Graphite coil shielding for TGSL problems

[Koala]

I was wondering

the Coil is made up of layers

top insulator
shield
insulator
coil
insulator
shield
bottom insulator


does the thickness of the insulator make any difference ?


the older thicker coils seem more stable. But its like comparing apples with pears as the older machines were less sensitive ad well

[billr]

Nice job on the coil housing.

[mateatek]

well done

the biggest test for any coil seems to be long wet grass. Even some of the big manufactures coils get twitchy

I made carbon fiber protector for my detector. The coils are shielded with aluminum.
Good for wet grass with carbon fiber protector. Not good without a protector.

[billr]

well done

the biggest test for any coil seems to be long wet grass. Even some of the big manufactures coils get twitchy

Thanks
I hope to address that with posts 10 and 18 spiral wire around the coil.

[billr]

Would you post more detail on your amplifier that you referenced?
Cheers

Gladly
Schematic attached. Not my design but a very good one. Far better than the three aftermarket ones I tried.
Couple of changes I made.
Volume pot changed to 1K with switch for on off control. This allowed more rotation of the knob than the 10K pot.
Gain pot was a small trim pot fitted to board, set and left as is on max or min (cant remember) it served no adjustable purpose as the audio was clear at that setting. And only the volume needed to be set after turning on.
C3 could be omitted to give 20 times volume. With it = 200 times. Gain vs volume adjustment, any figure in between that.
I fitted a 330R resistor in series at the speakers negative connection. Without it you would blow your ears out if wearing headphones. It also makes the volume more controllable as the knob is turned. A little like the gain vs vol reaction.

[Koala]

well done

the biggest test for any coil seems to be long wet grass. Even some of the big manufactures coils get twitchy

[Koala]

Is this your varnish ? https://www.ronseal.com/for-home/doo...tdoor-varnish/
it appears to be water based, I think that could be your problem
there are three different bases, oil, water, solvent,
its a whole science about how the varnish bonds the graphite together to become conductive
try a varnish that says wash your brush in mineral turps not water and it should work
1:1-----2:1 varnish to graphite, so for example two tea spoons of varnish to one of graphite.
emergency blankets are a different game.

its not the same tin. But then again I bought it about 10 years ago. Its now in the dustbin.

it had a gey background.


those blankets should be ideal. I guess when you wrap them the surface cracks.


if you could find a reliable way to connect a tap wire to them stuck to a circle of card I guess it would be possible to get a repeatable results. As they are available everywhere.

[surfdetector]

Although I suspect the range increase is due to the homemade amplifier I fitted to increase volume.

Would you post more detail on your amplifier that you referenced?
Cheers

[waltr]

Wow Billr,
good you got the coil working and thanks for the explanation of how you did the graphite shield painting.

[billr]

Success !!

Koala. Thanks so much for your input. Apologies for not getting back to you sooner. I was deep into solving this bugger, couldn't tear myself away from the building board.
Before explaining what I did. I want to say a huge thanks to everyone that through out their ideas and opinions in the thread.
Except Kt for calling me a squirrel and posting cat videos . Only kidding fella.

So the problem was obviously shielding as many said. The question was how to find the right amount?
I've listed and worded the steps I did to help those like myself with limited knowledge get through the process easily. There is probably an easier way but this worked really well.
I resolved it by doing the following:

1.Separated the two coils but beforehand marked the coils with a sharpie at the contact points where they both overlap. I marked approx 1/4" either side of the area where the coils touched.

2.Painted with none thinned guitar paint those areas on both coils. When dry I wrapped some electrical tape over the paint so it insulated any contact between the two coils when fitted together

3.After re nulling and bonding the coils with hot glue. I removed the complete coil assembly and painted ONLY the bottom of the coils with none thinned paint using the side of a paint brush so as to not go too much onto the coil sides. When dry it was returned to the jig I had made. I did this so as to be sure no thin areas of paint existed in the shielding that where flat against the jig. See pic of previous jig made for other coil using aluminum foil shield. This was made to keep shape and fine adjust during testing.

4.The coil was connected to the PCB and a very thinned coat of paint applied to it all except the contact points at the intersections and the bottom resting on the jig
After the first coat dried a quick resistance check revealed 13K ohms on average at the highest point (next to the shield separation point) The area where the shield is broken. Range was very poor.

