Hammerhead Question

[Ray-NM]

Carl
I was looking at the timing on the HH yesterday and wondered about the Gate turn on time for the Main delay. If a PI dectector needs to have the pulse shutdown in less than 8 usec to really detect gold, then why is the main delay turn on time at around 10 usec's? I have what I believe is usable signal at about 6 usec maybe 7 usec. Also to cut down on noise what is the minimum gate width that I should be able to utilize? I plan on making a few resistor changes to decrease these times just to see what happens.
The times I show are from an old B&K 30 MHz scope I have had for 20 years so my calibration is probably way off and may be giving me some erroneous timings.
Ray NM:confused:

[Reg]

Hi Ray,

I will jump in here and try to answer this one. First in my opinion, the HH is or was designed to provide a good basic knowledge of PI's and not designed specifically to find any one type of metal such as gold.

Next, a true delay of 10 usec is still very short and is probably shorter than that of a Minelab PI. So, a delay of 10 usec will work quite well for finding gold. Also, it is extremely difficult to build a coil that will readily operate at 10 usec and allow full coil current that occurs on the HH.

Now, it is nice to be able to reduce the delay even more, but to do so most likely will require you reduce the coil current also. However, operating at 10 usec will allow detection of gold down to size or weight of a couple of grains or so as it is.

Getting the delay shortened down to something like 7 usec or so will allow the detection of even smaller gold, maybe down to a grain or so in size, or detection of some of the special crystaline gold that is generally invisible to PI's. It will also provide a greater signal of all other gold targets, but on the down side it will also increase the ground response signal, thus making it more difficult to use, especially with higher coil current. Now, reducing the delay is simply a matter of reducing a resistor value.

Like you said, your scope may not be calibrated. However, if you have the vernier turned off, then the settings should be reasonable close. However, the best way to determine if the delay is short enough is to simply test the detection of small gold. I would start with something like a solid 3 grain nugget and see if it will detect it at a distance of an inch or two. If it does detect this size of small gold, then you know you have a PI that is quite sensitive to gold. This is really better than simply trying to obtain a desirable minimum calculated delay.

As for the minimum sampling width time, my guess is you will see a reduction in sensitivity if you try to reduce this to much less than 10 usec or so. You could try to reduce it a little more but I would check the sensitivity to see if there is any decrease at that point.

Generally, you can reduce noise more by providing a good coil shielding and changing the pulse rate, rather than reducing the sampling time.

Reg

[Carl-NC]

Thanks Reg.

Ray, when I designed HH, 12-15us min sample delay was the norm for PI, so I stuck with that. In the few years since then, more effort has been spent reducing the sample delay. Also, as Reg said, below 10us or so ground decay comes in, and for those who surf detect (I actually intended HH to be more of a surf detector than nugget), saltwater comes in as well.

Keep in mind HH is a learning platform, so I intentionally kept it fairly simple and non-cutting edge. But the design can be easily altered, including a reduced sample delay. But then you will have to put a lot more effort in a faster coil.

- Carl

[Ray-NM]

Oh well

Thanks Carl and Reg for the answers. I did decrease the main delay by 2 usec and the result was absolutely no increase in detecting small gold. I did notice though that the gate pulse width was best if I kept it at 10 usec's. I think all my effort in the future will be the coil itself or just buy a good PI coil. Then I will have something to start from and work my way back to optimize the HH. It works just great on lead and a nickel but a 24 gr gold nugget just does not give much of a signal more than an inch from the coil. So back to getting that coil capcitance down even lower.
Thanks again.
Ray NM

[Reg]

Hi Ray,

I would think you should be able to detect a 24 grain nugget at least 6" to maybe as far as 10" in an air test, with a minimum delay of about 10 usec.

I wouldn't think the distance would decrease much if the delay was at 12 to 13 usec. I would think that one could detect it at a distance of at least 6".

Now, there are a couple of extra tricks that you can do to increase the depth including increasing the gain of the differential integrator stage and/or the autotune stage. This will help. Also, if you use an external amplifier for your audio, then you can increase the sensitivity via R 35 that will help also.

