Voodoo Main Discussion

[SilverSurfer17]

That decaying signal from a target there is no way to slow it down or capture it or stretch it out to last longer ? Or make it echo

[SilverSurfer17]

Enough of my parts should be here today to get a coil going if i can find the correct cap out some ejunk i got sitting around ive already decided on my next coil build a 10 inch with a tx loop 10 inch of 100 turns of 20 gauge tapped every 5 turns the rx coil 5 inch 200 turns of 20 gauge tapped every ten turns im gonna see if a high voltage variable cap will work with lower voltage and mess around with like a 6 gang and id also like to build a rotary spark gap i think with all that i can test some theories from what ive read i know smaller wire more turns smaller cap better coil but tapping it would suck

[SilverSurfer17]

If you look at the last pic i posted it says around 500 turns the coil calc on here said 440 so it shorted me around 60 turns so either it is wrong or i typed something in wrong i don't know I'm just posting what i see

[Qiaozhi]

ok now i understand but that still means i did the calc right as far as i know and it didnt come out right this coil is 450 turns not 440 so it should of came out higher then i should of been able to remove turns to get it down to 35mh but its way below at 28.2 from what the calc said it should be with 10 less turns [ATTACH]54299[/ATTACH][ATTACH]54300[/ATTACH][ATTACH]54301[/ATTACH]maybe with all 3 pictures you will understand what i did wrong

Looking at your photos it is obvious why the inductance value is lower than you expected. You have left the windings loosely bundled. Once you tighten the windings the inductance will increase.

If you read Inside the METAL DETECTOR (ITMD), you will have a better understanding of metal detector technology in general. Also, Appendix A of ITMD describes the inner workings of the Air-Cored Coil Calculator software.

Inside the METAL DETECTOR - Second Edition - Published 2015

[surfdetector]

Well i figured out how to change the frequency by changing the cap or coil inductance so i guess ill just have to use different caps and see what works best

http://www.zen22142.zen.co.uk/Theory/tunedcct.htm

Since you already know what your resonant frequency is and hopefully your inductance just use the calculator to solve for C.

[SilverSurfer17]

Well i figured out how to change the frequency by changing the cap or coil inductance so i guess ill just have to use different caps and see what works best

[SilverSurfer17]

yeah its should of been more turns i dont think that coil calc is correct

[SilverSurfer17]

And my next question is can i remove some loops off the tx coil and drop it down the same amount of inductance to match the rx coil now and how do you know how to build a coil to get a certain frequency and is it best to match that to the detector frequency i sure hope you are writing all these down for your next book Metal Detectors for Dummies lol

[SilverSurfer17]

ok now i understand but that still means i did the calc right as far as i know and it didnt come out right this coil is 450 turns not 440 so it should of came out higher then i should of been able to remove turns to get it down to 35mh but its way below at 28.2 from what the calc said it should be with 10 less turns maybe with all 3 pictures you will understand what i did wrong

[MartinB]

I just noticed who ever made the diagram labelled it wrong. its DRS, but they make a machine called "Ground Exper PRO" which is what I search on, there is a lot of info on them.
Basicly with the coil, look at the marks, for any given TX coil, the RX length = the radius , the RX width = half the radius, so for an 8 inch TX the RX would be 4x2, for PI the TX can be 300uH, the RX can be 300-450uH, and adjust for IB .
For the Voodoo I will be trying TX 300uH, RX about 1.2mH plus cap and see if it will IB,
to print the ellipse shape I use https://www.rapidresizer.com/home/20...ne-shape-maker

Thanks 6666, really useful web site

[Qiaozhi]

In a concentric coil (for example) there are 3 loops - the TX, RX and nulling. The coil is the whole thing.
In a DD coil, there are 2 loops - the TX and RX.

[SilverSurfer17]

When you say loop you mean the inner radius of the coil being built right if so thats what i did so now that its 28.8 instead of the 35mh what does that change in the rx coil besides resistor and cap the frequency also now i dont know what to do to check the tx because i dont want to chop it off the roll i guess ill have to buy more 30 gauge and rewind it again with more loops and figure out how i got around 45 loops short from being where im suppose to be at

[Qiaozhi]

That was for the 30 gauge rx coil not the tx the tx is 20 gauge i think its in a 1/2 inch being off somewhere thats why i asked hes calling it a 8 inch coil but the tx is 7 1/2 and the rx isn't 4 inch its 3 1/4 Yours was the same in the book using 3 inch instead of 3 1/2 for that 7 inch coil why is whats the point of them being not what they are suppose to be and the coil calc where it asks for the inner radius is that the od of the coil form and the inner of what the coil is thats the only thing i can think of i did wrong 3 1/2 inches half is 1.75 inches converted to mm 44.45

The coil calculator requires the inner radius of the loop. It will then gives you the mean radius and the coil thickness. Obviously the outer radius is then equal to the inner radius plus the coil thickness.

[SilverSurfer17]

That was for the 30 gauge rx coil not the tx the tx is 20 gauge i think its in a 1/2 inch being off somewhere thats why i asked hes calling it a 8 inch coil but the tx is 7 1/2 and the rx isn't 4 inch its 3 1/4 Yours was the same in the book using 3 inch instead of 3 1/2 for that 7 inch coil why is whats the point of them being not what they are suppose to be and the coil calc where it asks for the inner radius is that the od of the coil form and the inner of what the coil is thats the only thing i can think of i did wrong 3 1/2 inches half is 1.75 inches converted to mm 44.45

[Qiaozhi]

[ATTACH]54293[/ATTACH] thats what i typed in and wound ten extra turns but ended up with 28MH ?

22 AWG wire is 0.6246 mm, not 0.254 mm.
If you put that into the coil calculator it gives an inductance value of 27.762mH, which is essentially 28mH!

20 AWG wire is 0.8128 mm.