PICKINI V4 - an easy to build, self adjusting PI detector

[MartyJ1963]

Hiya Bernard,,,hope you are well,,,,I thought those 1k resistors were to do with the audio so I learnt something new today,,Thanks Bernard. ,,,,@Rick,,,If you look through the Coils section of Geotech there are many threads and posts on how to build the 3dss and they are much better described than I could tell you,,,but like I say, aim for around 300uh, I use stranded wire to make mine, IE:- 0.5mm 16 strand wire 600v,,,it makes really fast coils and the sensitivity is great. Marty

[F117]

Marty,

there is probably some confusion about 1K resistors:
R16 and R17 = 1K in series with the audio output. You may decrease their value to have louder output.
R1 and R18 = 1K in parallel with the coil for damping.

[Rick]

Hi Bernard,
thank you for your help!
The 2n7000 is a small signal misfit that I've used instead of the BS170 because they are pretty similar.
For the DC offset transistor I've used a BC547C instead the BC549B that you use in the schematic, this could be a problem?
I've found a shop that has the LM318 today I'm going to buy it and replace the OP amp.

[qute1]

Aside from the higher gain on the BC547C the two are near identical but maybe the BC549B's gain rating was intentional, perhaps BC547C is "over-amping" things?

[Rick]

Quick update:
I've installed the LM318 and the behavior changed a lot: before also with maximum sensitivity and indoor the metal detector was quiet and maximum distance of a 2 euro was 13cm,
now indoor in order to get no false sounds I need to take sensitivity almost to minimum and the distance is almost 14cm ( so I definitely have to go outside to test with increased sensitivity!).
The bad news is that the waveform fed to the PIC is still different from the correct one and I have no clue on why
Waveform: https://ibb.co/dyG8Ze

Now on PIC Pin 13 I have the correct 2.0V (before was 0.7V) but on pin 3 of the Op amp I get 8.24V maybe due to the BC547C too high gain.

[F117]

Rick,

your waveform is correct.
The one I showed was from Pickini V3, just to illustrate that you should see this exponential decay.

It is quite normal that you get false signals inside the house. This has been a common remark from the start of the project.
Please take it outside, switch it on outside and let it calibrate without any metal near the coil.
The first time ever, the calibration routine may take a little longer, but next time, this value is taken from the onboard EEPROM as a starting value and calibration should only take max 10s

[MartyJ1963]

Hi Bernard,,,,Thanks for clarification on the '1k' resistors,,,I knew there was a 1k to do with the audio section but never knew about the damping resistor,,,learn somethng everyday!,,Thanks. Marty

[F117]

@Rick:

Now on PIC Pin 13 I have the correct 2.0V (before was 0.7V) but on pin 3 of the Op amp I get 8.24V maybe due to the BC547C too high gain.

This is OK. The DC offset voltage on pin 3 of the opamp is set up during the calibration phase.
Independent of the gain of the BC547. The actual steady state voltage is measured with the PIC ADC and the PIC DAC ouput is fed back into the base of the BC547.
The DC level of the flyback pulse = steady state voltage is set to about 4V at the PIC pin 9 = comparator input.

[Rick]

Thank you for the help and the explanations, now I just need some time to go outside far from the city to test in a lass noisy place!
I'll let you know if it's stable!

It's ok also in the countryside or only on the beach?

[MartyJ1963]

How to wind a 3dss coil

@Rick,,,,I found this file on 'How to wind a 3dss coil',,,hope it helps. Marty

[F117]

@Marty:
Thanks for the info on the basket coil.
I will also try it one day.

@Rick:

It's ok also in the countryside or only on the beach?

Both.
The main advantage of any PI detector on the beach is that it is not influenced by the mineralised wet sand. Searching on wet salt sand is almost impossible with most of the VLF detectors.
The main disadvantage of a PI detector in the field and on farmland is that it does not have discrimination. It will report any piece of metal with a preference for iron chunks. Sometimes these turn out to be a valuable artefact after cleaning but mostly these iron blobs are just junk. Meaning that you need to do a lot of digging on farmland ( depending on how clean it is ) to find anything valuable>
Another advantage on farmland is that a PI detector mostly goes deeper than a VLF detector. I have found items with Pickini on farmland that I had swept multiple times with an XP GoldMaxx Power, simply because they were deeper in the ground.

Have fun, and please report back on your findings in the field.

[WM6]

Meaning that you need to do a lot of digging on farmland ( depending on how clean it is ) to find anything valuable>

This is why life is beautiful.

So, dig all targets - you never know ......

and, at the end - your farmland will be clean.

[F117]

This is why life is beautiful.

Euuhhmmm, where I live it is called "Flanders Fields", the battlefields of WW1.
The fields are infested with lead shrapnel balls, which by the way also give a nice signal on an induction balance detector.
Another pain are all the copper grenade pieces that these fields are littered with.
As you say: a nice cleanup action and good if you are interested in militaria, but definitely a pain in the back at the end of the day...
We usually try to avoid these fields and search behind the front line.

[MartyJ1963]

Hi Bernard,,,,,Yes, those 3dss coils work brilliantly on your Pickini 4,,,I have tried it with both Copper enamelled wire and stranded wire,,,stranded wire working the best,,,I made 2 x 3dss test coils with single strand copper enamelled wire, one with 0.5mm and one with 0.7mm,,the 3dss with 0.7mm was more sensitive (by about 1" inch) than the 0.5mm wire,,,but both single strand coils, even though they were wound with the same geometry/configuration were 3"-4" less in depth than winding one with 16 strand 0.5mm (600v) wire,,,,,in fact, while I was testing all 3 of the coils the other day (on woodland paths near my home), I found a large women's gold ring that contained diamonds,,,it was at a depth of 12"-13",,,I made sure I covered the same area with all 3 coils and by doing that the 2 x enamelled wire coils had missed the ring, but when I went over the area a 3rd time with the 16 strand wire coil I got a loud target,,,I was surprised how loud it was at the depth I recovered it from,,I make all my P.I Monocoils with this method and wire and nothing else because I'm so pleased with them,,,,when I have time later, i'll clean the ring up a little more and post a photo of it before my partner claims the ring as hers!,,lol. It's a brilliant little detector and the 2nd one I made for my partner is as sensitive as the 1st so I'm really pleased. I think I'll be detecting a lot more of these country paths!,,The beach is too far anyway,,lol. Thanks. Marty

[6666]

even though they were wound with the same geometry/configuration were 3"-4" less in depth than winding one with 16 strand 0.5mm (600v) wire,,

interesting observation, I wonder why that is ?