two sweet spots on the delay pot. One will reject ferrous, the other non ferrous. I’m king of liking the ferrous rejection setting.
RX coil frequency can be played with quite a bit. As long as you can do less than 90 degree sample and cover the first 180 degrees of the first oscillation everything should work. I have not paid any attention the the load on the regulators.. I need to make some measurements to see that there is enough headspace but I have not seen a problem yet.
thx..
don

[green]

In a VLF design the problem with heavily mineralised non-homogeneous ground arises in the GB channel, because the selected GB sample point cannot handle the varying ground conditions. The same applies to the GB sample method used in PI detectors, because the target hole created by the GB sample also cannot handle the varying ground conditions. On the other hand, if the ground mineralization is homogeneous then either method would work.

Think I've read somewhere that ground X signal varies and ground R signal doesn't. I have some hot ground from California that GB(R signal)with subtraction at the same settings as ground from my yard. Tried looking at X signal when Tx on with IB coil. Requires different sample settings to GB. Recording I made awhile back shows ground decay slope changes with different Tx profiles but the two different ground R signals act the same. Anyone know where to look for some test information that shows ground R signal not the same with different grounds?

Chart Tx,(cc)constant current,(cr)constant rate

[Orbit]

There are definitely solutions for the soil

[Altair]

Ivconic,
the IRF9640 is a P channel, not N as in yours schematic and his S and D pins must be swapped.
Moreover, sometime you name -v the -5V.

[ivconic]

I corrected those.
Software for opening, editing and manipulating with this format is full free and can be downloaded here:
https://www.expresspcb.com/

[dbanner]

Don, I admire your work, thank you for your efforts. There is simply no way in which one can make a usable detector based on running up same street over and over. Let's face it. Probing the depths of GB with pi is futile, you may end up with a very sophisticated Analog device that only you know how to use.

[Altair]

I corrected those.
Software for opening, editing and manipulating with this format is full free and can be downloaded here:
https://www.expresspcb.com/

[ATTACH]54131[/ATTACH]

.... out pins 6,8,10 of 40106 must be left open!