Vallon VMH3CS Mine Detector

[Ferric Toes]

Coil looks odd plus 7 pin connector. Could it be two coil.

Some time ago, I bought six Vallon coils and shafts on ebay. These were ones that had been scrapped by the Army as faulty. Four of the coils were good with the faults being in the shafts or connectors. One of the dud coils was cracked where it had been whacked against a rock, so that one I cut open to see what was inside. It is a single coil around the periphery with the crossbar empty. The wire is solid single strand with an outer mesh shielding, but I have yet to measure the wire gauge. The 7 pin connector has the centre pin as live to the coax core and all six outer pins are wired together forming an outer shield and connected to the coax braid. The thin coax is coiled like a spring within the telescopic shaft. A lot of thought went into the design of this mine detector and my first impressions are that it will make a good top end hobby unit. For normal use as such it is almost indestructible and totally weather and dust proof.

[Carl-NC]

Hi Eric, nice to hear from you. I recently borrowed a Scheibel MIMID mine detector and was surprised at the performance, especially given its narrow long coil. I was far more interested in the package design and I didn't investigate how it worked. Had to return it, but now looking to buy one.

[Koala]

Coil looks odd plus 7 pin connector. Could it be two coil.

[Davor]

Oh, I'm so glad to see you back.

I've tried the machine, briefly, but I was impressed. It is slick, and I like vibration a lot. Led indicator is a bit childish, but works reasonably well. I found it perfectly balanced. And coil is inspirational.
It is designed mainly for finding small brass ... thing, and many small coins are found instead.

If the principle is extrapolated from the known parameters, it would be as if a "normal" PI machine is on for 10us, but complete cycle is 100us. Very tight, but efficient.

It was thoroughly tested near Obrovac on difficult terrain. Not so much viscous, but properly saturated with iron (terra rossa, bauxite) and riddled with all sizes of rocks. It excelled.

My recent thought was to investigate short Tx pulses against viscous soil using my viscous soil model, and see how far the Paltaglou principle may be stretched. The opposite, a looooong pulse, may straighten the viscous response to ideal 1/t, but in all accounts it is energy inefficient. So there must be another way. Perhaps Vallon already solved it.

[6666]

Welcome back Eric, very interesting subject.

[Asgard]

Hello,

lt's probably a pulse balance induction... Technology not employed in gold metal detector today.

But we're working on !

Eric I sent you a message.

Alexandre

[KingJL]

No more tests tonight - my wife is frowning at me.

Eric.

I think I know that picture extremely well.

[Ferric Toes]

Inductive coupling between scope and coil.
Inductive scope probe.

Actually it was quite easy as there is a cover that you can unscrew near the coil swivel. Underneath are two pins that connect the coax to the coil winding and it was easy to get the scope probe on. Result; it is pulse induction as I know it. The drive voltage to the coil is a 6v rectangular pulse 50us wide on both polarities. At the end of each pulse is a back emf spike of 250V nicely damped. The mystery remains as to how they get high sensitivity to small nuggets. Fully GB'd on ironstone a 0.3gm nugget gives a good signal at 1in and just detectable at 2in.

No more tests tonight - my wife is frowning at me.

Eric.

[ivconic]

Similar debate already happened on forum recently.
Is it PI or is it not PI?
I wander what KT315 have to say on this?
Never mind actually.
But if it is "pure" PI; than for certainly it extends the common definition and understanding of PI technology, we are used to accept here.
That's why i think this is very interesting that you have chance to investigate it more.
Maybe it is time to explore this subject totally and give some proper answers, once for all.
And who is the best man on forum for such task?
Exactly the one who brought it here!
Go for it, this will be very interesting topic.

[Ferric Toes]

Vallon describe the principle as "Digital Magnetic Pulse Induction". All the considerable literature on mine detector testing that I have read, classify it as pulse induction. I have designed PI detectors that run coils of 1mH and above but they have lots of series resistance to improve the time constant and run at at much higher repetition rate i.e 5 - 10kp/s. I tried a Vallon coil on a 'normal' PI circuit and the earliest I could sample was 25uS.

Eric.

[ivconic]

It is 530uS between positive and negative pulses. I have done this so far by placing a small ferrite cored coil near the Vallon coil. I am going to try connecting the scope directly across the coil to see what the drive voltage and back emf is. Not sure how I am going to do this as I don't want to muck up a working unit. I bought it as untested but the only problem I found was that the coil connector had bent pins because the user appears to have got mad at it and forced the connector into an orientation it wasn't supposed to go. Luckily I straightened the pins without then breaking off. By the way, they use a 7 pin connector for a mono coil. I think I figured out why, very elegant.

Eric.

Inductive coupling between scope and coil.
Inductive scope probe.

[Ferric Toes]

It is 530uS between positive and negative pulses. I have done this so far by placing a small ferrite cored coil near the Vallon coil. I am going to try connecting the scope directly across the coil to see what the drive voltage and back emf is. Not sure how I am going to do this as I don't want to muck up a working unit. I bought it as untested but the only problem I found was that the coil connector had bent pins because the user appears to have got mad at it and forced the connector into an orientation it wasn't supposed to go. Luckily I straightened the pins without then breaking off. By the way, they use a 7 pin connector for a mono coil. I think I figured out why, very elegant.

Eric.

[ivconic]

It is probably a CCTX, a continuous current bi-polar TX, where the coil is charged very rapidly at high voltage and then the current maintained for 50us.

Similar thing crossed to my mind when i saw "1.5mH" is coil inductance.
It's not a PI at all.
Most probably it is mixed technology.
Examining closer their web page and offered pdf docs there; is impossible to find any useful data and tech specs!
Only customer related and usual stuff are there... boring.
I watched the video on their page (flash) and it is noticeable that VMH3CS is producing quite different audio (pitch and waveform) with different coils attached.
Is it because of different coil or is is because of maybe different targets?
Would be interesting to test it with different coils.

[Monolith]

Hi All,

I have just obtained a Vallon VMH3CS mine detector and am surprised at how good the performance is. I've not done any detailed tests as yet, but it will detect coins at great distances in air and a 0.3gm gold nugget at 2-3 inches. This is without ground compensation. With compensation switched in (mineralisation on switch) it will cancel a substantial piece of Australian ironstone. The machine is fantastically rugged and runs off three D size batteries, so it feels a bit heavy, but is reasonably well balanced. The search coil measures 1.5mH and is 3 ohms resistance. TX pulses are bipolar and 50uS wide. It is not immediately apparent how such narrow pulses are effective using a coil of 1.5mH.

I have been off the forum for quite a long time and would be interested to know if anyone else has experience with this detector. I shall take it to the local beach next week and see how it performs there.

Eric

It is probably a CCTX, a continuous current bi-polar TX, where the coil is charged very rapidly at high voltage and then the current maintained for 50us.

[Ferric Toes]

I have just had the Vallon up on the bench to try alternative coils. Result, it does not like them. I tried a 300uH and a 1.4mH and the unit gives an alarm to say something is not right. The resistance of the 1.4mH coil was 10 ohms so maybe that flagged a problem. With a mine detector everything has to be right, obviously. When you switch on, the detector goes through a check sequence before coming to life. When I get time, I will wind a coil to the exact spec. of the Vallon one and see it that works. Ideally I would like an 11 or 12in one as the standard truncated ellipse one is a bit small for what I want to do. Battery life, according to an army test, is 30 hours on three alkaline D cells. It appears to run just below audio threshold but with a short beep every few seconds to show that the unit is still alive. Display is LED bargraph and/or speaker/phones, plus vibrator in the handle. No excuse for missing a mine.

Eric.