Source of litz wire - which has allowed a novice like me to make a very good coil fairly quickly....

[davy74]

The latest of 3 attempted coils for my first project (Surf PI) & was made by winding 25 turns of Litz wire obtained from the center yoke of a CRT computer monitor that I was scrapping - this was bound with PVC tape then spiral wrapped & ovalised before being fitted into a 10" x 5 " crossfire coil housing, currently unshielded.
With the pulse delay set just above minimum or (raised) until it just stops chirping, the threshold set at a low constant tone & a damping resistor arrangment set at 400 ohms, I'm easily getting 10" air test on a large copper Victorian penny & 6.5" on a tiny 1/2 penny. I've also now dug a bag-load of (mainly junk) but a couple of coins, from ground I've covered many times with a VLF machine so it works in the real world.
I'm waiting on an LRC meter so don't know actual inductance of coil but it looks like I've landed lucky so far.

Anyway the point of my post is that this easy to find source of quality litz wire seems to have allowed me with limited experiance, to make what looks to be a fairly good coil, fairly quickly & at no real cost other than labour. The litz wire is dense (heavy) it will be interesting to measure L & observe response once I've equipped with a meter & scope.
Photos show the yoke taken from the monitor & the coil housing complete.
Separate post shows air tests etc.

[davy74]

A short way off the beaten track today at an old ruined farmstead dating from the early 1800's, set by the side of a gin clear tributary of the river Ayr, we put up a tarp, My bro got a wee fire going whilst I run through some more tests with my Surf PI build. Initially found it very noisy & full of chatter - had to turn the T.H down to minimum; but decided to dig signals which could be pinpointed by crossing with coil @ 90° regardless. I was again impressed to find that almost every dig resulted in an old rusty tent-peg, nail or staple - prior to one dig I was getting a consistent double beep & thought 'looks like two items right on top of each-other' - so I was impressed to dig two intertwined fence staples, noted good depth & signal strength on a couple of ring pulls & beer bottle caps, OK so what am I excited about, I mean it's not exactly treasure; but in two outings, at least the machine is consistently & accurately locating iron & trash. However - one problem I did notice is that whenever I lower the coil from a couple feet high toward the ground, I get a sharp rising signal response in every single location -as if the coil is reacting to the ground - so am I correct thinking this means the coil definitely needs to shielded ? Is it normal to have to run the threshold at minimum?

[davy74]

Shielded the coil today by spiral wrapping with the foil wrap taken from some profibus coms cable, below the foil is a layer of (1.5mm thick) nylon spiral wrap. I added the copper drain wire shown in the photo which apparently made the coil deaf but I soon figured out it had just slowed the coil down far too much to the extent I had to run the delay near max... so I replaced it with a single hair thin strand of tinned copper wire again connected to the co-axe sheath. This allowed me to greatly reduce the delay - to summarize; Prior to shielding the coil I was running the delay @ 270° (3/4) turns from minimum on the 10 turn 100K pot, now I'm running it @ 720° (2 turns) from minimum (on 10 turn 100k pot) - I'm quoting turns on the delay pot as opposed to µ-sec because as I don't yet have a scope - I'm working on trial & error here.
Result of adding this shield appears to be that sensitivity /target separation is probably not quite as good as before but I'm getting towards my goal of making the coil much more stable & eliminating the chatter & rising response I observed in the field test when coil lowered to ground, threshold setting is now also more usable. Need to get another field test in to confirm, & of course I need to get a scope.

[davy74]

Not so sure about it being a good coil now that I've just got an inductance meter today & measured the coil at 108µH, the coil definitely does work in both air test & in the field but it's inductance is way lower than the specified 300µH for the Surf PI. What is the likely effect of using a coil with such a low inductance?

[Tim]

Hi Davy. Yeah, I love my SMPro. If you have 25 turns I would guess your inductance should be 300uH or higher, unless it is thick insulation on the Litz (?!) If not, I suspect your meter is dodgy, also the detector probably wouldn't work at all at 108uH. My limited experiments with foil shielded PI coils has been disappointing. My first decent home-made was Cat5 end-joined (3x8=24) 8 inch, and had shiny powdered milk packet (mylarised plastic?) shield on the bottom, with no earth wire, as no contact is possible between the wire and plastic.
But because I bought my board completed,(tacoma56 ebay) it was already set up for a DD. So I bought a MLcommander 10x5DD s/hand for it.
This is basically the Apberg XX board with DD points and a 390R damper, and it absolutely hammers!
Soon I will get a few XX boards professionally made, with a view to further SurfPro diddling. I want to make it VSAT for a start. This would help with ground signal from an un-shielded coil, and help to compensate for our brutal mineralisation here, I hope. A fast coil may allow for delay circuit adjustment to further ignore iron, and a variable/ higher frequency(adjustments around 555) for even finer gold(?).
...Early days with these plans though.
A while back I got some 24AWG tinned copper multi-strand-supposed to be comparable to Litz- but have had no success yet. The coil I made was 420uH for my Bara. Probably too high in retrospect.
But this spiderweb coil-6 inch 260uh is excellent on my 1.7g flour gold sample-about 4 inches on my Bara and NewhobbyNH-5(Attiny controlled). .5mm enamelled wound on 2 layers of shiny cardboard. Very easy to make. Be sure to have an odd number of slots, double-cut to .5-.8mm with sharp scissors.
Some points for you to ponder....

[homefire]

Dismantled at least 8 TV's this year after the DeGause Coil for wire. No Litz Wire found.

[hecbom]

G'Day Tim I just received the same board you have from (Tacoma). I became aware of this board a while back after reading one of your post. I have several coils I made (around 300uH to 340uH @2.2 ohms) and they seem to be working well. I have a commercially available 150mm (6 in) Coil from Miner John which is a Razorback coil. This works good and it is a folded mono design. I was wondering if you have tested a mono with your board and if so, how did it perform. I am also interested in the details of your Minelab 10X5 DD coil and how did you hook it up to the 2 connections on the Tacoma SM Pro. I am assuming you had to attach the Tx and Rx separate? or did you jump the 5 pin connector?

Thanks Mate!

BTW I lived and worked in Melbourne and Adelaide back in 2008!

[Tim]

G'day hebcom. It looks like Tacoma has gone away from his DD mod then-pity. I used the ML style connector, socket on board connected as per ML original. When using a mono coil, it is already hooked up in the plug as per below. I bought an 11 inch coiltek mono s/hand as well. But caveat emptor- it was a DD also! So I am still to obtain a factory mono for testing. From memory 2R2 might be a bit high(?) for the Surf. Tacomas board seem to be an Apberg XX copy In the pic below(sorry, blurry) the second coil connector point is near the FET. The TX and earth are obscured by the black/red wire. It is just a matter of removing jumper from R12 to FET(best to score around FET land leaving enough trace to solder pin there) and drill/solder the Rx pin to it. Most of my home-mades are AV plugs on the coil, so I have an AV-MLmono adaptor lead.
Although a bit fiddly, this will allow you to use DD's and monos.