Voodoo Main Discussion

[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

[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]

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

[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

[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.

[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

[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

[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]

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

[dfbowers]

Thanks Don, but having trouble seeing the meter movement, bit too far away, is that your home-made coil ?

One thing I noticed with Youtube. If you click on the gear symbol and change your video setting, you can change the resolution all the way to 1080p. That should clear up the video.
It took me a while to figure that out.
Don..

[6666]

One thing I noticed with Youtube. If you click on the gear symbol and change your video setting, you can change the resolution all the way to 1080p. That should clear up the video.
It took me a while to figure that out.
Don..

Thanks Don, did not know you can do that, will check it out,

WOW that 1080 makes a big difference, now on my 19 inch monitor with win10 PC, I can see the meter movement .
on my laptop 780 was as high as I could go before it locked up

[dfbowers]

Do you test with deep null or not?

I'm in the process of building my second concentric now.. One thing that Carl mentioned a while back but didn't pay much attention to is that many coils will null at a slightly different place at different frequencies. I find this to be true... With my coils, rearranging the fine tuning loop will bring things back into check if the frequency is changed. I think I nulled my first coil at 18kHz but it will run just fine at 10kHz too at reduced gain.

For example, the circuit I posted for my analog version is only running a gain of 5 for the preamp . I can easily increase the gain of the preamp to 220 by adjusting the fine tuning loop, and the disc channel becomes extremely sensitive. As long as the RX signal does not get clipped things are fine.. I don't think that kind of gain is needed for this project though as it's very easy to match the disc channel sensitivity of the PI section, even at low gain, which allows some flexibility. My fine tuning loop only needs to be move 1/4 inch or so to adjust the null from 18K to 10.5K. See attached..

Another thing that I didn't completely understand in the past is probably what I think is referred to as "null phase". By over coupling or under coupling it changes where the sample must be taken. It can change as much as 20uS because the polarity from the phase change will flip as it goes from under coupling to over coupling. It doesn't matter at all though as long as the sample delay can be adusted properly. With the TGSL project it didn't seem to matter, but here it does.

Also, with potting these concentric coils I find it useful to do multiple smaller pours as when things cure, small adjustment can still be made. About the 3rd pour I will finalize the position of the tuning loop. It all looks ugly at the moment but will look fine when finished.
Don.

[6666]

Thanks for the CC tip, what is the blue wire at the bottom of picture in the spoke next to the white object ?

[dfbowers]

Thanks for the CC tip, what is the blue wire at the bottom of picture in the spoke next to the white object ?

Haha.. You noticed. I went about 6 1/2 turns for the bucking coil until it nulled, and since it's an open center shell I could not return the wire directly to the ending point across the center . But.. I found out if I thread it through a spoke and double back same way that the main TX coil runs it has just a minor effect on nulling. The rest can be fine tuned with a small loop.

[SilverSurfer17]

On that post that i was coping that coil off of he said this For the tricky part - When the bucking coil is wound around the Rx coil opposite direction from the Tx coil, the frequency goes up by about 60Hz, so a turn or 2 will have to be added to the Tx coil to bring the frequency back down to 6.59kHz. After tuning and nulling, the inner ring is filled with slow cure epoxy.