Inside the TX-2002 pinpointer

[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.

[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 )

[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...

[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

[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

[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

[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 ?

[kt315]

the Project needs::

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

all the best

[kt315]

activation for initialization pic

[mr.sneezy]

What are you saying with the cryptic picture above ?

[sinclairuser]

hi been following this thread since the start, and cant help thinking that if you have managed to get it working, then you should continue.
kt315 is right about taking clear pictures, stripping the board and photoing again and testing components to make the parts list, but it did not work before, if you are going to reverse engineer a device, it makes more sense if it works in the first place.
a least now if you strip it take photos and make a parts list, we know it works and any mods done to it will improve it(or not), instead of having to mod the device after just to get it working.

[Thomas]

What are you saying with the cryptic picture above ?

I think he is showing that, you can use different coils instead of the original, and he has the characteristics of each shown. some have ferrite bobbins and some don't.

[walkman]

PCB maybe that?
Mistakes can happen. only suggestion

[Thomas]

unless some tracks are missing, the 4011 does nothing.

[mr.sneezy]

Final Parts List

PCB maybe that?
Mistakes can happen. only suggestion

Wow, impressive given the short amount of time. Yes, you have some missing tracks, but tomorrow (late) you will have HiRes images of the top and bottom faces of the bare board.

I have a plan to use a light table to backlight the bare board as well as do a regular image of each side. Backlighting really shows up thin tracks well.

Please see the final draft of the Parts List, as there are a few corrections in it now, plus 'as measured' values as all parts were removed tonight.

Looks at first glance that the 4011 is partly being used as an inverting buffer....

Martin

PS. What is the largest file size JPG image I can attach here ?