Surf PI - successful build

[godigit1]

If you want you can still adjust R6 without a scope. Its not perfect by any means but you adjust to best performance.
Using a resistor setup like in post #10 of this thread> http://www.geotech1.com/forums/showt...mping+resistor
That being said best performance for me is not perfect damping on the scope. Slightly underdamped as in the picture is where Im getting my best sens to small gold with the coil in scope picture.
I also am having better luck with coils in the 300 uH range and thinner wire for this kit .
I set up R6 on my board with D-sub connector pins for easy changing of the resistor and hook up of the adjustable set up.
What is your voltage at the coil connector If you dont mind me asking? Mine is at 40MV for this coil. I have tested higher and get a tiny bit better depth but pick up too much noise and sens to my ground sample . At this level I can set a palm sized peice of magnatite between the 4 inch coil and the amp does not saturate and it can still get a 1.2 gram nugget at 4 inches. The magnatite actually quiets the coil. Im wondering if the extra wire is in effect doing the same. (Partially damping the coil?)
As far as smaller coils being deeper in some cases this can be true as they see less soil volume and in hot ground or ground with allot of small iron targets may see deeper due to less interfearance. At least thats they way I understand it works but in the air, tests should still be true to expectations. Bigger coil more depth les sens.
What a great hobby....

[chrisw]

Hi Godigit1,
Sorry for the delay in answering I have been out of comission for a while, car problems.

I find I am more confused than ever now with these coils. The larger coil I just made gave good depth in the workshop, but if I put it on the floor outside it just doesnt work, so I this it must be picking up the floor. If I try it in the workshop and I put a small coin in the coil field. I thought it should still work being as the coil or the coin are not moving. But it doesnt work at all.

I managed to get my scope working a little and I managed to get the waveform perfect by altering the load resistor value. This made the depth better but it gets a little chattery now. With this load resistor, if I attach the smaller coil, the waveform isnt right but the coil works outside when I put it on the floor.

So now I dont know whether to make the waveform right or wrong, for the thing to work. I tried shielding the larger coil with a bit of foil then it didnt work at all. So I am reluctant to spend money on the copper tape that some recommend ( scotch 24), as its expensive. And I cant find any graphite here in the UK so I dont know what to try now.

By the way the voltage at the coil terminals is 35mV. I saw a good small coil on youtube with the swinging coin at a fantastic depth but there is no info on how he did it. I will have to keep trying.

[bernte_one]

hello Chris

give us a link to the Video with the small coil

[chrisw]

Hi Bernte,
This was my first vid; http://youtu.be/LO6XYJ6td3U

[bernte_one]

another queastion
what is a fantastic depth for you? thats not silly meant by me its a serious questions
http://www.youtube.com/channel/UC0FCYxDDMgOdhd8UOE0yWvA

take time and watch all Videos which contain a surf pi it is free from ads, i got no Money if you watch my vids

and than look this
http://www.youtube.com/watch?v=-jsbQ11ZBQE


i am sure you can pump the Performance up to 35-40 in air
but in real surface i think it is really good for surf pi if you get 25-30cm on a 2€ coin which lays flat

[chrisw]

I would say that fantastic depth for a little homemade circuit would be about 12 inches on a small coin or thin ring. I have had VLF detectors all my life and they have never given this type of depth. If I was looking for 40 gallon oil drums then I would think this little PI kit would give a depth of three or four metres.

[chrisw]

That second video that you posted is great depth and only a small coil, I would like to know how he did that but cant get any information.
How do I pump up the performance to get 40cm, I would be very happy with that.

I am still having trouble shielding my bigger coil. If I put foil round it , it stops working, so I think if I bought scotch 24 copper tape I would get the same effect so I dont want to waste my money. But if I place the coil on the paving slabs anywhere outside it just doesnt work at all, so I think it is detecting the floor. Also something that confuses is if I place a small copper coin close to the searchcoil it doesnt work with any other targets till the coin is removed, why is this? If the coil and coin are stationary I thought it should be ignored. The reason I tried this was I was going to use a steel bolt to secure the coil housing to the shaft, thinking that if both were stationary that the bolt would not be detected. So I will have to look for strong nylon.

It seems my knowledge about metal detector design is very lacking.

[godigit1]

Hi Chris,
It sounds like your coils may be fine but you are getting overload when metel is too close. The concrete outside may have rebar reinforcement. My coils do the same thing, overload and just go silent whereas my VLFs have a ring tone for this I prefer the silence!! If you check a clean area outside with your VLF you can sit the coil down on the ground and it should be ok, otherwise?? Ive been playing with copper shieldind a little and it works great but my method is pretty ruff and I am restricted to a minimal coil to shield spacing. I am lacing my coils into the shielding from my coil cable.
For the spiral coils Maybe if you cannot find carbon paint some conductive mylar (space blanket) might work. I have not tried the scotch 24 it looks a little thick on the guage.
Here is another way to experiment with a similar method. http://www.geotech1.com/forums/showt...-shield-method
This is a picture of what Ive been playing with for shielding. Fast coils wound inside of the shielding, .
Hope you get it figured out.

[chrisw]

Its strange, I just now went outside to the slabs outside and now its working ok, but maybe a little less depth, strange. I like your shielding idea I will give that a go, thanks.

[chrisw]

Hi Guys
I cant seem to get any shielding working. I tried first of all wrapping with some copper braid stuck to tape, but it was very thick and didnt work. Then I tried wrapped with foil. The waveform is below. But the detector made no noise at all. The only way I could get at working again was slowly changing the load resistor. It seemed to work ok but had a crazy ringing waveform. But this was the only way I could get it working. How can I shield this coil and get the proper waveform and depth?

[multieagle]

Maybe off base, but are you grounding the shield?

[bernte_one]

use graphit paint for shield, and how it is been told everywhere connnect the shield too

[godigit1]

Hi Chris
Also what is the coil to shield spacing? Capacitance between coil and shield? Are you leaving a gap in the shield? when I use the copper shielding it is from a 99 ofc free microphone cable and the weave is very fine. It has two wires and a shield wire for independant grounding. In the kit instructions it is stated that only solid core cable will work. This is not true. I had problems with coax. It is better to join the shield to the ground in the box. I drilled a new pin in for this. Also when I attach the shieldind I fold the wire over itself and use shrink tubing to lock it together. If I solder the shield at the coil I loose sensativity( coil goes dead ). The solder joint becomes a target and nulls my coil. My coil to shield capacitance is .31 nf I am using both teflon mega tape and silicon rescue tape as the insulator but the rescue tape has to be wraped tight so it might not work on a spider coil.
Hope you get it figured out. Oh Yea Just a thought on the foil are you cross hatching it before you wrap or using solid foil? See the hammerhead article how Carl preps his foil. That might help.

[kenny444]

Hello regarding the Surf PI and its hardware, I mounted mine a Project box with all the controls at one edge and the coil input connect close to the end but on the side, so I can use a right angle plug to keep it all compact, I also think I shall use it in a bag or on a belt mount to reduce swinging fatigue. As for the search coil I bought a cheap cake icing spinner and used that to house the coil. I do hope this image is here and useful.

[chrisw]

HI Guys,
The coil to shield spacing is two wraps of spiral wrap; I dont really know how to test the capacitance. But I have always grounded the shield at the coil not higher up, I dont know if this makes a difference. When I have used foil it is just normal sheet cooking foil which I have cut into strips. I cant seem to find the article where Carl preps his foil. Anyway, I have found and ordered some cheap graphite powder off ebay. I read somewhere about mixing this with varnish and painting it on, so I will give that a try.