5.More coats of thinned paint added, each time whilst the detector was running and adjustments to the null point made to cancel out as much noise as possible and max out detection depth which increased with each coat.

6.Approx 3/4 of the way into the job the shielding dropping to 1.6K ohms at the separation point after all the paint was applied.
More fine tuning of the nulling was carried out to reach max range without distortion.

7. A strip of paper 1/2" wide x 4" long was coated with graphite facing up but not connected to the circuit. This was placed over the coil edge at the starting point whilst listening to the noise emitted from the speaker. In low areas of the coil paint the noise lessened when the paper was placed over it. High areas it increased. At low areas paint was added. The paper was moved around the entire coil shape and paint added where required. This brought the resistance down to approx 1K + or - a few hundred at each break point per coil. The result of which was almost a completely silent coil, with the occasional click here or there every few minutes. Better than sounding like a distress call from the Titanic as it did before.

GB was able to be adjusted to 1/8 turn from closed position. Range measured using a 1 Euro coin was a tad over 14". Although I suspect the range increase is due to the homemade amplifier I fitted to increase volume.

8.If you find you have gone too far and applied too much paint making it difficult to regain the shield balance. Simply remove a small amount with a tissue damp with thinners.

Thanks again all

[6666]

yep. Goes to show you can't take anything for granted. I have tried Ronseal outdoor varnish gloss and that's a fail. Just grabbed the tin. No idea why I kept it for 10 years. Going in the dustbin.


I tried and tried with those emergency blankets. Never could get them to work. I tried over a tap wire under a tap wire different numbers of layers of insulation. One side measures a short. One thing I didn't try was just a large circle. It measures a dead short on one side from memory. I bet that would work ?

Is this your varnish ? https://www.ronseal.com/for-home/doo...tdoor-varnish/
it appears to be water based, I think that could be your problem
there are three different bases, oil, water, solvent,
its a whole science about how the varnish bonds the graphite together to become conductive
try a varnish that says wash your brush in mineral turps not water and it should work
1:1-----2:1 varnish to graphite, so for example two tea spoons of varnish to one of graphite.
emergency blankets are a different game.

[Koala]

Its strange isnt it, with the pva and graphite total open circuit, but I use wood varnish and works great.

yep. Goes to show you can't take anything for granted. I have tried Ronseal outdoor varnish gloss and that's a fail. Just grabbed the tin. No idea why I kept it for 10 years. Going in the dustbin.


I tried and tried with those emergency blankets. Never could get them to work. I tried over a tap wire under a tap wire different numbers of layers of insulation. One side measures a short. One thing I didn't try was just a large circle. It measures a dead short on one side from memory. I bet that would work ?

[6666]

Strange? should do cheapest White none waterproof type. Not sure why its different in Oz ?

Goes on almost dry.


I put two square of copper on carboard painted over them and measured between them.


your not bothered what it measures at the surface. Some things I have tried the graphite sinks down and is higher resistance on the top


Someone suggested yacht varnish but that made a nice insulator

Its strange isnt it, with the pva and graphite total open circuit, but I use wood varnish and works great.

[Koala]

Not all PVA glues are the same I have tried whats available here in Oz , but they dont work with graphite

Strange? should do cheapest White none waterproof type. Not sure why its different in Oz ?

Goes on almost dry.


I put two square of copper on carboard painted over them and measured between them.


your not bothered what it measures at the surface. Some things I have tried the graphite sinks down and is higher resistance on the top


Someone suggested yacht varnish but that made a nice insulator

[6666]

right I know this works.

lets see if its a shield problem

cereal packet carboard, cut two circles

stick a piece of copper slug tape down on it about an inch square and solder a thin wire to it

mix graphite powder with pva glue until its its a bit like tooth paste

paint over the card and the copper squares. You have to work quick it dries almost immediately. You don't have to leave a gap.

place the First graphite down, place coils on top but insulated from each other. then place the other piece of card on top graphite side up

Connect the bits of card together and connect to the PCB ground

thats how I make my concentric coils and works well

Not all PVA glues are the same I have tried whats available here in Oz , but they dont work with graphite