1st ever DD coil.

[hoadlies]

I always figured VDI was a gimmick and not accurate but then read where someone built one and
added it to his TGSL and could see targets before the audio went off. Audio has always been a
"less than ideal" area on the TGSL so I figured why not make a VDI and use it for audio?

I'm putting all the mods on the TGSL Quad now and then will investigate what a PIC might be
able to do to improve the audio on it.

The Silverdog boards are high quality but the spacing on the resistors is a bit tight. I didn't like
that I had to leave them tilted a bit above the surface. I ended up removing about half of them
with my mods anyway so no big deal. I'm waiting to see if he ever gets the IGSL boards back or
maybe some fancy PI on the order of QED or the SD2000. It's nice to start with a solid PCB...

HI,,S/D,,, Thanks for the link to the IGSL thread ''WOW'' a lot of info in there,,,i am half-way though reading all of that thread ..

YES the VDI would have advantages in that reguard ,,As I have an ELCHEAPO HA.Haar Chinese GC1006 , board looks A-Ok lots of chips & transitors & works reasonable [ & does discriminate] has 2x tones Plus an analog centred meter that indicated prior to sound !!!!..
The coil is an ''Omega-type'' , I feel it could perform better with a better DD coil ?????.I have converted it to use a 12v Gel-cell battery[I have plenty of those freebies from replacements on the local windfarms here] ..
I
The detector had 3x 9v batteries 2x for actual detector & 1x for the audio .. I fitted a 9v reg to supply the audio & altered a resistor for the detector circuit [circuit actully runs on 10.6v],,,TX -13.5khz..

I will make a coil for just to see if it can be improved, as an experiment..

OF course I will still build this IDX kit I have coming, & the TGSL later ?????.

QUESTION ,did you Mean to use the actual Mine-Lab coil $$$$$$$$.OR A diy copy of that coil ?????

I find your Centre tapped Coils idea's very interesting in your TGSL etc Quad THREAD ,Will be vert intersting to see your results once all is up & running ..

WELL MATE it very hot here in OZ at present High 30's & predicted 40+ next week ''WHEW''

CHEERS MATE,,,,,,,,,,,,BARRY..

[Silver Dollar]

I thought of getting one of those Chinese clones to play with but here they cost the same as the real thing right now!

The Minelab mod is a different transmitter circuit. It allows you to use coils wound to a different spec. You can use a
Minelab one or wind one yourself. Basically the receiver has more winding's for more signal and the transmitter is wound
with thicker wire and less winding's for more magnetic field. You'll find the info 4 or 5 pages into that IGSL thread.

[hoadlies]

HI S/D, & ALL ,,, THANKS for you help Catch you in the TGSL-QUAD thread..

All the Best Mate...................BARRY..

[hoadlies]

HI TO ALL ,, As I have not been able to get a TGSL PC-BOARD from SILVERDOG,, & wanted to get started on a DIY build I ordered a IDX=EDU KIT & its coming..

SO my original Coil former winder was for a DD coil to suit the TGSL,, But after reading the IDX thread I see I can use these dimentions to wind A coil for The IDX ..

OK i started with the TX coil using 0.7 Enameled wire 35 turns Gave me 1.7 ohms as per Intructions ,, My MH reading is close to instructions ,But not exact ,,
To Explain ..I have 2 meters that measure MH & both are reading differently my New one measures 0.57mH & my older one reads 0.62mH ????? & THATS not as yes potted ,Thats only tied with cord see pics..[I have the opportunity to remove turns before potting]
FRom what I have read here The mH reading may go higher once potted- [glued] any suggestion & or help would be appreciated ..

ANOTHER Query is Coil temperature as when I did the Measurements it was 42c here today ???? [does that affect readings??]

I dug-out my old Scope But will need some advice there as well ,Its an old unit [pictured below] single channel ..

DON has a message in the IDX thread of how to check the coil using a frequency signal & the scope But i do not relly understand all of it ???? I have my old signal-Gen & a counter that could help????

