Hi everyone.. new member with technical question on PI detection

[Beenthereuk]

Hi all, my name is Tony and I am new to metal detecting hardware.

I am trying to build a PI machine from scratch and have simulated a transistor circuit in LTspice that should detect and integrate the width of the coil's flyback voltage after triggering (see images below). My electronics work well, once soldered tightly on PCB (too many resonances on breadboard) but I cannot see any movement in the width of the pulse on my scope. Can anyone explain why or what I am doing wrong?

The dark blue line is the inverted pulse - its width drives the integrator in the following stage to produce a stable DC voltage for sampling.
The red line is the integrated voltage that is proportional to pulse width.
The faint blue line is the integrated voltage that is then buffered and held until discharged after ADC sampling.

Coil data: 0.8mm diameter, 50 turn enameled wire with 777uH inductance and 0.8 Ohm resistance.

Cheers,
Tony (London, UK).

PS.. sorry posted schematic twice and dont know how to delete 1 copy.

[Beenthereuk]

Clearly not the right place for it - can anyone recommend a good electronics forum with members who home-brew metal detectors?

[Tepco]

Your circuit wont work in PI metal detector. Pulse amplifier is useless, you need fast enough linear amplifier, to observe change in decay curve. On this forum you can find lot of information how these circuits work.

[Carl-NC]

Tony,

It's not clear what you are looking at. If it's the flyback pulse, then results depend on how you gate the integrator. Generally, you can't see the raw flyback width vary just from oscope observation. Nor will you see it vary if you gate off too early, and probably gating on too early as well.

Generally, for the first 8-10us you are looking at all coil response so you have to get that out of the way before you can distinguish the residual target response. The overwhelming choice for doing this is a fast-recovery linear amp followed by a delayed sampling integrator. You can do it with PWM as we included in our ITMD book, but even that design looks at the pulse width after a big linear amp where you can see some variation and does not do it with integration.

- Carl

[Beenthereuk]

Yes thanks for clarifying, Carl. I thought, naively, that I would be able to detect motion on the scope before the first amplifier. I'm new to metal detectors.

I can see clearly now (as the song goes) so have focussed on designing a more efficient linear amplifier. See my new post.

Tony.

[Qiaozhi]

I'm new to metal detectors.

Has anyone mentioned this book to you yet?
Inside the METAL DETECTOR - Published September 2012

If you order one, it will be shipped from the UK (assuming that's where you live).

[Beenthereuk]

No, hadn't come across it. Although there is plenty of information out there on their use and the general technical principles they work on so am reading up on this all now.
Might try a high frequency, high voltage BFO next, for detecting small conductive particles (something else I'm working on).
Yes I am in London, UK.

[Qiaozhi]

Although there is plenty of information out there on their use and the general technical principles they work on so am reading up on this all now.

Yes, there are lots of books / info available on how to use your metal detector, and even some that cover the general principles of operation, but ITMD is the first in-depth book on metal detector technology since 1927.

You might also want to look here ->
http://www.geotech1.com/cgi-bin/page...&file=main.dat
and in particular here ->
http://www.geotech1.com/cgi-bin/page...e=projects.dat
especially the Hammerhead article.

[Beenthereuk]

Will do. Thanks.

[Beenthereuk]

Just read the Hammerhead article and I can only imagine that whenever the author published that, PIC microcontrollers were very scarce and expensive.. that can be shrunk to 1/5 of the size using a small PIC for all the timing signals. Should be possible to make a smaller differential integrator using NMOS FETs too and do away with the need for negative supplies. Wouldnt bother with direct audio out, just integrate the output pulses and ADC the mean DC from that so the PIC can produce an output on display or frequency variable tone. I'll have to simulate it all as is when I get a chance.

[Carl-NC]

Hammerhead 1 intentionally avoided a micro to make it easier to build for a novice. Hammerhead 2 has a PIC with fairly easy C code. ITMD has several PI projects from all-discrete to all-PIC. Also has a clever PWM mode PI that Qiaozhi developed. None of these circuits are intended to be commercial competitors, they are "learning platforms" that teach the basics.