Graphite coil shielding for TGSL problems

[rickb]

What is the sheet resistance of the shell shielding? measure from opposite edges for very rough approximation. Did you thin the paint and test each layer before adding more? Are you using the paper shields and shell shields at the same time? You only need one type.

[waltr]

If I remember correctly, the graphite (polyurethane + graphite power) measure in the 500 to 2k Ohm range.

The coil shielding is NOT to reduce INTERFERENCE. It is to reduce to capacitance effect of triboelectric charges on objects, grass, etc.
You hand is such a source so good for testing.

The GB adjustment has NOTHING to do with shielding and triboelectric charge.

The GB is adjusted so when a piece of ferrite (minerialized ground) is waved across the coil there is no response AND Silver is still detected. If GB pot is turned too far TGSL will not respond to silver (coin).

I found that I had to very, very slightly tweak the GB pot in the field to eliminate the ground. Bob the coil up and down near the ground, if it beeps then increase the GB pot, if it doesn't and detects a silver coin then it is good.


Last: I can be Very difficult to evaluate inside a house, workshop, due to AC Mains EMI and EMI from many, many appliances, etc in and around the house. The EMI can cause the 'Morse code' sound (that sound like some piece of electronic/electric equipment in your house and when it sound is not there that piece of equipment is not emitting EMI. Been there trying to get my first detector working.
If the shield prevents this then it will also decease detection distance.

[billr]

So anyway. Update.
Took Ricks advice and removed all paint on each coil. Messy job.
Re nulled and placed paper coated with the paint above and below the coils. At first without grounding wires attached to them and then with to compare findings.
Tried several sheets. Each with different resistance values to them.

It did remove a bunch of interference noise almost to complete silence but the GB pot still needed to be turned 1/8th from fully closed to avoid any disturbance when the coil was touched by hand.
The cost however was a loss of range to what I had before paint removal. Dropped from 13.5 to 12.25". Approx the same as my working aluminum coil.

So for me at least, painting the shell with graphite and not the coil is a choice of noise vs range.
Flip a coin

[billr]

That is a good video.
That is also how I did a DD coil for my TGSL including the plastic null adjusting screw (this is really nice).
I did ensure a 'break' in the shielding on each coil and then insulated each coil with electrical tape before mounting in the coil shell.
DD coil works very well and has a little more distance than the Aluminum foil shielded DD I made.

Walt !!
Can you add a little info on how you went about doing the coils?
Like Resistance measurements at the start and finish of the paint. I was getting 1480 on average when checked either side of the break in paint. This dropped the more I moved the probe toward the staring point. Not sure if that was too high.
Also the wire around the coil pressed up against the paint. I tried that method but didnt notice any change in resistance drop as the guy on the video said he did it for. Did it anyway.
Other than that the coils were good for range 13.5" but boy where they noisy. How did you manage to calm them down. Mine sounded like an SOS signal regardless of how much I nulled.
Also agree graphite is better than the aluminum foil for range and doesn't need the disc pot turning to cancel it out.
Any info you can recall when making your coils would be so welcome.

[waltr]

Out of curiosity.
How did this dude manage to get his painted coils to run so silently when nothing was placed in front of em?
Mine sounds like Morse code.
Asking cuz I like to learn.

https://www.youtube.com/watch?v=XnrStRklcUQ

That is a good video.
That is also how I did a DD coil for my TGSL including the plastic null adjusting screw (this is really nice).
I did ensure a 'break' in the shielding on each coil and then insulated each coil with electrical tape before mounting in the coil shell.
DD coil works very well and has a little more distance than the Aluminum foil shielded DD I made.

[kt315]

general misunderstanding of electronics basics***

sorry

[Koala]

how confusing

one person says its too high one too low

I still think it will be all right. Especially with a spiral copper tap wire

[kt315]

2K sounds usable?
---
general understanding of electronics basics. you are testing in range of your batt setted in the tester.
this is 9 V batt. while we mean __the high_voltage_electrostatic field _charge-discharge process, in range 1...10+ kV (KILO-).
are you able to notice THE difference? your '2k' sounds nothing. and sure unusable.

[Koala]

2K sounds usable?

do you have you pcb in a metal case ?

I would try running a separate cable from the Rx coil shield direct to the pcb ground. Temporary leaving the Tx off. Just to test

[billr]

Out of curiosity.
How did this dude manage to get his painted coils to run so silently when nothing was placed in front of em?
Mine sounds like Morse code.
Asking cuz I like to learn.

https://www.youtube.com/watch?v=XnrStRklcUQ

[billr]

Ah now I see.
Didn't know you could do it that way. All the coils I've made have always had some form of shielding wrapped around them. I thought the graphite coating in the housing was for ...........actually I'm not really sure what I thought it was for until now. Some form of wet grass or water contact deterrent thing to stop false positives I guess.

LOL Rick must be tearing his hair out in frustration at my graphite fetish to cover everything including myself. Sorry Rick

Thanks Surf.
The file really explains a lot.
Armed with so much info I can really do some serious damage now.

[surfdetector]

You're confusing me. The coils need to be shielded or else they wont work. Yes? So how can they be shielded if there isnt any shielding added to them? Are you saying the coil shell itself is enough shielding to work? A little elaboration in detail would really help as I am only new to electronics.

Bill,
You might find this article to be of some use. You are doing a DD but the section on shielding is relevant to your situation:
https://www.hobbielektronika.hu/foru....php?id=139970

[billr]

You're confusing me. The coils need to be shielded or else they wont work. Yes? So how can they be shielded if there isnt any shielding added to them? Are you saying the coil shell itself is enough shielding to work? A little elaboration in detail would really help as I am only new to electronics.

[rickb]

The low resistance of the coil shielding makes your coils unusable. Remove all of the coil shields or make new coils.

Did you paint graphite directly on the bare coil? Stop adding graphite.

[billr]

Hey Rick.
This might be a long one but lemme explain. Read your posts and links. Appreciate the info in them and it kinda helped up to a point. Then it all went south.
Took the coils out of the shell and placed them on a board. Used a plastic adjuster screw to regain null. Got the null pretty darn close to silent when the thing was running with the intermittent light chirp every few seconds on audio. The GB knob did not correct this. Coin detection was back to 13.5" as it was before.
Made an error in my last post re the 2K shielding. It is in fact 10 to 20 ohms per coil as measured above. So I left the paint as it was.

Used a cotton swab with thinned graphite paint to spread an even layer on some paper. Placed them on top of the coils and all noise stopped. A slight tweak with the GB pot and nothing came through when the coils were touched by hand. Great........I thought.

So I coated the inside of the shell with a thin layer of paint and placed the coils inside. After securing in a few places with hot glue I set about nulling again.

That's where it all went out the window. It chatters like a crazy. Cant null it as before. Even placed the coated paper on the coil shell to no avail.

I dont know if the coil shielding is reacting with the shell shielding when in contact and causing over compensation but it's untunable. Did consider wrapping the coils in tape to insulate but after many hours of trial and failure ......... I'm pooped.

Real shame as these coils had better range than the aluminum foil ones I first made.

I need a drink to drown my sorrows.