Vallon VMH3CS Mine Detector

[Rivers rat]

I like the white Pulsepower coil the O shape is just so simple but beautiful



RR

[Ferric Toes]

Yes, that part onto which the battery cap screws is badly out of position. I will post the info later today on how to position it correctly.

Eric.

[TH'r]

What is the problem with the battery cap? Did you get a test piece with either Vallon?
I have not found any difference in performance with the Vallons I have had, which is about 12. I always check them out with the test piece. Have you tried changing the channel (presume pulse rate) to minimise interference?

Eric.

As you can tell by the exposed fine threads, the metal battery holder protrudes about .25 inch past the plastic thread where the battery cap screws on. I know that it must protrude a little (as in my other Vallon) to make negative contact, but at this distance the rather stiff coil spring has to be greatly compressed for the cap to travel that extra distance in just engaging the plastic threads where it bottoms out after about two threads. You can also see from that ring of Afghan dust that it has been some time (never?) since it screwed all the way to the seal O ring. Resultant here is a cap that takes excessive pressure while twisting around in trying to get engagement with the first thread. Since you have had these detectors apart I was hoping you could suggest some adjustment. Thanks in advance.

[kingswood]

That coil looks great Eric!...I hope it works well when it is finished!

[Ferric Toes]

Hello Eric; My wife had both thickness of RW baking foil. I cut some 1" squares and will share the results with you in an air test on my two Vallons, switched on in normal mode. EMI has been very bad lately in the evening (maybe from the Ham radio operator down the street). Trying again at midday gives much more stability with just an occasional #1 LED and audio grunt. Vallon #1 (has some scuff marks from duty and a problem with the battery cap I wish to discuss with you in a later post), did much better on the Std. Wgt. foil. The audio would scream at .5" and give a good signal out to 1.5" then fall off rapidly until no signal at 2". Vallon #2 (looks almost new) would only give a modest signal at .5" and none past .75". They would both give a good indication on the heavier foil out to 5" where it fell off rapidly with #1 being slightly the better. It is also somewhat more sensitive to other larger objects, in an air test, and much more sensitive to the EMI in the evenings. I swapped coils with the same results. I cannot tell a major difference in performance in my test garden between the two on Civil War type targets. I like the power and simplicity of the Vallon for hunting farm land where even iron might be a desirable relic. Otherwise, F75LTD.

What is the problem with the battery cap? Did you get a test piece with either Vallon?
I have not found any difference in performance with the Vallons I have had, which is about 12. I always check them out with the test piece. Have you tried changing the channel (presume pulse rate) to minimise interference?

Eric.

[Ferric Toes]

Hi Eric


That 15” coil is a great looking setup. I am looking forward to your test results.


You previously mentioned that the coil resistance and inductance is critical. Do you think a spider wound or similar coil with less capacitance and/or less capacitance in coax would pass the internal checks?


Have a good day,
Chet

Many things about this detector are outside the accepted knowledge of PI. The coil wire is 0.53mm solid enamel plated, wound in a tight bundle. The Scotch 24 tinned copper wire mesh shield is not spaced off to any degree. The capacitance of the cable to the coil is about 300pF which together with the 2mH inductance gives a lower resonant frequency than what would normally be used for a 'fast coil'. All this leads me to believe that low capacitance coils may not work, and if they do, there will not be any advantage in performance.

Eric.

[Chet]

Hi Eric


That 15” coil is a great looking setup. I am looking forward to your test results.


You previously mentioned that the coil resistance and inductance is critical. Do you think a spider wound or similar coil with less capacitance and/or less capacitance in coax would pass the internal checks?


Have a good day,
Chet

[Ferric Toes]

15 inch search coil for VMH3CS

This is what the VMH3CS will look like with a 15in search coil. The standard coil is there for comparison and I will be surprised if there is not a significant performance gain with coin and ring size objects. As yet there is no coil winding, as I have to make a winding jig and I expect there will be quite a bit of fiddling around to get it to work properly once I have made and shielded it.