WELL ITS my first of many trial coils I guess ,But doing this as I await the IDX kit to arrive from the UK -to Sth-australia..
sOME PICS of my set-up ..

CHEERS ,,,,,,,,,,BARRY IN a HOT AUSTRALIA..

[Silver Dollar]

My take on the whole inductance thing is that it's a moving target so easiest to wind the number of windings
with the correct size wire and don't worry too much about it. The tuning cap will most likely be off a bit too
so a fine tuning with additional caps will bring it into tune.

On some of my coils the resistance was off. I discovered ordering by AWG does not always give you the same
diameter wire. It's like how a 1/2 gallon of ice cream is now 1.75 - 1.5 liters (hey boss we can save $0.03 by
making the diameter smaller but calling it the same thing..).

It is nice to have a 2 channel scope for checking the Disc and GEB positions but a lot of work can be done with
a single channel. Frequencies are low so most any scope will do.

You can check resonance by sending a sinewave into the coil and seeing what frequency the highest amplitude
occurs at. I don't have a picture handy. You need to isolate the signal generator and scope so your only looking
at the voltage in the coil. So take your coil and put your tuning cap across it. One end is ground of scope and
signal generator. The other end goes through a 10K resistor to the signal generator and through another 10k to
the scope (so the 2 resistors connect one end to the coil/cap end and the other ends go to each device).

Fire everything up and sweep the signal gen looking at the scope. Look for the highest peak voltage. Stop sweeping
there and read the frequency. This is the frequency that your coil/cap is tuned to...

I'd like to do this with my coils but I don't have a good signal generator handy..

[Valters]

Is some definition or ''best ratio'' between H and W, when making DD coils?

[Silver Dollar]

I believe H = W gives the most field strength...

[hoadlies]

HI S/D & VALTERS,, I appreciate the Help, especially the description from YOU S/D Gave ,as I am a Dummy at the scope !!!thats a good bit of info to get me started on the Scope-sig-gen operation ..I understand what you say about the ICE-cream HA,Haar yes we here in Aust changed to Metric a good while ago & yes the converions seem to be the benifit of the producers of products ..

INcidently the wire I used came from AN old oil filled 12v IGN-coil primary winding Theres enough in that of 0.7 to do this particular TX coil .the wire is Clean & some off easily [Just washed the wire in Kerosene & cleaned off with water spray & let dry-out
I have trouble locating wire here in Aust !!! It Mostly is listed on US ebay & postage cost is very high !!!

I will try-out your set-up & report Back here ..

I have read most threads Now in the TECH-Forum but there so much to remember I print-out what I feel is revelent ,But I am slowly getting used to the general Idea's & Jargon ..

I am a pensioner-here & at least have some time to spend Searching the threads, but my Budget is small HAAaar..

WELL thanks & have a good Day fella's ,,,,,,,,,,,,,BARRY..

[hoadlies]

Let me know if you get everything to work. Lots of help available here!
One word of caution with your kit. Check every transistor and make SURE you get them in right way around as pinouts can be different when they come from Europe!!!
Also, if you need a shell I can flip you one cheap. If you used the 27cm pattern your coils should just drop right in.
Don

HI DON I tried to PM you But Your PMS are full I was to query the Shell offer price postage etc ,,As said I live in austalia & you I believe are in the US ????

CHEERS MATE............BARRY..

[dfbowers]

Hey Barry. Try sending me a PM again. I had 116 in my INBOX with 200 allowed. It's empty now.
Don

[golfnut]

Hi Valters that is a good question. In general RF the ratio is 1:1 for best form factor, best inductance and losses .. Q. Bundling turns as we do here Is frowned upon in general wound devices from mains transformer design to VHF air cored coils.

