Inside the TX-2002 pinpointer

[mr.sneezy]

Tested a DIY coil

I'm back at work, so now have access to an LCR meter again. The standard supplied coil measures at 1229uH, and a Q factor of 4.6 .

So, tonight I tested a DIY wound coil on the standard board, with interesting results. It points to the OEM coil being way off the mark for effectiveness...

To make the coil I used a length of 0.5mm enamelled copper wire of about arms length or so (it turned out to be 2.5m actually) and wound it on one end of a long bit of 10mm ferrite rod as a starting point.

The coil L measured at 424uH so I gave it a try. It worked better than the stock coil right away, so I then unwound parts of it and retested. By playing with the length over four goes I found the approximate sweet spot. Here is the results of each stage, the original coil is noted first.

With original coil.
Spec. 1229uH, Q 4.6, Fo 20kHz, Coil voltage 9V P-P (approx).
Coin test result. $1AU coin, face on = about 20mm, edge on = 3mm.

Test coil-1.
Initially 2.5m of 0.5mm enamel wire, on 10mm ferrite rod approx 180mm long.
Spec. 424uH, Q 10.7, Fo 36kHz, Coil voltage 9V P-P
Coin test result. $1AU coin, face on = 35mm, edge on = 20mm.

Test coil-2.
Reduced winding to about 1.25m of 0.5mm enamel wire, on 10mm ferrite rod approx 180mm long.
Spec. 103uH, Q 4.7, Fo 72kHz, Coil voltage 3V P-P
Result. $1AU coin, face on = 40mm, edge on = 25mm.

Test coil-3.
Increased winding back to about 2.0m of 0.5mm enamel wire, on 10mm ferrite rod approx 180mm long.
Spec. 232uH, Q 7.4, Fo 48kHz, Coil voltage 5.8V P-P
Coin test result. $1AU coin, face on = 50mm, edge on = 30mm.

Test coil-4
Increased winding back to about 2.25m of 0.5mm enamel wire, on 10mm ferrite rod approx 180mm long.
Spec. 335uH, Q 9.4, Fo 40kHz, Coil voltage 8.0V P-P
Coin test result. $1AU coin, face on = 55mm, edge on = 35mm.

Test-4's coil was 67 turns of the 0.5mm wire, and I've left it like that for now. That coil is WAY WAY better at coin detection than the OEM coil, so I think they stuffed up the coil spec they used for these units. Of course the unit might be a poor copy of another TX-2002, which in turn could be a copy of something else. Who knows when it's from the Chinese market...

Other news of interest. While measuring component values for the Parts List, I measured all the diodes to see if they were standard Silicon or something better, and it turns out that one measured a dead short in circuit. I removed it, and it measured OK. So I measured across the board pads, and there's a short. I then visually traced the tracks each side (looking for a short) and bugger me if it doesn't loop back on itself via the top board layer. So unless I'm going blind the board has some track layout errors on it !

What to do now, as the unit is much more useful with this DIY test coil. I'm open to ideas ?
I can strip the board as planned to photograph the bare boards for schematic creation (shorted diode included), or I could open up the OEM search coil casing and see if I can rewind it. There is some risk of damage but now there is little to risk on the old coil really, as it's rubbish.
Martin

PS. How does this detection range above compare to the better pinpointers on the market or DIY constructed ?

[mr.sneezy]

Moving along.

I've got started on the parts list. See if the PDF parts list looks OK so far as the column layout goes. Let me know if it should be changed for some reason. When finalised I'll attach the XLS sheet.

So far I have not removed anything much off the board but for a transistor because I still want to try a few coil windings to see what happens while intact.
Currently it's a bit warm in my home shed/workshop and will be for a few more days, so I think that won't actually happen till the middle of next week.
At least my PC is inside the house in relative comfort

[Qiaozhi]

Is there a link to an image of the probe circuit in the book you quote Qiaozhi ?

The book details are here -> Inside the METAL DETECTOR - Published September 2012

[kt315]

the Project needs::

1. board with components pics in hi resolution at sunny day at open air.
2. desoldered board. pics in hi res of both sides at sunny day.
3. full part list. every components must be measured individually, not color code only looking and values written. all transistors must be researched by haracteriograph, or tested on matter they are PNP or NPN structure.

all the best

[mr.sneezy]

I have some ferrite rod and enamel copper wire around some place, I'll try that before a strip down. Might be next week though now in that case.

Is there a link to an image of the probe circuit in the book you quote Qiaozhi ?

Edit:
Just had a thought. If I wind wire around the OUTSIDE of the original probe and leave the original winding open (disconnected) might that work as a fair test ?
The new test winding would be air gapped by 2-3mm from the ferrite inside...

[Teemo]

Thanks Teemo,
Is it fundamentally too high for a PI technique, or is there another reason ?
The thing being is if it's too high to use your PI mode then I'm going to have to make a different PIC circuit for it, or fix the original.

Yeah the inductance is about 10x too high for my circuit. And it seems it is too high for the original circuit also, the frequency being too low. Do the original circuit work when you connent it to coil with less inductance? (desolder some inductor from scrap electronics if you do not have other )

[Qiaozhi]

Thanks Teemo,
Is it fundamentally too high for a PI technique, or is there another reason ?
The thing being is if it's too high to use your PI mode then I'm going to have to make a different PIC circuit for it, or fix the original.

BTW, am I right in thinking that it's a VLF type unit (TX-2002), and the slight frequency offset with metal is the likely detection method ?

I'll try to strip the board in the next day or two and document regardless.

It's most likely an off-resonance detector similar to the probe circuit in ITMD.

[mr.sneezy]

Thanks Teemo,
Is it fundamentally too high for a PI technique, or is there another reason ?
The thing being is if it's too high to use your PI mode then I'm going to have to make a different PIC circuit for it, or fix the original.

BTW, am I right in thinking that it's a VLF type unit (TX-2002), and the slight frequency offset with metal is the likely detection method ?

I'll try to strip the board in the next day or two and document regardless.

[Teemo]

Teemo,
is there much hope that your PIC design can use the coil as it is (hi L value) ?
The coil is sealed with hot melt glue under the cap and some sort of glue down the sides, and I suspect the probe case will break if I try to extract it.

No my circuit can not use that coil. L is too high.
Teemo

[Silver Dollar]

That's how you can tell it's a Chinese knock off...

[Qiaozhi]

And it's surprising that no-one has noticed this ->

[Silver Dollar]

Well I see at least 3 transistors but ignored your comment.

The real problem is here;

The MD is missing. Just plug in a metal detector there and you'll be all set!

[Thomas]

It has a 358 and a 4011, from what I see and a single transistor. Not going to do much with that.

I was hoping someone would have challenged me on that. oh well.

[Qiaozhi]

Also, if anyone is keen and knows how to create a schematic from a pseudo net list (in Eagle etc), I could desolder the whole board. Documenting the part numbers and values in an XL table and then repost bare board images and the table ?
I'll only bother if somebody IS keen to do the CAD schematic side, as I don't have time for both.

If you're prepared to desolder the whole board and document the component values, etc., then please go ahead. I will create an area in the Project section where we can put the data. Then, as others get time to work on it, further information can be added. It's about time we had a small group project here.

[Thomas]

It has a 358 and a 4011, from what I see and a single transistor. Not going to do much with that.