The rod end for the coil swivel is a standard Whites type with a sleeve to fit the bore of the Vallon shaft. This shaft is off a broken coil and is the only one I have, so there is a problem making more coil assemblies unless I can get more broken coils with intact shafts. Hundreds must have been thrown away but I am searching around.

I will have to take the coil coax out of the coil and back into the shaft, keeping it sealed of course, but that should not be a problem. I will use a piece of the Vallon curly coax, as I have some of that spare.

Eric.

[Teleno]

Another idea that I tried in the 1970's was to use a thyristor to switch the TX current. I had a 200V supply that charged a 2uF capacitor which then discharged via the thyristor into the coil. This gave a half sine wave of current which gave an increasing derivative as the current approached zero and the thyristor cut off. Measured peak TX current was about 90A.

I've done en LTSpice simulation of a sigle pulse from a capacitor precharged at 400V. The response of a 1us target is clear (IB coil).

At the end of the pulse the voltage at the capacitor gets inverted (from 400V to -280V). Don't know how to proceed in order to repeat the pulse, eventually reusing the energy in the cap.

[TH'r]

Last Friday morning I took the Vallon to a local beach for tests. It is not the best beach as it has quite a slope with little difference between high and low tide. A storm the previous day left the surf rough and generating a lot of noise. It also piled the sand up close to the cliffs. We went to the waters edge at first to test the effect of a wet beach. On the default high sensitivity, non-mineralised setting, the detector worked fine with no false alarms. The first signal dug was a small copper item shaped like a bead and about 4mm diameter. Various other small bits of metal, some lead fishing weights etc, but nothing significant. We retreated higher up the beach as the noise of the surf made it hard to hear the built in speaker. Headphones would be best in this situation, but the one piezo phone on a headstrap as supplied is not the best to wear. I need to fix up a proper set of piezo phones for the Vallon.
The next task was to bury coins and the first was a UK 10p buried in the wet gravelly sand at 10in. No doubt at the strong response from that, both with the stock coil and my 11in one. We then raised the head five inches and the 10p was still audible. The sensitivity was raised to max. and with my coil we got 17in and Vallon coil 16in.
Using a US nickel; we recorded 13in with Vallon coil on default sensitivity and 15in on highest sensitivity. My 11in coil gave an extra inch on both sensitivity ranges.

Back home the Vallon coil would not detect a 1in square of aluminium baking foil but would detect a 1.5in square at 8in. My 11in coil would only detect the 1.5in square at 3in. The detection of low conductors seems to fall off very sharply and seems more dependant on coil size than I have noticed before, with other PI detectors. It will however, easily detect a 4mm diameter phosphor bronze ball and a 0.3gm gold nugget with the Vallon coil but the 11in coil gives less range on those targets.

It is early days yet and I need to go to a better, flatter and sandier beach. Also, I need to compare the Vallon with a couple of my other detectors such as the TDi and Goldquest SS.

[ATTACH]36273[/ATTACH]

Hello Eric; My wife had both thickness of RW baking foil. I cut some 1" squares and will share the results with you in an air test on my two Vallons, switched on in normal mode. EMI has been very bad lately in the evening (maybe from the Ham radio operator down the street). Trying again at midday gives much more stability with just an occasional #1 LED and audio grunt. Vallon #1 (has some scuff marks from duty and a problem with the battery cap I wish to discuss with you in a later post), did much better on the Std. Wgt. foil. The audio would scream at .5" and give a good signal out to 1.5" then fall off rapidly until no signal at 2". Vallon #2 (looks almost new) would only give a modest signal at .5" and none past .75". They would both give a good indication on the heavier foil out to 5" where it fell off rapidly with #1 being slightly the better. It is also somewhat more sensitive to other larger objects, in an air test, and much more sensitive to the EMI in the evenings. I swapped coils with the same results. I cannot tell a major difference in performance in my test garden between the two on Civil War type targets. I like the power and simplicity of the Vallon for hunting farm land where even iron might be a desirable relic. Otherwise, F75LTD.

