Surf PI - successful build

[chrisw]

Hi Guys,
Havent been here for a while. I finished my surf PI and went on to have a go at the Mirage. Finished that one now so I was looking for a new project. I found the crossbow on Svens forum, so I have sent away for a board for that. I believe the designer of that; "George Overton" is on this website. By the way I just read the book "Inside The metal Detector", by George and Carl; excellent book I enjoyed that. The reviews I read about the crossbow say it is quite a deep seeker, anyone on here made one?
Chris

[Silver Dollar]

Well the initial noise was not a problem as it went away and allowed for searching.
(if surf is a motion MD then it takes a little time for it to store the "background"
voltage then it shows a change from that)

The shield is necessary when it sounds off when there is no metal around.

The copper foil needs a longer delay to ignore it as a target so yes you are loosing
small targets with the delay adjustment.

[chrisw]

I have a question about shielding. When I finished my coil I air tested it indoors. I was getting 14" on a two pence coin. The coil did not sound off if I put my hand near it. So I put the coil in a housing connected it to the metal detector stem and took it out in the garden. When I lowered it to the ground the detector initially made a noise and then the noise stopped. But worked fine, found metal ok. But if I lifted it up from the ground then lowered it it again made a noise initially. Should a PI detector do this or is it that the ground shielding is no good?
So I shielded the coil with graphite powder in varnish. The result was exactly the same. So I shielded the coil with copper foil tape. This stopped the detector working. So to get over this I increased my delay control to about 5 on a scale of 10. The detector worked ok but with a slightly reduced sensitivity. But I still get the initial noise when lowering my search coil to the ground. Why is this?
1) Is this normal, 2) Is it not normal and the shielding is not working and I am detecting the ground?
Also when the copper foil is wrapped on the coil, the detector doesnt work unless I increase the delay, so I assume the delay sort of causes discrimination against the copper foil. So does this mean all small objects are then undetectable?
It is a 10" basket wound coil.
Any ideas?

[godigit1]

Nice Chris,
Glad you got her down.

[multieagle]

[QUOTE=chrisw;199740]My next project is a Mirage PI from Sven in Canada. So just waiting for the board to arrive./QUOTE]

Good choice. Loads of technical assistance on his forum, and great personal support from Sven.

[Chet]

Hi chrisw
Great job on both performance a packaging.
Chet

[chrisw]

Hi Guys,
Ive finished. My objective was I wanted around 12" on a coin and good sensitivity on small rings. Well, with an english two pence I am getting 13-14" in the air. My small gold ring, about 10-11". So I am happy with that. These are in air tests indoors.
Outside is another matter. I tried outside and the detector found signals everywhere, I could find a piece of quiet ground. If I reduced the delay it helped a little. So I am not sure if its doing this because my shielding isnt effective or just because its mega sensitive and theres junk everywhere. In the end I think I upset it because I just got a continuous tone, (sensory overload ). Its ok on the living room floor; it found some funny signals but gave me a good depth with my coins. So I was wondering if there was some test I could do indoors to test if my shielding is working. I used carbon powder in varnish and wrapped a few turns of wire for a few inches then taped it up. It doesnt detect my hands so I dont know if this is a good enough test.
Anyway Take a look at the pictures. My next project is a Mirage PI from Sven in Canada. So just waiting for the board to arrive.

[godigit1]

http://www.treasurelinx.com/mirage_pi.html

Thanks for the link Multieagle.
I will have to put out a email to him and see if he is doing any kits still. Looks like a fun detector project.
Be well!

[godigit1]

Thanks for all the info Chet

I have a joey coil that been acting up Im thinking its the small ground wire you mention that is faulty. When I get return home form the road I just might tear into it.

That was a interesting bit of reading. I will give a spider coil a try when I can get some Litz wire and may try some with the 30 guage poly insulated I have.
The patterns that KRinAz made are great Thanks go out to him as well!!

BBsailors article has been my staple so far to my coils other than size, shielding and insulation methods. I have some that perform pretty good. With better wire thay might perform a little better. So far Im not able to make anything bigger than 6 inches that will pick up my 1/2 gram 14 k gold stud earing with any effictive depth. I would be happy with 2 inches deep on a 8 inch coil. I get it at almost three with my 4 inch coil and the 4 inch coil will hit a dime right at 7 inches Quarter is less about six in the air and my stainless drafting ruler at 14" Thats pretty good for a 4 inch coil.
Thanks again

[multieagle]

Hi Chris
I cant seem to find the mirage as a kit or a schematic. Can you provide a link it sounds interesting.
Thanks

http://www.treasurelinx.com/mirage_pi.html

[Chet]

Hi godigit1;
It is common for Commercial VLF coils to have multiple twisted wires in the coil cable. Cable capacitance at low VLF frequencies (2-50 kHz) normally is not a problem.

Most commercial Pulse Induction coils use low capacitance coax cable. The flyback pulse represents a frequency somewhere between 500 kHz to 1500 kHz. At these frequencies stray capacitance from the cable, the coil wire and shield slow the coil down.


Think of a slow coil effect as tuning an AM radio, you are trying to tune in a station at 1500 kHz but the maker of the radio had poor construction habits and the stray capacitance he caused will only allow you to tune in stations around 550-650 kHz.


I have repaired a few commercial PI coils for friends. In each of three manufactures; they used unmarked coax cables. They evidently don’t want reveal information or it is probably cheaper to not have the coax labeled.


Two of the companies used paper attached to the coils with masking tape. The shield/ground connection is made with a long very small bare held in place by masking tape then graphite painted the entire surfaces. Before painting the small wire was routed through the center of the coil through small holes in the paper and stretched across and held in place on that side by tape and paint.


