[sdiy] Help, I'm Desperate! (Charge Injection with DG408)

Darren Shorsky dschnidsky at hotmail.com
Tue Jan 1 04:50:04 CET 2019

Wow David!!!  :O

congratulations.....  that sounds like quite the journey.

i look fwd to the video & happy new year.

From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of David G Dixon <dixon at mail.ubc.ca>
Sent: December 31, 2018 8:24 AM
To: 'David G Dixon'; 'Oakley Sound'; 'Synth DIY'
Subject: Re: [sdiy] Help, I'm Desperate! (Charge Injection with DG408)

Hi All,

Remember this thread?  Well, I have some progress to report...

I took Roman's advice and used SPST analog switches instead of multiplexers
for my scanner.  I just panelized the prototype board, and it all works
perfectly!  The switching is completely noise-free, as expected.  Also, the
board worked the first time with no layout corrections needed, which is a
bit of a surprise given that this is a 4" x 6" one-sided PCB which is
completely stuffed (12 ICs and a pile of resistors and diodes).

This was not a terribly straightforward redesign.  First of all, the CV
circuit was originally designed to convert a -5V to +5V ramp to four
truncated triangles from 5V to 0V (5-0-5-0-5-0-5-0-5).  This was perfect for
9 channels, which my original multiplexing scanner did.  However, with the
SPST scanner, each channel requires its own switch, and the switches come
four to a chip (DG444), so this means only 8 channels.  Hence, I want my
triangles to only go 5-0-5-0-5-0-5-0 (no final 5).  This means that I want
to convert the ramp to 3.5 truncated triangles instead of 4.  This took some
serious thinking, but was ultimately pretty easy to achieve, with only a
couple of additional opamps.  I designed it so that if the incoming CV
signal is lower than -5V, the scanner will output Channel 1, and if it is
higher than +5V, the scanner will output Channel 8.

As for the switching circuit, it required 6 comparators (LM339 + LM393), 8
XNOR gates (2 4077), and 8 SPST switches (2 DG444).  I used hysteresis on
the comparators, and wired things up using Roman's "Fat Dot" concept.
Actually, the channels fall on two CV busses -- A and B (A is on when the
2164 CV is 0V and B is on when the 2164 CV is 5V).  Channels 1357 are on the
B bus (5V), and channels 2468 are on the A bus (0V).  Rather than thinking
of the XNORs as providing fat dots, if one simply sees channels 1357 as one
circuit, and 2468 as another circuit, with the XNORs alternating between
them, the one has two 4-channel logic circuits with regular skinny dots.
Either way, it works.  Also, to clarify Roman's schematic, the top and
bottom comparators are not necessary.  The top one can be replaced with
Ground, and the bottom one with 5V.  Hence, only 6 comparators are needed.

Also, I opted for "negative logic" in this circuit, and thus used XNOR gates
(4077) and DG444.  However, it would work just as well with "positive
logic", which would use XOR gates (4070) and DG445.  These are
pin-compatible with the others, so either could be used on the same board.

Finally, the "client" had originally wanted LEDs to indicate the channels.
I had deemed this impossible with my original circuit.  However, it is very
straightforward with this new circuit.  The XNOR outputs provide "ground"
for the LEDs, and the truncated triangle CV signal drives the LEDs.  Since
only one XNOR output from either the 1357 or the 2468 side is on (at ground)
at any given time, only one LED from each bus will shine.  The "inactive"
LEDs have 5V instead of ground at their negative end, and therefore cannot
conduct.  The triangles cause these two active LEDs to fade one to the other
in exactly the same fashion as the audio signals themselves.  Really, this
works better than I could have hoped, and it was very easy to do.  I just
had to invert and level shift the truncated triangle CV signal so that one
goes 5-0-5-0-5-0-5-0 while the other goes 0-5-0-5-0-5-0-5, and use these two
different signals to drive the 1357 and 2468 LEDs, respectively.  If
"positive logic" were used instead (with 4070 and DG445), then one would
simply have to swap the CV signals around and wire up the LEDs the other

I'm going to make some kind of video to show how this works, and I'll put up
a link here for it.  It will probably be tomorrow.  I also plan to create a
big long thread on Muff Wiggler explaining the entire circuit in detail with
schematics and stuff, as a sort of tutorial on how I approach design.  I
think that might be pretty useful for other DIYers.

Thanks again for all your help, especially Roman!!


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