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**Carl-NC** · 03-05-2004, 12:11 AM

Re: Hammerhead waveforms

If you probe the main output of the 4066 switch (IC7 pin 2) only the signal in the sample window should move up & down. The rest of the signal is ground.

At TP5 you should not see pulses, only a varying DC. If you are seeing pulses there, then IC8a is not integrating, check your hookup.

Next time I have the PI running on the bench, I'll try to photograph IC7-pin2. However, my noise problem will make it look crappy.

- Carl

**Ray Huffman** · 03-05-2004, 11:04 AM

Re: Hammerhead waveforms

Thanks for the info Carl. Your answers were what I expected. Now that I know for sure I can check out IC7. I thought the output should be steady except for the gate window. I have tried several 4066 IC's so its not the chip itself. Possibly a problem with my power supply? really not sure at this time will check it all after I get off from work tonight. "At TP5 you should not see pulses, only a varying DC. If you are seeing pulses there, then IC8a is not integrating, check your hookup." - This may be related to the incorrect floating of the output of IC7. Possibly a ground is not referenced to all the other grounds - a voltage drop, High resistance solder connection ?? After I fix that I may have something else to relate that you might find interesting.
Thanks Ray

**Ray** · 03-07-2004, 12:35 PM

Re: Hammerhead waveforms

Circuit seems to work fine now. I rebuilt the Vco section. The other had an intermittent noise problem that would come and go as I tapped the PCB. I resolderd all connections but it would not go away. Could be a bad film resistor or cap causing this. So I just rebuilt it.
I can detect a 22 gr nugget about 2 inches away now and thats with everything laying out on the desktop. I found out it was also detecting the lamp and my metal shelving which was a couple of feet away, I had the coil hanging in the air from some fishing line. So I need to box it all up and take it outside and see how it works.
My waveforms are just fine it was me looking at them at too low a level on the scope.
Let me know when and where to send payment for the new boards.
Ray

**Russ_NY** · 03-07-2004, 01:16 PM

Re: Fishing line

Ray
Thought I was the only one that did that with the fishing line. I have the same problem. to much metal in the shop area,, shelves, benches, duct work, etc. Anything of any size within about 3 feet and probably more. It may not show up on the scope but I bet it effects the circuit and probably causes the adjustments to be off a bit. My somewhat modified version reliably detects a dime at 12 inches in the shop. it was only about half that till I built a better coil.

Russ

**bbsailor** · 03-07-2004, 04:48 PM

How did you change the coil?

Russ,

What were the design parameters of your first coil and then your improved second coil?

How many external controls did you put in your design? Would you do it the same way again? What is the most critical control to adjust to get optimum results on your dime?

I used 50 feet of Radio Shack wire-wrap wire which has a thin isulation to slightly increase the coil wire spacing. My coil has a free resonance (only connected to signal generator and scope) of 500KHz (including shield), is 10.5" in diameter (to fit inside the Hays Electronics 11" coil form) and is slightly less than 300 uH. I used high quality audio coax with a stranded center conductor with a total of 100 pf for the 5.5 ft length. I am still in the middle of building the circuit board.

It is good to hear sucessful Hammerhead results. I suspect that once in the field, away from interferance and metal, you will get a little better results.

bbsailor

**Ray Huffman** · 03-08-2004, 11:54 AM

Re: Fishing line

Russ I agree about the adjustments being off. I think my threshold has to be set to high since it sees the other metal around me. Probably even my glasses as they have metal rims around the glass and that would be picked up very easily. With all of that it is no wonder that the gold can only be detected a short distance away. But I am still learning about all this so as I learn more the better it will be. I did use cloth thread to hang the coil but it broke to easily so I dug out some fishing line to replace it.
Ray(NM)

**Russ_NY** · 03-08-2004, 03:20 PM

Re: How did you change the coil?(Long)