I would recommend you modify the HH to accept a DD type coil if you haven't done so already, or modify the unit so it has a form of ground balance. Otherwise, there will be serious issues with ground signals in the field when the sensitivity is increased.

Now, coil building is an art and there are tricks that work to allow one to reduce the delay. Also, there are component changes that help also. Changing types of wire insulation make a difference and/or coil wrapping design as does changing the FET to a faster one. Both can reduce the capacitances involved.

Also, there have been some pics posted recently by Woody that show windings similar to those done in the Chemloc PI. It appears this design has merit in both improving the speed of the coil by reducing capacitance and increasing sensitivity as a bonus. Unfortunately, using the single layer coil design, shielding becomes a bigger problem.

Care must be taken to use a decent shielding material also that doesn't create problems or cause excessive "signal" droop. This becomes even more important when trying to shorten the delay because one doesn't need the shielding to be detected and that becomes a bigger issue if exotic winding techniques are used. Now along the lines of shielding, I am about to test some rather strange material for shielding. My guess is it will work fine and can be installed in several different ways. The material of all things is a metalized Mylar used for reflecting light and is sold by hydroponics stores. One just has to make sure the surface is conductive. The order I received does meet my criteria. My guess is some brands may have a second layer of Mylar and may not have a conductive surface available, but I dont' know that for sure. The beauty of it is, the material is relatively cheap compared to some of the other alternatives.and at least the type I have appears to be invisible to a detector, but I do need to conduct more experiments to be sure. I bought mine off Ebay.

Reg

[Ray-NM]

Thanks for the suggestions

I had thought about increasing the gain in Differential Integrater, but have not done so yet. On another note I thought about the mylar from the packaging from a laserjet toner cartridge but it was not conductive at all. So nixed that idea. I think first I will get it to detect the small nugget from a reasonable distance then work on the shielding. I have a trip planned next month to do a little nugget shooting and wanted to try out the HH PI on it but will just stick to the VLF for now. It will find gold down to a 1/10 of a gr. I will get the HH PI to work the way I want it to eventually.
RayNM

[bbsailor]

Good source for mylar: mylar balloons

Ray-NM, Reg and Carl

Here are some tips for using a mylar balloon for a coil shield.

A good, common source for a coil shield is the mylar balloon. This is the type that is filled with helium and used to celebrate a variety of events.

Make sure that the surface is conductive by measuring with an ohm meter. With probes touching the mylar balloon surface, a few inches apart, you should get a low ohm reading of a few ohms.

A good way to apply the mylar balloon to the coil is to:

1. Measure the circumference of the coil wire bundle cross-section.
2. Divide the wire bundle circumference by 2.
3. Use this divide by 2 measurement and measure that exact distance from the inside edge seam and mark an inner circle to cut.
4. Cut the baloon as indicated in 3 above.

You will now have a pocket that will fully encase the wire coil bundle. To add a ground, just spiral wrap about 4 turns of bare wire secured with electrical tape. Continue wrapping tape around the entire coil to secure the mylar so it can not move relative to the coil. Make sure you leave a small gap where the coil wire leads enter and exit the coil.

My tests show that the mylar balloon is minimally detected at 8uS to 10uS.

Once the shield is applied to the coil, connect the coil to your PI machine and see if your hand is detected when it is placed on the coil housing. Your hand should not be detcted indicating that the shield is working properly.

I have also been playing with Scotch 24, a tubular wire mesh that when flattened is 1" wide. This too is minimally detected at low delays. See the following link: http://www3.3m.com/catalog/us/en001/...er/output_html

I hope this helps.

bbsailor

[Reg]

Hi Sailor,

Thanks for the info on the 3M shielding.

As a followup on the reflective Mylar I mentioned earlier in this thread, I found out that it will work fine if cut in a strip and used to wrap the windings, like what is normally done with lead foil or other foil tapes, but it is a little too conductive if used as a full solid piece the size of a 11" coil housing and the delay is at 10 usec or less. It is slightly detectable and could cause too much droop. Now, this large piece apears to work and appears to be invisible at something between 15 usec and 20 usec, depending upon its size.

