A Ground Balance Circuit (build and field tested)

[Digger429]

There are many ways to skin a cat, but they all end up with the same result. This circuit is not revolutionary in that it achieves the goal that has already been discussed in several other threads, but it is perhaps interesting in that it uses circuitry that anybody who has built a Minipulse Plus has already seen and is familiar with, and does not require the cutting of tracks or any circuit modifications to the Minipulse Plus kit at all. Just 5 wires tapping onto the existing board (plus 2 more for power supply).

The general concept of PI ground balance circuits is that they take a Ground Balance (GB) sample very soon after the main sample, and then at a much later time also take an Earth Field (EF) sample of equivalent sample time to counteract any EF effects. Because there is already a EF sample built into the Minipulse Plus, people have discussed addition and subtraction calculations, often with microcontrollers to do the work of determining the correct length of the resulting EF sample.

My design doesnt do any of that, it leaves the standard Main and EF pulses alone, and just places new GB and EF samples in parallel at the appropriate times. The key to inserting samples in parallel is to use a transistor OR gate, as seen here:

http://hyperphysics.phy-astr.gsu.edu...angate.html#c2

and as seen on the circuit:

https://i.imgur.com/wdrVVoB.jpg

I'm not sure the type of FET is critical as long as it is similar to the J113. I had some MPF102s lying around, so I used those.

To generate the new GB and EF pulses, you need to tap a wire onto the existing test point TP7, and make a new copy of the kit circuit that creates the pulse train:

https://i.imgur.com/oYFuQwz.jpg

For the new circuit, pulses that went to the integrator at point C now go to D, and pulses that went to D now go to C... because the ground balance circuit works to cancel signals.

---

At top left there is a potentiometer on the circuit diagram, but in my final build I eliminated that and just used a 1K resistor with a 5.6pf capacitor for C8, so the ground balance sample is taken 3 uS after the main sample is done.

The circuit diagram mentions 192uS for the time between GB and EF, but thats just what I used so the timing of the later EF pulse would not impinge on the next transmit pulse. Make it as long as you can, depending on your sample pulse rate.

R11 (and C11) controls the width of the ground balance sample. It also acts effectively as a "strength" control in that a longer time means more ground balance effect. With a potentiometer, mine can vary between 10 - 150 uS. You will need to experiment a bit here to see what works for the soil you have locally.

I also put a dual gang potentiometer for the outputs that go to C and D to control the ground balance strength. With field testing I think I'll throw that out and just put a switch, because the potentiometer at R11 does the same job.

---

It is not a miracle worker, but it is better than nothing. Its the difference between giving up in frustration at that damned ground and going home, and being able to stay out and swing the machine some more. Field testing at sites around Ballarat (Victoria, Australia) lets me cancel the ground completely if I have a main pulse width of about 30uS and a GB sample width of about the same. For a sample of very very mineralised soil I took from Wedderburn, I need to wind the GB width dial all the way up to 150uS to cancel it.

As other people who have built GB circuits have noted, a strong GB balance circuit can also cancel out targets making a "hole" in response so they wont be detected at all, and some large targets can be "negative" in that they make the metal detector go quiet instead of beeping. So use the minimum amount of effect you can get away with.

[Qiaozhi]

Well done.

That was on my "planned list" of future projects. Now it looks like we just need an add-on PCB to help all you nugget hunters.

[6666]

This is interesting thanks, how much attenuation of the target signal is there with GB on and off ?

[Digger429]

Attenuation entirely depends on the target.

eg. Aluminium:
A piece of drink can 10m x 10mm shows no obvious difference.
A solid mass that weighs 1 gram can be invisible.
A solid mass 11 grams in weight can go negative. (Detector goes quieter)

I am conscious of the fact that if I go gold hunting, there may be nuggets of a particular size that do not register at all. But like I said, its either that or not go out at all because mineralised soil drives me crazy (or buy a Minelab machine). I normally go to the areas near Ballarat and that was bad enough but just manageable without ground balance, but a few weeks ago I visited Wedderburn and every place on the ground was so mineralised it gave full strength signal everywhere. Had to pack the Minipulse Plus back in the car after 5 minutes. Was unusable. This circuit might give me a chance.

[Wahyu]

Hello Digger429, can you please upload image at geotech, i can't open image at that link

Thank

[homefire]

Any chance someone can do a quick sketch of a Schematic ? This portion of the Original timing needs duplicated ?