Have a look at a Helmholtz coils where the field out of the end gets projected, by stacking spaced coils - maybe good for MD coils - although bulky

[golfnut]

Barry - from experience,,, steer away from IGSL as it can be a handful. That is a 2nd year project.. Steve

[hoadlies]

Barry - from experience,,, steer away from IGSL as it can be a handful. That is a 2nd year project.. Steve

OK THANKS G/nut ,, I will build this IDX EDU [As you recomended to me in another thread thanks]
If this goes Ok I will have a better Grasp of the way they operate, & go from there ..
BY what I have Read here the Coils play a big part in the efficiancy of the detector ..
I am at the stage of potting the TX Coil & hoping the super-Glue [recomended] does not affect the out-come figures ..
Question What is most important ?? The Resitance reading of the coil [In this case 1.6-1.7 Ohms ,,,OR the mH reading ????,The Resistance should not alter with potting????..
After potting ETC I was to wrap the coil with poly Wrap & I bought a roll of aluminium foil [self stick very thin for the farrady shields..

CHEERS BARRY..

[golfnut]

Bind the turns tooth floss is good. or the waxed whipping twine sailing guys use.)

Add a few drops of cyano to stiffen it up!. Dont flood as you may need to bend the shape later without fracturing the enamel covering the wire!!!


The L and R does not alter with the cyano.


The L goes up a few % when you bind it - as the turns get closer - but dont stress as the final frequency is not a big issue. Plus you can tweak the freq, with the cap value later if you feel the need to get 6.57688976kHz ( I run mine higher to see smaller items 12.5KHz Tx with no issue )



Wrap the coil post glue, in pvc insul tape, then foil with a fine drain wire then pvc tape again - Dont forget, foil needs a gap or the Mag H field wont get out. Make sure you have a shell that will take your coils when you got all these wraps on them.



Dont pot the coils in the shell untill you know it all works - as wasteful. I would say if you just put coils in a shell and potted you would have a 5% chance of success.!!!


Tack Tx coil in the shell with, say 4 small spots of hotmelt glue -- then place Rx - dont glue this one.


Location in room is critical - dont sit your coils/shell on anything with metal in it -- trust me



Put scope on Tx coil and trigger scope on that ch - you will have approx 16v peak to peak +/-4v and about 6.5 kHz not that critical 6.5 to 7kHz is fine
( I use 12.5kHz Tx no issue)

Dont worry if slight kink in the sine wave as all have this minor non linearity - (if you view the AC mag field with a clamp on, its much cleaner)

Put Ch2 on the Rx preamp output. You will probably see 1-2volts pp initially. Scale both traces so you see both plots as similar amplitudes on screen

Adjust the timebase so you see only 1 or 2 cycles on sreen - so you can see the phase offset between the 2 traces..


You need to do a few things together here

1)Move the free coil about in the shell, while viewing the scope

You will learn quickly the how the amplitude (and phase) of the coupled signal varies with its location in the shell.


You need to priortise the phase relationship over amplitude

Builders who have working kit say that the Tx and Rx need to be 180 deg out of phase. Ive seen a few, and this phase relationship is common to all of them. You have approx a 20 degree slip either way.


So you need this phase offset AND a low amplitude Rx signal - again fortunately the Rx signal level is not super critical. ( I got lots of small coins with a 2voltpp null one time)


Rule of thumb is 200mV to 300mV of preamp output - if its lower, good.


If you get your 180 degree shift and a reasonble null - progress to testing

If you cant get the Rx sig phse anywhere near, desolder the Rx coil and swap the coil connections over.
Re adust in shell for 180

If your good,

Make sure not in pin point mode..

try to reject ferrite - use geb pot to find cut off.

try to reject clean small iron/ steel ( not plated , or ring, or large) using the DISC pot (cut the head of a small bolt perhaps, dont use iron that may be magnetised!)


If reject ferrite and iron then try on gold bronze silver etc.

you should get all alloys other than ferrite and iron.


If sens to mid size coin is about 40cm, and struggling to reject ferrite - add 2n2 to Rx coil/cap combo - to move Rx tank down away from Tx freq.


these are the starting points I use. It can be a real pain - some of us like the fight. S

[Koala]

Lots of good points and tips for building and excellent coil. Thanks Golfnut for taking the time to post a step by step explanation.