Now as a followup, I did take 11" round large solid piece of the reflective Mylar which was about the size that would fit and cover the bottom of a solid 11" round coil, glued this piece of Mylar to a piece of thin plastic, and then made simple slice type cuts in the Mylar with a utility knife. Now, the Mylar is still one complete piece but now has thin slits, something like spokes expanding out from the hub. Now, there is no one large solid piece but all pieces or strips are still connected. When this was done, the stuff became invisible again when tested with the PI ieven below the 10 usec delay range. Care must be taken to make sure to not cut the Mylar piece completely and it is still 1 piece. but is not a fully solid piece now.

I did this with an elliptical shaped Mylar piece also. In this case, I glued it to a thin piece of plastic also and then simply made cuts in the Mylar perpendicular to an imaginary center line down the long side. I offset the left side and right side cuts to assure the Mylar piece was never cut in two, but was simply slotted on both sides. The result was the piece became invisible to testing at less than 10 usec also. I have attached a very crude simple pic of what I mean on the elliptical piece. The pic is crude for two reasons. First, I am lazy and second, the cuts do not have to be precise. They just have to break up the surface of the mylar film but not cut the Mylar shielding into multiple pieces. If the Mylar is cut into multiple pieces, only the piece connected to the wire will act as a shield.

Now, once the shield is made, it can be tested by passing it over a working detector. If it is still detectable, one can simply add more slits to the Mylar. They do not have to be as long as those shown in the pic, but just simple slits that break up any larger surface areas of the Mylar material.

The wire needed for connecting the shield to the ground lead had about 2" of insulation stripped from it and the bare portion was taped to the Mylar along the center line using a good quality tape. Now, if for some reason the Mylar film does get accidently cut in two, a jumper can be placed connecting the two pieces using a similar wire as what is used to connect the ground conncection to the Mylar tape. So, there is no need to panic if the film is glued and then accidently cut in two.

I plan on trying to simply glue the Mylar to the inside of the coil housing and making the slits while in place. This will simplify things. If the slits are close enough and the shielding is "invisible", then flexing of the housing shouldn't generate a false signal. However, it is a good idea to fill or at least coat the bottom of the housing with epoxy, two part foam, or some other lightweight solid filler.

The use of the metalized Mylar for shielding appears to be simple, reasonably inexpensive, and effective.

Reg

[Tinkerer]

Coil shielding

The following pictures explain the importance of shielding the coils.
Coil with 310uH, acts like an antenna and picks up electro magnetic waves from the environment.

# 1 coil with no shield and no cable
# 2 coil with no shield and no cable and with my hand on the coil
# 3 coil with shield and 2 conductor shielded cable
# 4 Coil with shield and cable and my hand
# 5 coil with shield and cable and ground
# 6 coil with shield and cable and ground and my hand

Settings: 5uS div and 200mV x10 div.

Tinkerer

[bbsailor]

Fast coil making tips

Here are some tips that when combined will make a coil that will operate at 10uS or less. These tips are cumulative, meaning that all of them combined contribute to making a fast coil. The invisible things, like dielectric constant of the wire insulation and coil spacer are important considerations. Household aluminum foil is NOT a good shield at low delays. Reg made some recommendations about good coil shield material. To test your own shield material, just ensure that the shield material is minimally detected at the lowest delays. If you use an unshielded coil to make this test, place the shield material on a stick to keep your hand away from the coil as your hand will cause a response near an unshielded coil at short delays.

