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Wow David!!! :O</div>
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<br>
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congratulations..... that sounds like quite the journey.</div>
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i look fwd to the video & happy new year.</div>
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<br>
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d.<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Synth-diy <synth-diy-bounces@synth-diy.org> on behalf of David G Dixon <dixon@mail.ubc.ca><br>
<b>Sent:</b> December 31, 2018 8:24 AM<br>
<b>To:</b> 'David G Dixon'; 'Oakley Sound'; 'Synth DIY'<br>
<b>Subject:</b> Re: [sdiy] Help, I'm Desperate! (Charge Injection with DG408)</font>
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<div class="PlainText">Hi All,<br>
<br>
Remember this thread? Well, I have some progress to report...<br>
<br>
I took Roman's advice and used SPST analog switches instead of multiplexers<br>
for my scanner. I just panelized the prototype board, and it all works<br>
perfectly! The switching is completely noise-free, as expected. Also, the<br>
board worked the first time with no layout corrections needed, which is a<br>
bit of a surprise given that this is a 4" x 6" one-sided PCB which is<br>
completely stuffed (12 ICs and a pile of resistors and diodes).<br>
<br>
This was not a terribly straightforward redesign. First of all, the CV<br>
circuit was originally designed to convert a -5V to +5V ramp to four<br>
truncated triangles from 5V to 0V (5-0-5-0-5-0-5-0-5). This was perfect for<br>
9 channels, which my original multiplexing scanner did. However, with the<br>
SPST scanner, each channel requires its own switch, and the switches come<br>
four to a chip (DG444), so this means only 8 channels. Hence, I want my<br>
triangles to only go 5-0-5-0-5-0-5-0 (no final 5). This means that I want<br>
to convert the ramp to 3.5 truncated triangles instead of 4. This took some<br>
serious thinking, but was ultimately pretty easy to achieve, with only a<br>
couple of additional opamps. I designed it so that if the incoming CV<br>
signal is lower than -5V, the scanner will output Channel 1, and if it is<br>
higher than +5V, the scanner will output Channel 8.<br>
<br>
As for the switching circuit, it required 6 comparators (LM339 + LM393), 8<br>
XNOR gates (2 4077), and 8 SPST switches (2 DG444). I used hysteresis on<br>
the comparators, and wired things up using Roman's "Fat Dot" concept.<br>
Actually, the channels fall on two CV busses -- A and B (A is on when the<br>
2164 CV is 0V and B is on when the 2164 CV is 5V). Channels 1357 are on the<br>
B bus (5V), and channels 2468 are on the A bus (0V). Rather than thinking<br>
of the XNORs as providing fat dots, if one simply sees channels 1357 as one<br>
circuit, and 2468 as another circuit, with the XNORs alternating between<br>
them, the one has two 4-channel logic circuits with regular skinny dots.<br>
Either way, it works. Also, to clarify Roman's schematic, the top and<br>
bottom comparators are not necessary. The top one can be replaced with<br>
Ground, and the bottom one with 5V. Hence, only 6 comparators are needed.<br>
<br>
Also, I opted for "negative logic" in this circuit, and thus used XNOR gates<br>
(4077) and DG444. However, it would work just as well with "positive<br>
logic", which would use XOR gates (4070) and DG445. These are<br>
pin-compatible with the others, so either could be used on the same board.<br>
<br>
Finally, the "client" had originally wanted LEDs to indicate the channels.<br>
I had deemed this impossible with my original circuit. However, it is very<br>
straightforward with this new circuit. The XNOR outputs provide "ground"<br>
for the LEDs, and the truncated triangle CV signal drives the LEDs. Since<br>
only one XNOR output from either the 1357 or the 2468 side is on (at ground)<br>
at any given time, only one LED from each bus will shine. The "inactive"<br>
LEDs have 5V instead of ground at their negative end, and therefore cannot<br>
conduct. The triangles cause these two active LEDs to fade one to the other<br>
in exactly the same fashion as the audio signals themselves. Really, this<br>
works better than I could have hoped, and it was very easy to do. I just<br>
had to invert and level shift the truncated triangle CV signal so that one<br>
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<br>
different signals to drive the 1357 and 2468 LEDs, respectively. If<br>
"positive logic" were used instead (with 4070 and DG445), then one would<br>
simply have to swap the CV signals around and wire up the LEDs the other<br>
way.<br>
<br>
I'm going to make some kind of video to show how this works, and I'll put up<br>
a link here for it. It will probably be tomorrow. I also plan to create a<br>
big long thread on Muff Wiggler explaining the entire circuit in detail with<br>
schematics and stuff, as a sort of tutorial on how I approach design. I<br>
think that might be pretty useful for other DIYers.<br>
<br>
Thanks again for all your help, especially Roman!!<br>
<br>
Cheers,<br>
Dave<br>
<br>
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