The third company sprayed graphite paint on the inner surfaces of the top and bottom coil covers with the very small ground/shield wires held in place by the paint.


All three companies wound their coils on Styrofoam molded forms. The forms created approximately 1/8” to 1/4” space top and bottom and around the circumference between the coil and the shield. The space is to reduce the capacitance across the coil.


Air is the best dielectric so air gaps are good. Styrofoam and Teflon are good dielectrics. Dry materials should be used, glues and sticky tapes are not as good.


Your wrapping ideas will help reduce capacitance. The small center coax conductor is also good. It reduces capacitance; the small added resistance is not a problem. If necessary any added resistance in a coil circuit can be compensated for by a small lengthening of the charge time (transmit pulse width).


You should review the Basket weave coil in the; Coils Forum _ under Chance PI around Posts #110 -----126.


Also review;
http://www.geotech1.com/forums/showt...-the-MPP/page1 KRinAZ has some improved Basket weave templates in the last pages.

This paper by bbsailor / Joseph J. Rogowski is a must read for making fast coils.

http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=/projects/fastcoils/index.dat

Chet

[godigit1]

Hi Chris
I cant seem to find the mirage as a kit or a schematic. Can you provide a link it sounds interesting.
Thanks

[godigit1]

Hey Chet

"If the two coil wires are not twisted or shielded they act as two antennas. The interference hits them unequally. This produces a noise voltage between the wires that at times can be stronger than the weak Eddy Current signals from the target."


"If you twist the two wires together the external noise hits both wires at many opposing turns which creates opposing voltage polarities, thus self-canceling the external noise with little effect on the weak target signals."

Ok, Thanks This kinda gives me a little insight to a question I have been pondering and that is why the big brand coils do not have a shielded coil cable? Just a shielded coil and they use twisted wire that is twisted inside the cable.
When I tried Coax I was looking at it wrong and got the biggest center conductor I could find. Im also thinking that coil type and construction may benefit more from one cable type more than another. Say a fast coil with a high resistance may benefit more by a lower resistance cable than a lower capacitance attribute. Also dielectric material used for shield spacing how much capacitance your coil to shield allready has.
I wonder about spiral wrap beacuse of the air gaps in wraping It. Do those gaps effect anything ?
My Dialectric is a pain but I think it is good.
One wrap white teflon to tighten the coil bundle. One wrap of rescue tape but it ends up two or more thick and then a wrap of teflon mega tape then shield then another wrap of megatape.
What should a maximum acceptable coil to shield capacitance be?

Hi Chris,
As far as the Hammerhead Kit I got the rev d board from his site. It cost I think around $70. I built the option one and with no mods just calibrated per instructions it performes really close to the SMPI. I would give the edge to the HH as it seems a little quiter than my SmPro. Pluse with the mods you can do to it I think will be better. The board is tight and I dont want to riun it so I am real hesitante to do any mods to it. I have started making my Franken Hammer.
I have allready converted the Rev d to 4 seperate PWB's so I can mix and match or mod any part of the board. It also will be a hammerhead 2 as I am making seperate PIC board and dual front end board. I am configuring them for both 1 and 2 options. and from there I also intend a Uno shield.
End disire would be touch screen programming and three to five preset timing programs with Usb support and ground balance.
Currently I have biult it to the clocking and waiting on parts. So far so good. When it is done and tested I will put the boards on the forum for anyone that might be interested .
I have not looked into the mirage I will check it out when I get a chance.

Thanks!

[mohandes]

Hi Guys,
I have been following all these posts about the surf PI kit for months now and you all inspired me to have a go. So I bought a kit off the internet and built it. It all works great, and all the information about coil building was really helpful. I seem to have good depth now. I also made a vid on youtube. Just a matter now of making a shaft and fixing the box on. So thanks to everyone on this wonderful site.
http://youtu.be/LO6XYJ6td3U

hi
can you send here correct file and pcb .
regards

[chrisw]

Hi godigit1; chrisw ;
Shielded coax or twisted pairs are used to reduce external interference which the atmosphere is full of; radio, power lines, ignition noise, lightning from around the world, etc.


If the two coil wires are not twisted or shielded they act as two antennas. The interference hits them unequally. This produces a noise voltage between the wires that at times can be stronger than the weak Eddy Current signals from the target.


If you twist the two wires together the external noise hits both wires at many opposing turns which creates opposing voltage polarities, thus self-canceling the external noise with little effect on the weak target signals.


Coax cable prevents one wire from being exposed to the external interference. This prevents external noise voltages from being generated between the shield and the center conductor.


An important factor is capacitance per length. Twisted pairs of small (22-24) gauge wire with Teflon or thick insulation has less capacitance than large gauge wires.


Coax with a small center conductor with a low capacitance insulation is best. Stay away from any coax or microphone cable that does not provide a capacitance per ft. specification. For our purposes two conductors shielded has no advantage and would only increase the capacitance across the coil.


With either choice consider keeping the length as short as possible.


I use Blue Jeans LC-1 Audio Cable at 12.2 pF/ft, it gets close to matching small gauge twisted pairs. It is very durable and handles better than twisted wires. And it looks better. If you plan on making a number of coils it is cheaper to buy one long cable and cut to length as you use it. It does come with high quality audio phono connectors on each end.
Regards,
Chet




http://www.bluejeanscable.com/store/audio/

Hi Chet, thanks for that info that clears much of my confusion. I think I will try some twin screened cable and remove my coax because my circuit is a little noisy and see if that helps. Thanks for the link.
Chris