1. The peak coil current is based on the resistance of the coil, the on-resistance of the MOSFET, and any other resistance in series with the coil such as a series resistor or wire leads. Lower peak coil currents and lower total coil capacitance allow faster sampling, so look for ways to minimize coil current and coil capacitance such as:
1.1 Use thin Teflon insulated wire such as AWG30 single strand wire that makes a thin coil cross-section bundle for 18 turns on a 10.5" diameter coil. This small cross-section contributes to making a coil with less shield-to-coil capacitance. My coils using AWG 24 stranded wire make much thicker coil cross-sections with higher coil-to-shield capacitance just due to the larger surface area of the shield circumference around the wire bundle.
1.2 Use a coil spacer that has a low dielectric constant such as Polyethylene (PE) instead of PVC. Look on the internet for 1/8" OD (1/16" ID) PE spiral wrap. This will ensure that the wires are held tight and the coil-to-shield capacitance is kept as low as possible. PE has a 2.2 dielectric constant, while PVC has anywhere from 4 to 6 dielectric constant. The dielectric constant is a multiplier meaning that if a PE spacer offers a 50pf coil-to-shield capacitance, then a PVC spiral wrap spacer will have about two to three times higher capacitance. Teflon spiral wrap has a dielectric constant of 2 and is very expensive so use PE as it is close to Teflon and much better than PVC spiral wrap.
1.3 Keep the coax between the coil and the PI control box short. A 30" to 32" cable length keeps the capacitance down. Look for coax cable that is about 25pf per foot. Each 100pf reduction can gain you about 1uS in coil speed. If you want to make the control box light, just mount the circuit board and controls in the box and place the batteries in a remote body-carried box. Use four conductor cable (two for battery voltage and two for headphone so the audio/headphone connection can be placed on the battery box so only one wire needs to be connected to the PI control box.
1.4 Use a low output capacitance (COSS) MOSFET (under 50pf).
1.5 MOSFETS with higher on-resistance are OK to help lower the coil current. I found an IXTP3N120, 1.2KV, 3A, 4.6 ohm on-resistance. This MOSFET has a little higher COSS than some of the lowest COSS MOSFETS I have seen. If you want to easily swap MOSFETS without needing to resolder these connections use a home-made MOSFET socket. Simply obtain from Radio Shack or mail-order, the inexpensive, flat-pin type 8-pin DIP socket. Using an X-acto saw, cut the socket length wise and the cut off one pin cross-wise. This will give you a 3-pin socket that will perfectly fit the MOSFET pin spacing. Once you find a MOSFET that works you can leave it in the socket or just remove the socket and install the MOSFET. This technique will allow experimentors to quickly try different MOSFETS and will save your circuit board. Do not use the round-pin sockets as the MOSFET leads will not fit.

2. Here are some coil winding tips that will make fabricating your coil much faster.
2.1 Accurately measure the ID of the coil winding so it will fit your coil housing when you add the thickness of the wire bundle (OD) plus the thickness of the spiral wrap spacer and shield.
2.2 Obtain 16 2" tall screw-in "C" hooks. Place these 16 "C" hooks in a circle on a piece of plywood so the the distance between opposite hooks equals the desired ID of the coil. Place a screw in the middle to hold the start and end coil leads.
2.3 For a 300uH coil, wind 18 turns, about 10.5" ID and while the wire is still on the hooks, add the spiral wrap. The actual ID using the 16-sided coil form will have slightly less diameter (about 1/8") than making the coil on an actual circular coil form. You can add the spiral wrap and wind it under the 16 places where the wire crosses the "C" hooks. You will need a length of 1/8" OD (1/16" ID) spiral wrap about 1.75 times the coil circumference to account for the spiral wrap expansion to fit the wire bundle.
2.4 Once the coil shield is added, the measured inductance of the coil will be about 10 to 15uH higher than the inductance without the shield.
2.5 For the technically curious, here are the coil parameters using the above method: Wire AWG 30, .024" OD, .1 ohm per foot resistance, 1.1 MHZ self-resonsnce (coil alone no shield, no coax), self-resonance with shield added is about 900 KHZ. When you measure the coil-to-shield capacitance will get a different capacitance value from each coil lead. This can differ by as much as 10 to 15pf.

