Surf PI - successful build

[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.

[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]

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

[bernte_one]

hello Chris

give us a link to the Video with the small coil

[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.

[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, Yes I know r6 should be adjusted, but not been able to check prevents this. So for now I just have to leave this and experiment with things I can change. The bigger basket coil has the same amount of notches; I just scaled it up a little, I dont know if it will be any better or even work, but it gives me something to try.

I understand what you say about the hand test, mine was always being detected until I removed my watch haha.

[zelkos]

Hi Chris,

I'm not sure, but I think you're wrong...
For now, you're trying to find the optimal coil for your circuit.
Different coils have different resistance and inductance. Every time you change the coil, you should adjust R6 to get optimal performance. R6 is a damping resistor - 2W. Of course, for that you must have a working oscilloscope.
After adjustment you can compare coils, and see which is better.

By the way, how many notches is on base of your bigger basket coils at the outer edge?

I have a spiral coil as you, and this coil "sees" only my left hand, because I have a wedding ring ...

[chrisw]

Thanks for the test point godigit1, I put my scope on this but I couldnt keep the trace still. But the scope has been in the attic for 30 years so it probably duff. But I could catch a glipse of the waveform, and it seemed to look ok.

New video about my coils is now on youtube. http://youtu.be/bIGa3vhjd5I

[Qiaozhi]

You can set you damping resistor from pin 6 of the 5534 and Ive been looking at the flyback on the input side of R6. Im not sure why it is inverted yet.
[ATTACH]31673[/ATTACH]
Scope is set at 5us, 5 volts top trace, 5vx10 bottom trace.
Post #5 in this thread has some good info as well for set up and tuning.

http://www.geotech1.com/forums/showthread.php?21466-Coil-for-the-Barracuda-and-SurfPI

Hope that helps with the mess..

Looking at the scope image, I don't think the signal is inverted. I believe you've mistaken the overshoot as the inverted signal. The top trace (preamp output) starts off following the lower trace (TX) until the opamp rails out. Once it comes out of saturation, the signal again starts to follow the lower trace, but overshoots slightly due to being underdamped, and then settles at 0V. If the value of the damping resistor is reduced, you will see the overshoot decrease in amplitude, and the two signals will match (except for the point where the opamp is saturated).

[godigit1]

You can set you damping resistor from pin 6 of the 5534 and Ive been looking at the flyback on the input side of R6. Im not sure why it is inverted yet.

Scope is set at 5us, 5 volts top trace, 5vx10 bottom trace.
Post #5 in this thread has some good info as well for set up and tuning.

http://www.geotech1.com/forums/showthread.php?21466-Coil-for-the-Barracuda-and-SurfPI

Hope that helps with the mess..

[6666]

I bought the kit of ebay from Tacoma56, I saw him mentioned on one of the threads. £30. Shipping cost £19 though, bit expensive, but it was a good kit.

And yes, I was born in B'Ham, but live about 20 miles north of B'ham, little town called Cannock.

Thanks, its good to know what people are building and what works, looking for your new video.

[chrisw]

I havent tried it with very small targets, just my wifes small gold ring. On the spiral coil the depth for the ring wasnt too good but it was good with coins. But the problems I had with the spiral coil was it picked up my hands so I thought it would be too much trouble trying to shield it. So I have concentrated more on the mono coils, then my latest basket coil. I have got an old scope, but I dont know any test points to connect to. I did connect it to a few places but couldnt get the nice square waveforms everyone else shows. All my waves looked a mess.

[chrisw]

I did look on silverdogs website first, but I wanted the square board rather than the long thin one. Also I wanted the for pot one. I couldnt seem to find that. I will try to tone the accent down a bit ha.

[godigit1]

Nice job on the build and the coils. I bought the same kit and it performs vary well I think. I also have the hammer head Rev d Vco opt And straight out of the box with Carls cal settings and no mods yet. Both perform very close with only a slight edge going to the hammerhead as it seems to handles Emi better. You appear to be in a pretty quite enviroment, Or do you have the machine cranked way down for the indoors? I dropped R34 down to R100 It helped a little with the volume. I have a 4 inch coil operating at 8us (If Im measuring right) it has good sensativity and depth. Please do post your mod for the coils when you can.
What kind of sensativity to very small targets are you getting from your spiral coil? Say something like a 1/2 gram lead chunk or gold stud earring.
Have you been able to scope your fly back yet? The reason I ask is Mine seems to be inverted??? Im wondering if this is specific to the kit or I messed Up!
Mine spikes down at 250 volts. Must be inverted somehow. Seems to work fine though.

Thanks for the video and good luck with the rest of your build!