[Teleno]

I'm not convinced that's how the Vallon works. According to their patent, they switch in the damping resistor during a later part of the decay curve. If you play around with that idea in SPICE, and switch it in about half way down the curve, you can get it to sample at less than 10us. The hard part is getting it to work using real components.

The later you switch the damping resistor on, the larger the derivative and the closer that derivative is to the sampling point.

On the other hand, the later you switch the damping resistor on the lower its value has to be. At some point the decay of this resistor gets too long, cancelling the other benefits.

[Qiaozhi]

The first PI's in the 1960's used to discharge into a capacitor, otherwise the power transistor would instantly die. Then, epitaxial planar transistors came out that, with care, you could run in avalanche mode. Finally came the Mosfet.
Eric.

I'm not convinced that's how the Vallon works. According to their patent, they switch in the damping resistor during a later part of the decay curve. If you play around with that idea in SPICE, and switch it in about half way down the curve, you can get it to sample at less than 10us. The hard part is getting it to work using real components.

[kingswood]

The first PI's in the 1960's used to discharge into a capacitor, otherwise the power transistor would instantly die. Then, epitaxial planar transistors came out that, with care, you could run in avalanche mode. Finally came the Mosfet.

Another idea that I tried in the 1970's was to use a thyristor to switch the TX current. I had a 200V supply that charged a 2uF capacitor which then discharged via the thyristor into the coil. This gave a half sine wave of current which gave an increasing derivative as the current approached zero and the thyristor cut off. Measured peak TX current was about 90A. A bipolar half sine TX with energy recovery might be interesting to look at.

Eric.

I have no idea what this means, but I am in total awe of you !!
Its interesting what you wrote above about the decay of 10uS for the titanium screw yet the Vallon can still detect it......How do we explain that??

[Ferric Toes]

A normal PI discharges the coil on a resistor which causes an exponential decay. The derivative of the decay decreases with time.

Te Vallon discharges the coil on a capacitor, which causes a quarter-cosine decay. The derivative of the decay increases with time.

Targets with shorter tau respond to the derivative of the decay. Compared to a regular PI, for the same sampling delay the Vallon provides a derivative that's larger and therefore a larger signal for short tau targets.

The first PI's in the 1960's used to discharge into a capacitor, otherwise the power transistor would instantly die. Then, epitaxial planar transistors came out that, with care, you could run in avalanche mode. Finally came the Mosfet.

Another idea that I tried in the 1970's was to use a thyristor to switch the TX current. I had a 200V supply that charged a 2uF capacitor which then discharged via the thyristor into the coil. This gave a half sine wave of current which gave an increasing derivative as the current approached zero and the thyristor cut off. Measured peak TX current was about 90A. A bipolar half sine TX with energy recovery might be interesting to look at.

Eric.

[Ferric Toes]

Hi Eric


I totally agree that the shape and thickness affects the detection ability. The 0.65 gram nugget is a chunky D shape. Its greatest dimensions are 7.06 mm x 5.33 mm x 2.28 mm. I will test some more nuggets of different sizes later.

I am interested in your new 15” coil results. I have the following old coins for possible comparison. Please let me know if you have any of these since they are probably not in circulation. I just ordered a 1990 set of UK coins on eBay which should arrive within a week.


1920 Half Penny
1943 Half Penny
1967 Half Penny
1965 One Penny
1942 Farthing
1955 One Shilling
1959 One Shilling
1963 Two Shillings
1957 One Pence
1956 Six Pence


Have a good day,
Chet

I expect that I have most of those coins and some even older.
It will be a little while before I have the 15in coil up and running. I have the shell and swivel ready, but the winding is the tricky bit as if it is slightly out, in inductance, resistance, and possibly resonant frequency, then the detector indicates a coil fault and will not work.
Vallon make a 60cm (23.6in) coil for the VMH3CS but unless one comes up on ebay for £50 or less, I wont be investing in one for obvious reasons. In my estimation, a 15in coil is about right for maximum depth on coins and most rings, proved the area is relatively free of trash.

Eric.