Geotech Baracuda - REV-A - Coils

[Chiv]

I have started a Barracuda build so far so good with the bad shakes for soldering, I'm at the stage of building a coil to test at step 4, now with building my coils yep built lots of proto type for experiments (use the calculator) plus using the little LC100A tool for checking UH and PF. I am amazed at what sort of reading that you can get from loose windings to tight windings, from spider web coils to basket, even tried lan cable that I stripped to make 3pair twisted 10cm coil (even at 48 wraps could only get 105uh got quite fat that coil did). Now I have built a coil that is 556.9uh and 162pf from .4mm enamel coated, now if I elongate the circle to 70mm dia this bring 556uh to 476uh and 162pf to 149pf. ( to be expected ) Now If I make a figure 8 with this same coil I get487uh and 147pf, now the fun bit, if I squash the Figure 8 together and then cross the centres over to make another Figure 8 in the centre this drops UH to350uh and 105pf, now I have read in many places that a figure 8 coil self cancels itself out for emi in vlf (good for salt lakes) I'm new at this PI side of things so my question is; would a double figure 8 work on a PI set up, when you look at the lay out there is no real dead spots in the coil like a standed figure 8 coil as far as I can see, I'm interested in what other people have to say before I resin it together for a test when I finish this Barracuda

[green]

I have started a Barracuda build so far so good with the bad shakes for soldering, I'm at the stage of building a coil to test at step 4, now with building my coils yep built lots of proto type for experiments (use the calculator) plus using the little LC100A tool for checking UH and PF. I am amazed at what sort of reading that you can get from loose windings to tight windings, from spider web coils to basket, even tried lan cable that I stripped to make 3pair twisted 10cm coil (even at 48 wraps could only get 105uh got quite fat that coil did). Now I have built a coil that is 556.9uh and 162pf from .4mm enamel coated, now if I elongate the circle to 70mm dia this bring 556uh to 476uh and 162pf to 149pf. ( to be expected ) Now If I make a figure 8 with this same coil I get487uh and 147pf, now the fun bit, if I squash the Figure 8 together and then cross the centres over to make another Figure 8 in the centre this drops UH to350uh and 105pf, now I have read in many places that a figure 8 coil self cancels itself out for emi in vlf (good for salt lakes) I'm new at this PI side of things so my question is; would a double figure 8 work on a PI set up, when you look at the lay out there is no real dead spots in the coil like a standed figure 8 coil as far as I can see, I'm interested in what other people have to say before I resin it together for a test when I finish this Barracuda

I use two round coils, one inverted to make my figure 8 coils so I maybe looking at your example wrong. I don't see the difference between your double figure 8 and one starting out with twice the turns and half the diameter and twisting for a figure 8. You can test with the coil before potting. Just swing the targets on a wood or plastic ruler. Verify the ruler doesn't cause a response without the target.

[homefire]

I think your going to find the figure 8 coil also cancels out target signal on a PI.

[green]

I think your going to find the figure 8 coil also cancels out target signal on a PI.

You are correct. I was thinking about the null in the center and forgot about having to add an absolute value circuit to my PI, also needed when adding GEB.

[Chiv]

well my testing of the double figue 8 coil had come to abrupped end, Yep i let the smoke out on LT1054(U5) slipped with test pin belonging to the oscilloscope, now waiting for local shop to get a few in for me, time to build a few more coils for my vhf while I wait

[Chiv]

Have finished the board with the every thing in place, tested for volts and found negative rail only getting -4.7v is that a problem? In step 7 trying to get the volts to -2.7v can not make that voltage only -1.7v now I'm thinking it because of the -4.7v supply!! After reading forum over and over I have decided to buy a oscilloscope to help with this build as my automotive oscilloscope is not good enough to dial in my coils to the barra. the barra dose pick up sig with every thing connected but only 10cm on screw driver which I'm not to concerned about until I can hook up on the oscilloscope. my main question is- dose the -4.7v rail a real problem that need to be repaired so it is a true -5v

[Qiaozhi]

Have finished the board with the every thing in place, tested for volts and found negative rail only getting -4.7v is that a problem? In step 7 trying to get the volts to -2.7v can not make that voltage only -1.7v now I'm thinking it because of the -4.7v supply!! After reading forum over and over I have decided to buy a oscilloscope to help with this build as my automotive oscilloscope is not good enough to dial in my coils to the barra. the barra dose pick up sig with every thing connected but only 10cm on screw driver which I'm not to concerned about until I can hook up on the oscilloscope. my main question is- dose the -4.7v rail a real problem that need to be repaired so it is a true -5v

I'm guessing that you didn't read the part in the Build Document where it says:

----------------------------------------------------------------------------------------------------------------------------------
If you go through the following steps carefully, and in order, you will have a greater chance of
ending up with a detector that actually works. Please do not proceed to the next step until you have
solved any problems encountered in the current step. If you get stuck at any stage during the build,
then seek help in the Geotech forums before moving on. Do not simply populate the whole board;
discover it doesn't work (which will be the most likely result) and then post in the forums: “I've
built the Geotech Baracuda, and it's not working. What's wrong?” .... as you will receive little
sympathy from other Geotech members.
----------------------------------------------------------------------------------------------------------------------------------

I would advise going over each step in a methodical manner, and fix any issues you find before proceeding to the next step.If the power supplies are not measuring close to 5V, then you'll need to fix that first. An oscilloscope will very useful.

