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

[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?

[BC547]

@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.

[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

[BC547]

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

[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.

[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]

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.

[BC547]

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.

[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

[BC547]

The 2 x 1K in parallel over the coil is the damping resistor of 500 ohms. There are 2 resistors of 1K in parallel to reduce the dissipation per resistor. If you use 5X100 ohms in series this should give the same result.
You should be able to detect a 2 euro coin at 25-30cm with an 8 to 10 inch coil.
The differences will probably relate to the different opamp you used.
LM318 is quite common and should be easily obtainable ?
BC547 is also very common and is used to set the DC level of the amplified flyback signal.

What is the 2N7000 ?

The wave form you are showing is not correct. Probably because you did not use the opamp that was designed in.
It should look more like this:

[Rick]

Thank you so much for the help Marty!
I'll download and try to build this kind of coil, I have some MDF but after using it as a support I can leave it or I have to remove it from the complete coil?
Wich diameter and number of turns you suggest me to use?
Thank you again for the help!

[MartyJ1963]

Hiya Rick,,,,I think the two 1k resistors are to do with the Audio section of the detector because I remember others asking Bernard how to raise the volume of the speaker and I think he replied to them telling them to lower the value of these two resistors,,,I know it's somewhere on this thread. The 3dss coil takes quite a while to make as you will need a lot of patience to wind it, but it has the advantage of not needing to add a 'shield',,,I don't know fully how the 'Self shielding' works but I think it has to do with the Geometry of the coil. Basically, you decide what size coil you would like, eg:- 10" Diameter,,,then you cut out a circle of 10" diameter from material such as 'Lexan' or stiff plastic (I use 1/8" thick MDF board) etc,,,You then measure from the outside of the circle 'inwards' about 1 and 3/4" inch and draw another circle. You then divide the outer circle's circumference into 37 segments or increments and 37 segments on th 'Inner' circle's circumference. There is a thread on how to wind the wire on this kind of form in the Coils section of the forum,,look for 'Chance' coil,,,or 'Racetrack' coil,,,but there is quite a lot of information available on the 3dss coil on Geotech. I only use 3dss coils now on all my PI's because they are really fast and don't need the tedious job of shielding etc,,,I'll see if I can find out what I have on them for you and I'll post them for you to download. I would send F117 a private message about trouble you're having with the DC offset,,I'm sure he won't mind helping you cos' he's a great guy. Here's two PDF files with some templates to give you an idea. I'll see what else I can find for you. Marty

[Rick]

Hi Marty,
thank you for your response. I've built the Pickini as close as the original I was able to but I hat to use some different components:
I've used a OPA1612 because I cannot find the LM318, the 2n7000 and the BC547 and I've used 5 100ohm resistor instead of two 1K in parallel so that should not be a problem I think. The Coil I've built is from the instructions on the Pickini web page, could you please tell me what is a 3dss coil and how to wind it?
I've also seen the voltage table and instead of 6.8V as DC offset I get 11.5V and that should be a problem but I don't know how to fix it

Thank you

Riccardo

[MartyJ1963]

Hi Rick,,,,The best person to ask your questions to is F117 (Bernard) who actually designed the Pickini 4,,,he's a great guy and I'm sure he wil give you all the help you need as he's helped me plenty with the 2 x Pickini 4's that I've built. Did you build the Pickini 4 to the specifications that Bernard provided for this project? I followed the whole build from Bernards webpage, IE:- I etched the original board, I used 1% resistors, all components were to Bernards spec's etc, and I have 2 x perfectly working Pickini's (Thanks again Bernard). I think your coils inductance is bit too high though at 500uh,,,I use 300uh 3dss Monocoils with both my detectors and they both detect a 1 Euro at 14 " Inch with the sensitivity up about 80%. Also, I didn't add any other parts in my build (5 x 100 ohm resistors??). If you build a 3dss coil around 300uh I'm sure you will have better results and you'll find you won't need to shield the coil either,,,I've never had any problems with interference etc using a 3dss. But of course, the man to ask would be Bernard. All the best for a successful build. Marty

[Rick]

Good Morning Guys,
I've built on perfboard a pickini V4, it detects a 2 euro coin at about 11cm in free air with the sensitivity set to maximum.
As damping resistor I use five resistor 100 ohm (1W) in series for a total of 500 ohm 5W. The coil is 25cm diameter, 500uH and shielded with aluminium tape.
I've measured the DC offset bias and is 11.38V with a Voltage supply of 12.55V is it correct?
I've also attached the wave coming out from the op-amp because I don't know if it's correctly dumped



Picture:https://ibb.co/nyJb0K

Could anyone helps me? It's my first metal detector

Thank you so much Riccardo