If you use all the techniques mentioned above, you will make a coil that will operate at 10uS or less. The total TX circuit resistance will be about 10 ohms, considering the MOSFET on-resistance and wire lead resistance. If you are happy with the PI performance with your existing MOSFET, you can lower the peak current by adding a 4 to 5 ohm resistor in series with the coil. Just add this resistor between the coil connector (on the +12V side of the coil connection) and the circuit board connection point. With 10 ohms total resistance, the peak coil current will be about 1A and less with a lower TX pulse width.

A final note about about DD coils. DD coils require a separation of the RX circuit from the mono coil connection point by either: (1) connect the coil end of R12 to the new RX coil connection or (2) break the circuit board connection of R12 to the coil as suggested by Reg in a different post a while back. A 4-pin microphone jack and connector set are available at Radio Shack and works very well for this configuration. Only combine the RX and TX ground at the circuit board to obtain the best performance. Why is a DD coil better for speed? Separating the high TX pulse from the RX circuit by having a null or close to a null on the RX coil, IC6 (first amplifier) saturates less and will settle down a few uS sooner thus allowing a little faster sampling. The RX circuit will not see the MOSFET capacitance and will have typically a higher value of damping resistor compared to the TX damping value, increasing the sensitivity to small targets.

Please comment on these tips.

bbsailor

[Reg]

Hi,

bbsailor has added some great information on coils and building one that will operate at a short delay.

Now, one thing difficult to do is to add the spiral wrap. I have found it easier to add the wrap after it is removed from any device used for winding. However, it is difficult to get spiral wrap to go on smoothly and not have bunching of the wires unless the wire is held tightly before you try to apply the spiral wrap.

Now, I like to wrap or tie the wire windings snug using half hitch's and using a string that doesn't come loose easily. I will also tie tight knots as I go to make sure things do not come loose.

What I found works best is something I found on EBay and that is beeswax coated kevlar thread, or coated spectra. This stuff will stay tight much better than regular thread and is extremely tough, so you can pull it snug without fear of it breaking. Here is a link to the stuff I used.

http://cgi.ebay.com/Kevlar-Tex-92-So...QQcmdZViewItem

He also has the beeswaxed spectra which I have also used. It works well also.

Now, I have used the 30 awg teflon coated wire and normally use it for the receive winding on a DD coil. Unfortunately, the windings are quite small cross section wise and require a person to use 1/8 inch spiral wrap as bbsailor mentioned. I will use it and then add a second layer using the 1/4" spiral wrap so get the shielding a little farther away. The smaller spiral wrap is needed so the larger wrap isn't loose on the wire. All should be snug.

A person might want to try both a single wrap or the double wrap to see if they see any difference. When trying to get the delay down to less than 10 usec, a lot of tricks have to be used. I doubt one will see much of a change but there should be a little.

Unfortunately, it does take time to wrap each set of windings but that is part of the process and why some coils made by manufacturers are expensive.

On a little different note, I was talking to Bill Hays of Hays Electronics recently and he mentioned that during his testing of shielding on his Mark1 and Mark2 detectors, he used carbon based paint for shielding at one time and found it to cause some sensitivity loss. He said the effectiveness of the shielding was dependent upon the paint thickness also.

Now, I didn't conduct any tests at the time I tried the paint because I was not aware of any potential depth loss, but I didn't see any obvious problems. So, I have not verified this on a PI and it is something I should do when time permits. If anyone does check it out, I would appreciate knowing what they find.

Also, even though I had tried tinfoil, mylar balloons and even some mylar used in capacitors, again it was Bill Hays that put me on to the first reflective coated mylar I used with much success. It was a reflective Mylar taped on to sheets of foam insulation. This worked well in his detectors also. Unfortunately, it became difficult to obtain. One thing he mentioned was this reflective Mylar used on insulation sheets did not cause any sensitivity loss and did an excellent job of shielding to reduce the ground capacitance problem.

So, I went looking for a second source of the reflective Mylar material. It is surprising just how many different types of reflective Mylar there are out there and many of them are coated such that the conductive surface is under a plastic layer and impossible to get to. So, one has to be careful.

