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Offline xavier
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« Reply #30 on: July 24, 2011, 04:57:24 pm »
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Hi Tommyb

1. Is offset adjusted without a coil (or pulse set to 0)? Are we adjusting the offset so that the V at the output Pin 6 of the opamp is 0V? I know this is simple in theory but I am coming the learning curve.

That depends some are quite different in setting than others you will have to get the data sheet for that particular Op-Amp.
I set mine in this manner remove the coil test pin1 to pin 6 and pin 5 to pin 6 (LF357) I get 1.31v.

As for the delay I have set mine while presenting a gold coin to the coil there where I found the best result is where it was set it did take me quite a while as I also tried different frequencies. (Theory is good but at most time it don't work as well in practice).

If I recall I have seen that done before placing the cap and MOSFET by the coil but if I were you I'd first get it working then do all the mods that you like that way you will know if it's going better or not and you can always come back to your old configuration also only do one mod at a time (If possible)

All the best
Xavier   

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So many questions so little time

Offline tommyb
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« Reply #31 on: July 25, 2011, 08:10:42 am »
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Xavier,
Thanks for the reply(s).  I now need to put everything in a case and get out in the yard for testing.

The bias and delay both seem to be fairly insensitive or should I say have a wide window to function. I can surely adjust them so as to make the unit barely respond to an object, but within the working range it is pretty hard to quantify the difference in detection sensitivity vs huge changes in the traces as the pots are adjusted. Maybe my workshop is noisy?

Based on the detecting sensitivity results, it sure seems that only the volume and tune pots need to be available in the field?

Ill try the jumbled coil next.
Cheers,
tommyb

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Offline xavier
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« Reply #32 on: July 25, 2011, 01:16:37 pm »
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True but if you are in a highly mineralized area it could be handy to have the other pots. I'm not an expert in electronics but one thing that I did find out is that even though they say that PI detectors are not affected by mineralized soil they are. I have proven it time and again this is why I now use two coils one unshielded for the beach and one shielded for in land.

All the best
Xavier

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So many questions so little time

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