[green]

There are many ways to skin a cat, but they all end up with the same result. This circuit is not revolutionary in that it achieves the goal that has already been discussed in several other threads, but it is perhaps interesting in that it uses circuitry that anybody who has built a Minipulse Plus has already seen and is familiar with, and does not require the cutting of tracks or any circuit modifications to the Minipulse Plus kit at all. Just 5 wires tapping onto the existing board (plus 2 more for power supply).

The general concept of PI ground balance circuits is that they take a Ground Balance (GB) sample very soon after the main sample, and then at a much later time also take an Earth Field (EF) sample of equivalent sample time to counteract any EF effects. Because there is already a EF sample built into the Minipulse Plus, people have discussed addition and subtraction calculations, often with microcontrollers to do the work of determining the correct length of the resulting EF sample.

My design doesnt do any of that, it leaves the standard Main and EF pulses alone, and just places new GB and EF samples in parallel at the appropriate times. The key to inserting samples in parallel is to use a transistor OR gate, as seen here:

http://hyperphysics.phy-astr.gsu.edu...angate.html#c2

and as seen on the circuit:

https://i.imgur.com/wdrVVoB.jpg

I'm not sure the type of FET is critical as long as it is similar to the J113. I had some MPF102s lying around, so I used those.

To generate the new GB and EF pulses, you need to tap a wire onto the existing test point TP7, and make a new copy of the kit circuit that creates the pulse train:

https://i.imgur.com/oYFuQwz.jpg

For the new circuit, pulses that went to the integrator at point C now go to D, and pulses that went to D now go to C... because the ground balance circuit works to cancel signals.

---

At top left there is a potentiometer on the circuit diagram, but in my final build I eliminated that and just used a 1K resistor with a 5.6pf capacitor for C8, so the ground balance sample is taken 3 uS after the main sample is done.

The circuit diagram mentions 192uS for the time between GB and EF, but thats just what I used so the timing of the later EF pulse would not impinge on the next transmit pulse. Make it as long as you can, depending on your sample pulse rate.

R11 (and C11) controls the width of the ground balance sample. It also acts effectively as a "strength" control in that a longer time means more ground balance effect. With a potentiometer, mine can vary between 10 - 150 uS. You will need to experiment a bit here to see what works for the soil you have locally.

I also put a dual gang potentiometer for the outputs that go to C and D to control the ground balance strength. With field testing I think I'll throw that out and just put a switch, because the potentiometer at R11 does the same job.

---

It is not a miracle worker, but it is better than nothing. Its the difference between giving up in frustration at that damned ground and going home, and being able to stay out and swing the machine some more. Field testing at sites around Ballarat (Victoria, Australia) lets me cancel the ground completely if I have a main pulse width of about 30uS and a GB sample width of about the same. For a sample of very very mineralised soil I took from Wedderburn, I need to wind the GB width dial all the way up to 150uS to cancel it.

As other people who have built GB circuits have noted, a strong GB balance circuit can also cancel out targets making a "hole" in response so they wont be detected at all, and some large targets can be "negative" in that they make the metal detector go quiet instead of beeping. So use the minimum amount of effect you can get away with.

I have suggested a couple times in the past that GB couldn't be added to the MPP integrator. Was thinking sample time effected integrator cutoff frequency not integrator gain which is normally controlled. Was thinking the integrator needed to be modified also. Maybe you are proving me wrong.

[6666]

Attenuation entirely depends on the target.

eg. Aluminium:
A piece of drink can 10m x 10mm shows no obvious difference.
A solid mass that weighs 1 gram can be invisible.
A solid mass 11 grams in weight can go negative. (Detector goes quieter)

I am conscious of the fact that if I go gold hunting, there may be nuggets of a particular size that do not register at all. But like I said, its either that or not go out at all because mineralised soil drives me crazy (or buy a Minelab machine). I normally go to the areas near Ballarat and that was bad enough but just manageable without ground balance, but a few weeks ago I visited Wedderburn and every place on the ground was so mineralised it gave full strength signal everywhere. Had to pack the Minipulse Plus back in the car after 5 minutes. Was unusable. This circuit might give me a chance.

Hi , I was not being critical, ( I called it target attenuation, but will now call it depth), I was just wondering about the usual loss of depth with ground balance circuits for PI, I was experimenting with Davor's GB circuit for a while untill he mentioned he got some feedback that his system had the greatest loss of depth, so I was just wondering if you had done any compassion depth tests on say something like a dollar coin to compare depth with GB on and off.

[green]

Hi , I was not being critical, ( I called it target attenuation, but will now call it depth), I was just wondering about the usual loss of depth with ground balance circuits for PI, I was experimenting with Davor's GB circuit for a while untill he mentioned he got some feedback that his system had the greatest loss of depth, so I was just wondering if you had done any compassion depth tests on say something like a dollar coin to compare depth with GB on and off.