Finally, I sent the information regarding the type of reflective Mylar I tried that appeared to work to Dave Emery (Pulse Devil) and he wrote back about several of different types that I had not heard about. The type I found was coated on both sides with a conductive material. I am not sure just what the coating is, but it appears to work fine and not oxidize. Unfortunately, I have not had it long enough to determine if there are long term problems, but since it is used in hydroponics, I suspect it has been tested for a while. Now, to complicate things, Dave mentioned there are single coated types and ones with titanium coating, so now we have a wide range of different materials to try just in this one general area of shielding material. Its getting to be more fun but also more complicated to find the ideal material. Anyone trying similar material is welcome to post their findings.

So, thanks to Dave and Bill we have a few more ideas including one important fact and that is, we should check to see if the shielding causes any noticeable depth loss. Unfortunately, this is going to be difficult to do on a PI since the shielding does reduce the noise pickup, thus causing a sensitivity increase because of reduction in noise level. So, maybe we should start out with a shielded coil and then add a second shielding to see what happens? What do you think? If the shielding is transparent, then we shouldn't see any depth loss or other problems. One thing's for sure, I need a better workbench setup to do any really accurate testing along these lines.

One final note, Bill Hays informed me his going to quit making his Mark 1 and Mark 2 detectors at the end of the year. For those of you interested, he has posted the schematic and more information on building the coil for this detector. Now, he indicated that someone might continue on building the Mark series but that isn't confirmed yet. Bill said it is time to slow down a little. Well, what do you think, should we let him slow down. After all, he is only going to be 80 this year!!!!. Jeez, he is still a young kid, right?

I think we owe Bill Hays a big thanks for all of his contributions to the field of metal detecting and a special thanks for making the coil housings many of us use. He has been in the detector business for 54 years now.

Reg

[Ray-NM]

A lot of info here

You all give me a lot to think about and try, Thanks.
With a lot of fine tuning and watching the scope I have gotten the PI to see a nugget at about 2 inches. Now this is an irregular shaped nuget of about 24 grains. I dont have any that are really flat as I have been using a nickel for comparison on distance and signal. The detector sees a nickel just fine at several inches. I increased the the gain of the Diff/integrator to about 200 to see if that would help. It did just a little bit. I did not want to go much higher I figured that it would cause instability if I did. I think using all the tips in the last couple of posts will help me get a faster coil. If all else fails then I will either borrow from a friend of mine a Coiltek coil or buy one.
Thanks for the help.
RayNM

[Carl-NC]

Bill said it is time to slow down a little. Well, what do you think, should we let him slow down. After all, he is only going to be 80 this year!!!!. Jeez, he is still a young kid, right?

I think we owe Bill Hays a big thanks for all of his contributions to the field of metal detecting and a special thanks for making the coil housings many of us use. He has been in the detector business for 54 years now.

Reg

Wow, I didn't realize he was hitting 80! So this brings up a concern... Bill is the only source for coil shells right now, that I know of. I can't imagine that it's really worth his time to make them, so it wouldn't surprise me at all if he decided to drop them as well. Any thoughts on a new source?

And, yes, thanks Bill!

- Carl

[Carl-NC]

The gains I chose for the stages in the HH were based on bench measurements, in a fairly noisy environment. So on the bench, it's pretty chattery. But on the beach it runs fairly smoothly, so there might be some room to up the gain.

[Reg]

Hi Carl,

Bill informed me he still plans on making the coil forms, but you are right, we do need a back-up source. One bad fall and we are in trouble.

This is why I have decided to stock up on coil form sizes I think I will need.

Carl, you are right, he is not making any money on the coil forms at the price he is selling them for. He told me some time back that making them at a very low price was his way of paying back to the hobby that had been kind to him.

For those of you who do not know him, Bill Hays is truly one of a kind. he is a really special person who has contributed a lot over the years. Most of what he has done technically is behind the scenes type work. Most people do not know it, but he has made many prototype coils for some of the more sohpisticated and popular detectors in use today.

Personally, I am honored that he considers me a friend.

Reg