BMC040 dual logic CV mod

KOSMO MODULES
21

Here is my first draft of the schematic :

dual-CV-logic
dual-CV-logic1480×1021 24.2 KB

Q: now that I added both a 100K input R, and the 100K R to GND for non-infinite impedance, I have a voltage divider at the input.
The threshold is now 0.24V instead of the original 0.12V.
Should I adjust the comparator reference (R1 & R2), or remove the input Rs (the original schematic doesn’t have any), or do we just don’t care as it is supposed to process gates which are 5V minimum ?

Some Comments:

  • I left A4 & A5 free in case somebody prefers an OLED instead of the LED bars.
  • The LEDs are all on the same port, so we can use port output instead of the slow writeDigital().
  • I haven’t drawn the power part yet, it will have a 7805 to provide the 5V bus.
22

What’s the purpose of the 100k input resistors on J2, J3, J6, J7? I don’t see any need for them. No current’s going to flow into the op amp, so there’s no current to limit other than that going through the resistors to ground. Anyway, 100k would be very high for current limiting.

You need the 100k on J4, J8 as input resistors for the inverting inputs, and since the inverting inputs are virtual grounds you want those to be 100k for input impedance, but for the comparators it’s different.

If you really want them there you could put the 100k to ground to their left, then there would be no voltage divider. (As it is you now have 200k input impedance instead of 100k.)

Likewise I don’t understand the 100ks after the comparators. You do need a series resistor for the Schottky and you do need a pulldown but the series could be maybe 1k and the pulldown should certainly be 10k or less.

23

Thank you,

If there is no need for them (don’t remember where I picked this from, it’s drawn on my notepad, without the 100K to GND…), I’ll just remove them.

The Rs after the comparator where taken directly from Barton’s schematic, I didn’t have an Arduino compatible digital input on my notepad…

input-stage
input-stage732×287 35.5 KB

So let’s change these too.

24

Bringing it back to the original BMC040 modding, I got a reply from Michael Barton on the switching points:

.84V
1.68V
2.52V
3.37V
4.21V

Seems like my original circuit was alright in this respect.

1 Like
25

Are you going to make a board/circuit that attaches to the original BMC eurodesign, or are you going to recreate the whole thing as a new board?

1 Like
26

I will either make little daughter boards which connect to the original one (one per channel), or build the whole thing on a proto board. I’m actually leaning towards making pcbs, cause wiring all those leds will lead to spaghetti…

2 Likes
27

cool I would be on board for 1 or 2 of those so make extra when you get it all worked out .

1 Like
28

Here’s what I ended up with:

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I’ll send them off with my next JLCPCB order

4 Likes
29

We need to start up a category for “CV modifications to Barton designs”. I hope to have one done by end of next week. (I took longer… started around June 2020…)

4 Likes
30

Which Barton module are you modding?

31

BMC040 dual logic. I’d link it to you, but his website is set up such a way that that’s impossible :wink:

32

Sure you can :slight_smile:
https://www.bartonmusicalcircuits.com/logic/index.html

2 Likes
33

Sorry…I should have been more specific. That question was for @analogoutput :slight_smile:

1 Like
34

Poking around and looking for logic modules, I found this - any new developments? I would love to see/hear it in action.

1 Like
35

I actually have all pcbs assembled here… but currently don’t have access to a drill press to make the panels (nor to my synth, for that matter). I should be able to get everything set up after my holidays!

2 Likes
36

How difficult would it be to add cv control to the selector pot?

Asking for a friend.

37

Super simple, it’s exactly what I did. Just need to make sure to stay within the supply voltage of the mcu. I used MCP600x for that

2 Likes
38

Oh! I think I was totally misreading that schematic.

I was confusing the ins and outs.

2 Likes
39

soooo, did this PCB mod work well for you? I was literally about to build my BMC040 up and was thinking about adding a daughter board or two anyway…

if it did, do you have any left?

\cheers,
Lance

40

I’m ashamed to admit it’s still in my ever-growing pile of half-finished products… reason was a house move which meant I lost access to my drill press, and couldn’t make the panel. I should just draw one up and order from JLC.

Anyway, yes I still have some! Send me a message if you’re interested.