Vallon VMH3CS Mine Detector

[green]

This is getting way off topic, but OK, it is relevant in many ways and would no doubt improve the detection of mines with only a tiny amounts of metal.

I couldn't resist getting out my ultra-fast PI and taking a picture of the response. The coil is 5.5in diameter, 330uH and wound with Teflon insulated 0.25mm solid wire. Resistance is 5 ohms and the coil is shielded with copper fabric tape spaced from the coil with polythene spiral wrap.

TX pulse width is 12uS, delay 4uS and 1st sample 4uS. pulse rate 25,000. Coil switch on TC 10uS. [ATTACH]37137[/ATTACH]

Included in the the damping circuit is a cermet preset resistor for fine trimming.

This only works at such a short delay if there there is no cable to add extra capacitance. With 1m of good cable 5uS should still be attainable. Alternatively, the inductance could be dropped to, say, 200uH.

Eric.

Nice looking trace. With a mono coil mine looks closer to Teleno's. Would you mind listing peak coil current, amplifier gain, amplifier frequency response, damping resistance, coil self resonance and anything else that might help us get closer to a perfect decay?

[Ferric Toes]

This is getting way off topic, but OK, it is relevant in many ways and would no doubt improve the detection of mines with only a tiny amounts of metal.

I couldn't resist getting out my ultra-fast PI and taking a picture of the response. The coil is 5.5in diameter, 330uH and wound with Teflon insulated 0.25mm solid wire. Resistance is 5 ohms and the coil is shielded with copper fabric tape spaced from the coil with polythene spiral wrap.

TX pulse width is 12uS, delay 4uS and 1st sample 4uS. pulse rate 25,000. Coil switch on TC 10uS.

Included in the the damping circuit is a cermet preset resistor for fine trimming.

This only works at such a short delay if there there is no cable to add extra capacitance. With 1m of good cable 5uS should still be attainable. Alternatively, the inductance could be dropped to, say, 200uH.

Eric.

[crane]

Sampling that portion of an under damped waveform should result in a significant response from a lump of soft ferrite or non-viscous magnetite. Probably doesn't matter though if the detector is only used on non-mineralised beach sand.

[Teleno]

It has been noted in the past that using a slightly under-damped response can lead to increased sensitivity to low conductivity targets, and your results seem to indicate that this is true. Have you noticed any undesirable side-effects from this setup?

There's a background growl in salty wet sand, but objects still cause a distinct beep.

Sensitivity is so increased that it can detect an axial resistor with its leads cut short. I tried a 1cmx1cm piece of aluminium foil and it was detectable at 2 - 3 cm.

[daverave]

i under damped my last detector coil like the waveform above and i got better sensitivity to small items with no adverse effects

[Qiaozhi]

Hi George,

I finally got the time to run the scope.

Blue line: main sample pulse (TP7)
Yellow line: preamp output (TP3)

I'm sampling at 5.5us just as the preamp is recovering from saturation. Tx coil is slightly underdamped.

I've been adjusting R1 and the minumum delay for the best sensitivity to a test gold chain.

It has been noted in the past that using a slightly under-damped response can lead to increased sensitivity to low conductivity targets, and your results seem to indicate that this is true. Have you noticed any undesirable side-effects from this setup?

[Teleno]

I'm sure you've already checked this ..... but I have to ask:
Is your coil actually capable of being sampled at 6us, and it's not just sampling while the opamp is still saturated?

Any chance of posting a scope picture showing the preamp output versus main sample pulse?

Hi George,

I finally got the time to run the scope.

Blue line: main sample pulse (TP7)
Yellow line: preamp output (TP3)

I'm sampling at 5.5us just as the preamp is recovering from saturation. Tx coil is slightly underdamped.

I've been adjusting R1 and the minumum delay for the best sensitivity to a test gold chain.

[Skippy]

On the off-topic subject of Lithium batteries/cells, I was just this week looking into using AA (14500) Lithium rechargeables for a project (bicycle light), and it's really hard to find any cells that are genuine quality ones. eBay is just one continuous list of unbranded or xxxxFire junk with vastly overstated capacities. The best cells in this size are 850 mAh (Sanyo / Panasonic) , but there's no shortage of Oriental 1200 mAh etc stuff to trap the unwary.
So it's not just confined to 18650 size cells. Quite a number of AA Lithiums have built-in protection circuitry, which comes at a price - the capacity is reduced, and the cost is much higher. It seems that a common use for them is in high-power LED torches.

Funny stuff: Whilst doing my research, I came across a forum post, stating that these xxxxFire cells should be re-named:
"Guaranteed-to-start-a-Fire".
Sounds right to me.

[6666]

My real 18650 2600maH batteries arrived this week, they weigh 49 grams,
did a charge and discharge test
Discharged at 500mA over 5 hours, and recorded around 2525 milliamp hours capacity
lowest discharge voltage was 3.22 volts, before safety timer stopped test, so happy with that.

[Orbit]

I think no. Look here http://www.vallon.de/products.lasso

Thank you !

[mschmahl]

I think no. Look here http://www.vallon.de/products.lasso

[Orbit]

Is this Vallon?

[Ferric Toes]

What a difference a couple of Lions make.

Using the voltage MT3608 converter board now in a little screened box, I can now dispense with the 10AA batteries in my viscosity meter. The reason I used 10 was to get enough overhead to regulate at 12V for the TX so that the signal from the soil would not lessen as the battery voltage diminished. I could of course use a current regulator instead for the TX stage. This instrument is used both in the field and the laboratory with different sensors. In the lab. a wall plug 15V power supply is used instead of batteries.

With this converter I can run for about 8 hours on two 2400mAh cells in parallel and it is much more convenient to change just two cells instead of 10. Another advantage is that one version of this converter board has a mini USB connector so that it can powered from a laptop, or other computer, with which the meter is often used.



I can see that I shall be using this type of power supply for other detector projects.

Eric.

[6666]

The coil wire is AWG23 enamelled copper. I would not have used solid wire because of cross sectional eddy currents resulting in a 'slow' coil. Nonetheless it works.

Eric.

Thanks Eric.

[Teleno]

This is my monopolar version with energy recycling and real components.

I'm sure you've already checked this ..... but I have to ask:
Is your coil actually capable of being sampled at 6us, and it's not just sampling while the opamp is still saturated?

Any chance of posting a scope picture showing the preamp output versus main sample pulse?

I didn't check the amplifier state, but since it works I guess the preamp is not saturated. The peak of transient at the Rx coil is about 3V, decaying into the mV range at 6us. It is a very well balanced DD coil. My guess is it balances out the salt water signal just as it does with the ground.

Give me a week or so before I can find the time to get the scope pics.