[homefire]

Emmmm Where do you get this -2.7 voltage from ? It calls for -2.1 and your -1.7 is quite close. Did you short PL2 ?

""STEP 7
Sampling integrator (see Fig. 10):
Fit R19, R20 (270k), R26, R27 (1k), C10, C11 (470n), C16, C24 (100n), Q2, Q3 (2N5485) and U7
(LF412A) .
Monitor U7 (pin 1) with an oscilloscope or a voltmeter, and confirm the DC voltage level increases
when a target is placed near the coil.
If you did not make the adjustment to R14 (Step 6) using an oscilloscope, then do it now while
monitoring U7 (pin 1) with a voltmeter. Set the DC voltage level to around -2.1V. Make sure you
attach your coil and short pin2 of PL2 to either pin 1 or pin 3 (before adjusting R14) otherwise the
voltage will not change.""

[Chiv]

thanks Qiaozhi and Homefire, MMM I don't know why I put -2.7v in my statement just not thinking to straight when typing this up,should have type -2.1v. When I was building the project I thought I was doing it step by step. every thing was right on the $$$ with the build to as written out in the pdf, Untill I stuffed up and shorted a couple of pins out on U5, so got a new LT1054. now I only get -4.7v now I'm not the bright spark in this type of electronics but step 6 lead me to step 7 with out oscilloscope, so with the voltage with in a few .V of where the preferred v should be lead me to think that is why I cant get -2.1v on pin u7 pin1 and I shorted out pl2 pin 2 shorted to pin 3, I can make voltage change not a problem just could not get the -2.1v and the instruction states, other than that the detector work quite well. hope this made more sense on what I was trying to get across

[homefire]

I use 1% resistors on all my projects and still have quite some variations at times. The Differences your seeing would be fine I think. March On.

[Qiaozhi]

thanks Qiaozhi and Homefire, MMM I don't know why I put -2.7v in my statement just not thinking to straight when typing this up,should have type -2.1v. When I was building the project I thought I was doing it step by step. every thing was right on the $$$ with the build to as written out in the pdf, Untill I stuffed up and shorted a couple of pins out on U5, so got a new LT1054. now I only get -4.7v now I'm not the bright spark in this type of electronics but step 6 lead me to step 7 with out oscilloscope, so with the voltage with in a few .V of where the preferred v should be lead me to think that is why I cant get -2.1v on pin u7 pin1 and I shorted out pl2 pin 2 shorted to pin 3, I can make voltage change not a problem just could not get the -2.1v and the instruction states, other than that the detector work quite well. hope this made more sense on what I was trying to get across

When you were following the step-by-step instructions (before you shorted something out) was the negative supply voltage at -5V?

[Chiv]

Yes it was at -5v , could of U6 have been damaged by the short? or C22 C23. I know in the automotive game relay and alike do burn contacts and give false readings to what they should be with age or too higher current, after a google to find how these (u6 U7 type of chips work) they are basically a multi switch of sorts. any way I like to problem solve I just have to wait for oscilloscope to turn up, as you all say it is a invaluable tool to tune and problem solver (maybe)

[Qiaozhi]

Yes it was at -5v , could of U6 have been damaged by the short? or C22 C23. I know in the automotive game relay and alike do burn contacts and give false readings to what they should be with age or too higher current, after a google to find how these (u6 U7 type of chips work) they are basically a multi switch of sorts. any way I like to problem solve I just have to wait for oscilloscope to turn up, as you all say it is a invaluable tool to tune and problem solver (maybe)

What is the voltage on U5 pin5 (LT1065)?

[Chiv]

Battery pack down to 11.5v U5 pin5 -11V...................so found new battery and now 12.8 U5 pin5 -12.1, now with the agm battery hooked up I get-4.9V and just to clear up all so get 5V on other rail, all so u7 pin 1 now get -1.9v so between the battery pack not so new battery's which they are have not lasted more that 1hours worth of testing at 12v, and the board likes a good clean supplied 12v has helped with the V situation

[Chiv]

just an update on this project as its all working sweet, good depth with the mono coil as in image with graphite shielding. 9kt mens ring in the beach sand 220mm .Have a flat spiral coil @250mm dia that pick up the gold ring @390mm in beach sand, so happy days. thanks to all who helped along the way to build my first PI detector. Now to silver dog in the new year for my next kit or may be kits