PI sensitivity Problem

[xpcam]

I dont have my signal stable (yet) but i can detect my gold ring now on 12 cm (almost 5 inch) on my oscilloscope!!! And the strange thing is that i used only 8.5 volt as input voltage! Im comming close!!

http://imageshack.com/a/img661/2417/8gPwAv.jpg
http://imageshack.com/a/img673/7540/PD5ULK.jpg
http://imageshack.com/a/img538/3859/SSjLcE.jpg

[Qiaozhi]

unfortunately the Pi - 5 schematic was pointed out to be drawn wrong months ago !!!

the arrangement around the back to back diodes send all fly-back to the rail splitting op-amp.......
it may not like it..........

poss best print another diagram, sorry George.

This is actually the first time anyone has mentioned a potential issue with the PI-5 schematic, but (eventually) we've got to the bottom of it ->
http://www.geotech1.com/forums/showt...325#post201325

Thanks Dooley. We'll fix it in Edition 2.

[xpcam]

This is actually...

Qiaozhi what is the average depth that you can see a gold ring on a PI detector, on the oscilloscope?! I'f reached now 12 cm on my gold ring! Is that good? exelent or poor?

[DOOLEY]

no problem George,

spotted it shortly after the book came out,

thought it'd already been mentioned ages ago.

was a bit cautious about mentioning it........
just in case I was wrong.

[Qiaozhi]

Qiaozhi what is the average depth that you can see a gold ring on a PI detector, on the oscilloscope?! I'f reached now 12 cm on my gold ring! Is that good? exelent or poor?

It depends on the size of the gold ring, the diameter of the coil, and (of course) the detector. Typically, you should be able to get 12" (30cm), and maybe even more, using the audio response. Simply looking at the trace on the oscilloscope will reduce this by quite a lot, but I'm not sure by how much. It's possible that 12cm is ok.

[xpcam]

YES! I got my signal stable and it still detects my ring at 12 cm on the oscilloscope!!

See: http://imageshack.com/a/img909/9604/pvxwZZ.jpg

Either way, as Qiaozhi said, the preamp alone does not give sufficient gain to see the detection of a coin properly. You are lucky you managed 2cm with that alone.

If you are using the oscilloscope to look at the preamp output as an indication of the presence of a target, this may be the issue. After sampling the signal from the preamp, it is normal to amplify this sampled signal further to increase sensitivity. Typically, you can detect a coin several inches away from the coil, even though you will see no change on the oscilloscope by monitoring the preamp.

I Can see my ring at 5 inch on my oscilloscope the gold ring is 1.5 cm and 7 grams.

[Qiaozhi]

YES! I got my signal stable and it still detects my ring at 12 cm on the oscilloscope!!

See: http://imageshack.com/a/img909/9604/pvxwZZ.jpg

I Can see my ring at 5 inch on my oscilloscope the gold ring is 1.5 cm and 7 grams.

The fact that it's a ring will help in detection. Eddy currents like to go round in circles.

[xpcam]

Even a 1 euro coin do i see at 12 cm! Also my roman coin is now visible :-) i also tested a 300gr iron block and detected at 35-40 cm. i hope to get my comperator working on the arduino to get some noise out of the thing!

[Beenthereuk]

So show us your new circuit then - the one that works so well...

[xpcam]

So show us your new circuit then - the one that works so well...

Works so wel??? 12 cm for a euro is not that deep... But i cant show my schematic at this time while patenting... see http://www.geotech1.com/forums/showt...g-on-8-5v-10mA

Christian

[6666]

xpcam,

your feedback resistor is still going to the non-inverting (+) input of the (2nd) op-amp,
needs to be to the inverting (-) input.

you need to power the op-amps (usually) with more voltage than the signal it's going
to see,
ie: more than 12v+

can't see the need for the 1st op-amp as a voltage follower.

and your upper 10k to +12v doesn't need to be there,
remove and link to +12v

please try either the non inverting or inverting "standard" set-ups like I posted
earlier.

there's lots of ways to arrange a Pi front end that work,
the ones I have posted are but just a few.

oh, your bottom 10k just before the diodes will work,
but from experience its going to be as "noisey as hell"
most designs use 1k, or less.

Hi Dooley, put me on the right path here but where you say "you need to power the op-amps (usually) with more voltage than the signal it's going
to see, ie: more than 12v+" because of the diode clipping arn't we only supplying a target signal of .7 of a volt to the op amp ? or are you talking about the DC path through the diodes to +12V ?