PI-5 Working Version

[Qiaozhi]

As promised (quite some time ago) here is a working version of PI-5:

Please see the attached PDF file for all the details.
I've also included the schematic as a separate image, and an audio file so that you hear what it sounds like.

Enjoy!

Edit: Please ignore the attachments below and use the ones in post #4.
I have left these here to maintain continuity with the discussion.

[surfdetector]

This comes at the ideal time as we head towards Winter. Many thanks for yet another good project to look forward to.

[6666]

Nice audio, well done, so a series resistor is no longer required between the coil and diodes ?

[Qiaozhi]

Nice audio, well done, so a series resistor is no longer required between the coil and diodes ?

Well spotted.
Of course there is a series resistor there, as you can see on the solderless breadboard image. I've updated the schematic with the correction.
Thanks. If you spot any other errors then please do not hesitate to point these out, as it's very difficult to check your own work.

[Andi68]

What a coil will be used, George?

[Qiaozhi]

What a coil will be used, George?

Any standard mono PI coil should work. If it works with Baracuda, Surf-PI, or MPP, then it should be ok with PI-5.
I even used a 700uH 4" diameter coil for testing.

PI-5 is an experimental circuit that could be developed further, and that's obviously the sort of thing we like to do here.

[Andi68]

The circuit diagram shows connections with 0V
Where are they connected ?

[Phantom]

They are connected to pin 7 of U2b. This point is the half voltage reference created by operational Amp. U2b

There are 6 volts from 0 V point to +VB point, and - 6 volts from 0 V point to -VB point

Regards

[Qiaozhi]

The circuit diagram shows connections with 0V
Where are they connected ?

They are connected to pin 7 of U2b. This point is the half voltage reference created by operational Amp. U2b

There are 6 volts from 0 V point to +VB point, and - 6 volts from 0 V point to -VB point

Regards

Correct. The ground (0V) is the virtual ground referred to in the PDF document.

[Andi68]

Ah, now i undertstand- thenk you

[Qiaozhi]

I've done some more experiments with this circuit to check out its potential. My first thought was that increasing the preamp gain would be a good step, but it made the detector go unstable, so I next experimented with a modified MPP that included a PIC processor. In this case it used a dual-stage preamp with an overall gain > 1000. The PIC was used to generate the TX pulse, and read an external pot to allow adjustment of the blanking pulse. One of the comparators in the PIC was used to turn the preamp output into a logic signal which was ANDed with the blanking pulse using a couple of transistors. The circuit was reasonably stable in operation but the result (which was to be expected) was not good enough to compete with a standard PI sampling circuit.

In conclusion I think this concept would make a satisfactory PI pinpointer with a VCO-type output, but it's probably not worth pursuing as a fully-fledged PI detector.
Anyway, it's an interesting experiment if you have some free time on your hands.