bbsailor,
Carl put a tremendous amount of time and effort into his design, and continues to do so. I have no indication that the changes I have made to his circuit are any better than what he has done. I am also certain that his circuit uses less current. I like to tinker with things and also like to try to use what I have on hand where possible. I also like to layout my own boards so that I can have room to make modifications. I use a syncronous power supply to obtain the +5 supply. The TX pulse is triggered from the +5 supply oscillator. I also have additional series resistance in the coil circuit. This is the second prototype I have built and I have a third more compact board in the works. The adjustments as Carl describes them are all pretty critical for optimal performance. I consider this a learning machine and will probably make some of the normally internal adjustments accessable from the outside. The first coil was about 300uH and 8 inches in diameter, and the shield was applied directly over the coil windings. I also used RG59 for the coil cable. The latest coil is 23 turns of 26 AWG (7x34)stranded bare copper also 8 inches in dia. I salvaged the wire from a long length of 4 conductor flat telephone line cord. The measured inductance is a bit over 200uH. After winding I wrapped the coil tightly in teflon plumbers tape. I then wrapped the coil in spiral wire wrap. I don't recall where I got it from but it looks like a plastic tube cut in a spiral so you can wrap it around small bundles of wires to protect and keep them in place. I think this is a critical factor. It spaces the shield away from the coil reducing capacitance. I also put another coating of teflon tape over the spiral wrap. Everything must be tight so nothing can shift around. The shield is regular kitchen aluminum foil. I used a spray contact adhesive to adhere it to the spiral wrap. It is kind of tedious to apply the foil since it tears easily but you don't want it too thick at any one spot. Try to keep it as close as possible to a single layer, and be sure you leave a gap in the foil or it won't work. I left a small approx 1/4 inch gap opposite side from where the coil wires come thru. I then used a length of uninsulated tinned stranded wire to make an open spiral wrap directly over the foil. About 20 wraps total beginning on one side of the gap and going around to the other side of the gap. a couple close wraps at each end of the wire and a bit of solder hold it in place. I also used Mogami SVHS cable for the coil lead on the advise of Reg Sniff and it seems to work very well. To complete the coil you connect one end of the coil to both the wire that wraps the shield and the cable shield. and the other end of the coil to the center conductor of the cable. I use a plastic wire tie around the coil at the cable connections to help prevent flexing and wire breakage. Just leave the pigtail on the tie a bit long and heat shrink the cable and loose end of the wire tie together. I also then put yet another layer of teflon tape over everything. I have only tried this coil on the bench. Never out in the real world, but I think it will be fairly good. I hope this information helps.
HH
Russ

**bbsailor** · 03-08-2004, 04:08 PM

Re: How did you change the coil?(Long)

Thanks for your detailed response. Carl did a great job of making this PI project available to us. Those of us who have attempted or are attempting to build this project probably have more questions for those other brave folks who have actually build it. Thanks for keeping us informed.

You said" I then used a length of uninsulated tinned stranded wire to make an open spiral wrap directly over the foil. About 20 wraps total beginning on one side of the gap and going around to the other side of the gap. a couple close wraps at each end of the wire and a bit of solder hold it in place."

Comment. Be careful that you don't make a complete closed loop around the spiraled direction of the coil (about 3/8" dia). This could cause some currents to flow. I would think that you only need to wrap the spiral ground wire around the aluminum foil for a few inches and then secure with a tight spiral wrap of plastic electrical tape.

Eric mentioned that getting a thin shield is better for a quick coil. Household foil may still be too thick at very quick delays (if I remember correctly).

While ordering a sandwich recently at a local WaWa, I was served the sandwich wrapped in a foil lined piece of pre-cut foil-bonded paper. The I took the foil wrapper home, and removed a piece of the foil to measure it's thickness. It was about 1/2 thousandth thick, bonded to a thicker paper backing. This would make a good materiel to create a coil ground and not be thick enough the generate any currents.

Next time you go into a fast food place, look to see what kind or wrappers they use. Look for what I described. It measures 16" X 14". It also has the following numbers on the paper side: 602 12-27-03 (later numbers are probably a batch date).

bbsailor

**Russ_NY** · 03-09-2004, 10:14 AM

Re: Foil

bbsailor
Thanks for your comments. I used the spiral wrap wire over the shield because I was concerned about maintaining a reasonable connection to the shield. I think of it like a drain wire in a section of aluminum shielded cable. I think I understand your comment about not having the drain wire make a shorted loop around the cross section of the coil. I'm not really sure if I see that it would create problem since the shield does effectively the same thing. I mentioned once in Erics forum the possibility of using Gold Leaf as a shield material, which he had several problems with from a conductivity standpoint, and of course the cost. But they also make fake gold leaf which I understand is very thin aluminum colored in some fashion to look pretty. I wonder it that might not work. The entire shield issue has been rehashed several times on both forums, and I'm really not sure what the best solution is for the home builder. Aside from the conductivity of the shield material I see it as 2 issues, how does one apply it consistently for best results, and how to make a reasonable electrical connection to it. When I finally get a unit finished to the point where I can try it out in great outdoors is when the shield will really come into the picture. Family comittments have really cut into my bench time over the past several months and I hope to have something ready to try outside by the time spring arrives.
Thanks again and HH

Russ