I would be interested also. Carl mentioned in another thread using layers of aluminum foil between 2 layers of clear packing tape to hold in place. A test using 25x25mm squares of regular aluminum foil with my detector(1C integrator). Recorded integrator out instead of distance because it's faster for me. Recording distance might be better since distance isn't linear with signal strength, closer to log. Changing delay time 6(A), 10(B) and 14(C)usec) and adjusting GB(target sample) to cancel ground effected how many layers caused minimum signal(hole). Need more layers for 10(B) and 14(C)usec delay. Cutting the foil to correct size and folding works best for me. 25x50mm folded once(2 layers), 50x50mm folded twice(4layers),as examples.

[Digger429]

Since a few have asked about attenuation, I just ran some indoor tests (where there is admittedly a lot of interference) and I've come to the same conclusion I had last week - that it is so highly dependent on the settings that I cant give a clear answer.

eg. A 1 euro coin (although measurable at greater distances) gives a "decent" signal at 22cm with GB off, and a main sample length of 50 μS.

This does not change if I turn GB on and have a GB sample size of 30 μS. But that is a very low value and would hardly have any effect on ground. It could only be used where mineralization was almost non existent.

As the settings are turned up further for more compensation (more GB length and less sample length) the distance gets worse until at a main sample length of 20 μS and a GB sample length of 150 μS the coin is invisible. This represents the sample of highly mineralized soil I took from Wedderburn.

So whats it going to be for the soil where you live? Its a local issue, with attenuation results depending on the settings needed to get rid of the mineralization you have in your local area.

---

But that was a quick indoor test. I'll go out sometime in the next week or so and bury some objects for a field test.

[6666]

thanks interesting result.

[green]

Since a few have asked about attenuation, I just ran some indoor tests (where there is admittedly a lot of interference) and I've come to the same conclusion I had last week - that it is so highly dependent on the settings that I cant give a clear answer.

eg. A 1 euro coin (although measurable at greater distances) gives a "decent" signal at 22cm with GB off, and a main sample length of 50 μS.

This does not change if I turn GB on and have a GB sample size of 30 μS. But that is a very low value and would hardly have any effect on ground. It could only be used where mineralization was almost non existent.

As the settings are turned up further for more compensation (more GB length and less sample length) the distance gets worse until at a main sample length of 20 μS and a GB sample length of 150 μS the coin is invisible. This represents the sample of highly mineralized soil I took from Wedderburn.

So whats it going to be for the soil where you live? Its a local issue, with attenuation results depending on the settings needed to get rid of the mineralization you have in your local area.

---

But that was a quick indoor test. I'll go out sometime in the next week or so and bury some objects for a field test.

If a 1 euro coin falls in the PI hole with target sample(20usec) and GB sample(150usec). What are a couple targets detected at similar distance with GB on or off with target sample(20usec) and GB sample(150usec)?

[Digger429]

ok, more indoor testing, but for this one I put my "better" 8inch coil on for a bit more distance.
To get the 1 Euro coin to blank out with GB on, I had to turn the main sample size down to 17 μS, which is probably a bit lower than you'd want to use in the field. Also worth mentioning that even though the 1 Euro coin can go invisible, the effect varies across the diameter of the coil. If the coin was within 1 or 2 cm of the ground surface, you'd still detect it when the edge of the coil was over the top of it. It only went undetectable under the middle of the coil.
The "notch" to remove the coin from detection is also very narrow. At 17 μS main sample size it was invisible (with GB on, 150 μs), but at 20 μS it was being detected as normally as all the other samples.

Detection distances in cm.

[green]

ok, more indoor testing, but for this one I put my "better" 8inch coil on for a bit more distance.
To get the 1 Euro coin to blank out with GB on, I had to turn the main sample size down to 17 ìS, which is probably a bit lower than you'd want to use in the field. Also worth mentioning that even though the 1 Euro coin can go invisible, the effect varies across the diameter of the coil. If the coin was within 1 or 2 cm of the ground surface, you'd still detect it when the edge of the coil was over the top of it. It only went undetectable under the middle of the coil.
The "notch" to remove the coin from detection is also very narrow. At 17 ìS main sample size it was invisible (with GB on, 150 ìs), but at 20 ìS it was being detected as normally as all the other samples.

Detection distances in cm.

Thanks for the test. The 10mm and 25mm aluminum. Regular, heavy duty aluminum foil or something else?

[Digger429]

The 10mm and 25mm aluminum... cut from a Red Bull drink can.