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

[F117]

@Saltydog

I think that with judicious use of the slice level, you can discriminate even with just the one comparator, I intend to change the code to give a different tone when the current period is < than the average (non-ferrous)
compared with current period > average (ferrous). (as you currently have it)

Still no idea how you can do this with a single slicing level. I always thought that you needed at least 2 pulse widths at different voltages to have a directional coefficient of the slope, that is an indication for the conductivity of the metal target.

@ Hammerhead : I assume your semi-kit did not arrive in Egypt. This may have to do with the mail service quality and/or the corona crisis. I will refund your money.

@ Goaty:

@ All: keep it safe for the next weeks/months...

[SaltyDog]

@Saltydog

Still no idea how you can do this with a single slicing level. I always thought that you needed at least 2 pulse widths at different voltages to have a directional coefficient of the slope, that is an indication for the conductivity of the metal target..

Non-ferrous will decrease the pulse width compared to no metal, Ferrous will increase pulse width ... simple. If you have a look at the graph again, you will see at level 0.4v you can clearly note
that you will get less pulse width, with non-ferrous than no metal .. I don't think you have to measure the slope at all ..

Anyway, I will try it and let you know how I get on ...

[waltr]

Hi Bernard,

Back to discrimination ... I have managed a coil/front end to have a discharge curve of <2uS .. this makes the coil extremely sensitive to small gold (my aim).
(For example .. my ring is detectable 300mm away )

I think that with judicious use of the slice level, you can discriminate even with just the one comparator, I intend to change the code to give a different tone when the current period is < than the average (non-ferrous)
compared with current period > average (ferrous). (as you currently have it)

This can only be done with a fast coil as I have just built .. and operating in the correct part of the discharge curve (i.e slice level)

When my semi-kit arrives from you , I will test my theory out .. here's hoping it arrives soon ..

I have attached the curves again .... think of the slice level being around 0.4v on the vertical axis.
[ATTACH]49741[/ATTACH]

Non-ferrous will decrease the pulse width compared to no metal, Ferrous will increase pulse width ... simple. If you have a look at the graph again, you will see at level 0.4v you can clearly note
that you will get less pulse width, with non-ferrous than no metal .. I don't think you have to measure the slope at all ..

Anyway, I will try it and let you know how I get on ...

I have tried this when I built my first PI detector and found that the graph you posted is THEORY only. Real world objects do not always do what that graph shows.

Please do experiment and post your findings.

Also check what I did in PIC code on my Hammer Head to do GEB and high verses low conductor discrimination here:
https://www.geotech1.com/forums/show...ake-on-the-HH2

This works well for well defined objects like coins but rusty (ferrous) objects came in all different kinds of sizes, shapes and alloys which makes ferrous verses non-ferrous discrimination impossible.

[SaltyDog]

I have tried this when I built my first PI detector and found that the graph you posted is THEORY only. Real world objects do not always do what that graph shows.

I have found that "Real world objects" behave just as the graph, AS LONG AS you have a fast enough coil ..

As I said, I have a coil that produces a discharge curve in 2us .... I think that is the difference between discrimination or not ...

When you tried this, what was the speed of your coil? ...

[waltr]

I have found that "Real world objects" behave just as the graph, AS LONG AS you have a fast enough coil ..

As I said, I have a coil that produces a discharge curve in 2us .... I think that is the difference between discrimination or not ...

When you tried this, what was the speed of your coil? ...

Standard bunch wound, slowish, Pi coil where I could get a minimum sample delay of 10us.
I can't wait for you to get this to work and see your results.

[SaltyDog]

Standard bunch wound, slowish, Pi coil where I could get a minimum sample delay of 10us.
I can't wait for you to get this to work and see your results.

Yep ..10uS is too slow ... will keep this thread posted with my testing (Still waiting on hardware though, so might be delayed ... dang Covid-19 ...)

Keep safe

[6666]

As I said, I have a coil that produces a discharge curve in 2us

What type of coil construction did you make to get that speed ?, thanks.

[SaltyDog]

What type of coil construction did you make to get that speed ?, thanks.

Hi, I built a spider weave coil that is constructed from copper wire (0.5mm) which is cotton covered by 1mm cotton. It is the cotton wire in combo with the spider
weave that gives very low capacitance. (I have had a lot of experience building there for high voltage/current Telsa coil applications)

The coil is 160uH and 1.6E ...

[nickel_n]

constructed from copper wire (0.5mm) which is cotton covered by 1mm cotton.
The coil is 160uH and 1.6E ...

do you have a link to a supplier?

[SaltyDog]

do you have a link to a supplier?

Sorry .. It is a bobbin of wire I have had for some time .... I have no idea when and where I got it ..
I suspect I got it when I was working, but now I have been made redundant, no longer have access to corporate emails ..

However, if you look for cotton covered copper wire of the dimensions I mentioned you should be all good .. you do need at least 1mm thick cotton ..

[SaltyDog]

Hey Bernard,

Glad to see forum is back up .... I have a couple of questions for you.

1. Can you explain why the op-amp gain needs to be >1000 when the input voltage range is 0..0.7v and the output range is 0..5v
Seems to me only a gain of 7 is required ... is there something I am missing?

2. Could you please explain the function of the op-amp offset and calibration function, and why it is essential ..

Many Thanks,

PS: I am thinking of implementing your design hardware completely in an FPGA .... I already have the FPGA tracking a 2..5us slope, and reporting the result ..

[6666]

Hi, I built a spider weave coil that is constructed from copper wire (0.5mm) which is cotton covered by 1mm cotton. It is the cotton wire in combo with the spider
weave that gives very low capacitance. (I have had a lot of experience building there for high voltage/current Telsa coil applications)

The coil is 160uH and 1.6E ...
[ATTACH]49751[/ATTACH]

Thanks we used to use it here in Australia back in the 1960-70's , have not seen it for long time.

[waltr]

1. Can you explain why the op-amp gain needs to be >1000 when the input voltage range is 0..0.7v and the output range is 0..5v
Seems to me only a gain of 7 is required ... is there something I am missing?

Since most desired targets are small and the returned target signal is very small only to lowest few uV of the decay curve has target information.
Therefore, a gain of ~1000 is needed to get the few uV up to a few mV.

If you build and measure then it will become clear.

[SaltyDog]

Right, but since this is a"temporal" application, we are looking for sensitivity in the time domain, not voltage ... So, it's more about resolution in measuring pulse width..

[waltr]

Right, but since this is a"temporal" application, we are looking for sensitivity in the time domain, not voltage ... So, it's more about resolution in measuring pulse width..

Give it a try and post results.