Arduino Nano PI Main Discussion

[Qiaozhi]

Can you post an image/drawing of your connections to the coil socket?

[green]

Does either coil false signal when your hand is placed near the coil?
Which damping resistor are you using, first, second or third picture?
What first delay time are you using?
Do both coils have a similar decay picture with a quarter placed on the coil center?

[Altra]

Yes, the second set of photos look normal.

[Carl-NC]

I agree with Bklein. Post 471 the scope shot looks to be over damped. The clipped part of the flyback is almost 15us at 4us/div. I'm guessing the first sample is pegged to one of the rails. So only the second sample is being indicated. Which might explain the reverse signals on metal and ground noise. You might try a 470 ohm damping resistor or check the input diodes for damage.

That sounded like the best explanation so far until I looked at ding's latest scope plots. The GPX coil looks every bit as fast as the basket coil.

[bklein]

GPX wiring diagram

Here?s a diagram I suspect is correct for the coil wiring. I referred to this for my GP Extreme.

[dingbatca]

1. Doubt the ground is that bad. None of the rest of my detectors are putting up a fuss.
2. Dry dirt... I live on the Oregon, USA coast. It's been raining for 30+ days. ;-)
3. I can dance, with passion, with the ANPI in the air. No false signals.

Over damped... Maybe? Scope readouts attached. Tested against both my know good basket coil and the GPX Mono coil. GPX_Mono2 has the 350 ohm. I think that's the sweet spot for this coil.

[Altra]

I agree with Bklein. Post 471 the scope shot looks to be over damped. The clipped part of the flyback is almost 15us at 4us/div. I'm guessing the first sample is pegged to one of the rails. So only the second sample is being indicated. Which might explain the reverse signals on metal and ground noise. You might try a 470 ohm damping resistor or check the input diodes for damage.

[Carl-NC]

If I can pull of a sweep with almost zero height change between the coil and ground, the ANPI is mostly quite.

So that definitely narrows it down. It could be:

1. Ground mineral pickup -- might go away with a larger delay unless it is a viscous effect. If it's viscous ground, you will need a PI with GB.
2. Ground capacitance pickup -- only likely if the coil shield is bad. If you are swinging over grass then move to a dry dirt-only spot. Don't rub the coil on the ground.
3. Earth field -- is easy to tell just by raising the coil and twisting it in the air.

I'm still bothered by this statement:

When I do pass over a metal target, the signal suddenly goes away, until my sweep changes direction and I start sweeping back in the other direction.

This might happen if the SAT circuit gets stuck on a bounce-back signal. But then you say a home-brew coil works fine. Puzzling.

[dingbatca]

I would not be surprise if the coil was/is picking up ground minerals. Sure feels like it. I swept my test location with two other (VLF) detectors to make sure it's clean of obvious targets. I have extensively messed with the delay and threshold trying to find the correct combination. No luck.

Any other ideas?

[ionut_mtb]

Swapped the coil wires and does not seem to make a difference. This is a know good ANPI build. With a simple home brew coil, it works great. So I am 98% sure this is all related to the new GPX coil.

Lets see if I can better describe the issue. As best as I can tell, this is some kind of interaction with the ground. When I hold a target up in the air and sweep over it, everything works as expected. If I can pull of a sweep with almost zero height change between the coil and ground, the ANPI is mostly quite.

Does anyone have the wiring/pinout for the GPX mono coil. Or first hand knowledge of what's inside said coil?

i think the gpx coil is more sensitive and you are detecting minerals in the ground. Did you try to adjust delay higher to see if still have false signals?

[dingbatca]

Swapped the coil wires and does not seem to make a difference. This is a know good ANPI build. With a simple home brew coil, it works great. So I am 98% sure this is all related to the new GPX coil.

Lets see if I can better describe the issue. As best as I can tell, this is some kind of interaction with the ground. When I hold a target up in the air and sweep over it, everything works as expected. If I can pull of a sweep with almost zero height change between the coil and ground, the ANPI is mostly quite.

Does anyone have the wiring/pinout for the GPX mono coil. Or first hand knowledge of what's inside said coil?

[green]

If the coil is correctly damped and works well on the bench, but it misbehaves in the field, then it sounds like a shielding problem. Either the screen is not connected or you have the wires swapped over so that the ANPI is transmitting on the screen.

Earth field, ground signal and coil shielding can cause false signals. Any others? https://www.geotech1.com/forums/show...094#post292094 appears to be ground signal. What tests can be done to best determine what the problem is? Maybe a problem is something stupid. Was testing on the bench the other day and wasn't getting what I expected when I moved a target close to the coil. The problem was a new shirt I got for Christmas, had metal snaps on the sleaves instead of plastic buttons.

[Qiaozhi]

If the coil is correctly damped and works well on the bench, but it misbehaves in the field, then it sounds like a shielding problem. Either the screen is not connected or you have the wires swapped over so that the ANPI is transmitting on the screen.

[Carl-NC]

What do you mean by "ringing?" Part of you symptoms sound like bad SAT behavior. It would help to post an audio recording.

[dingbatca]

Scope pic is on the bench. Damping resistor set to 350 ohms, after a bit of tuning to find the sweet spot.

GPX mono coil wiring. So I think I have that right. "Think"

This is a new coil, so I think it should be good.

Coil rings in both direction of the sweep. Does not feel like loose cables or wiggling cables.