I can pick up an Adafruit MCP4725 here locally. I just checked and the store here has several in stock. They are still open with limited hours. I have all of the other parts listed except for the optocoupler. The one on that site suggests “6N138” but there are several listed on Mouser. Is there a specific one on Mouser you can recommend?
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6N138 is a JEDEC part code so they all have the same basic specs. Just grab one in the right package (DIP/PDIP for through hole, not SMD).
Note that 6N138 appears in a lot of internet schematics, but strictly speaking it’s not fast enough to match the MIDI spec requirements, even if that probably doesn’t matter for normal use.
(I use 6N137 instead, which is much faster, has a logic gate output instead of a darlington transistor which is nice since you’re connecting it to a logic gate input, and is readily available from Tayda and others. See schematics here.)
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indeed!!! the module I’m making is around the midimuso anyway! which is using an at mega 328, and pretty much all of the pins as Pwm tbh I really am not sure how he has managed it but he has. it has a total of 12 CV outputs and 6 Trigger outputs all from 1 atmega chip. I have used it a number of times for a few projects and decided to use it cus I’m deffo not gunna be able to beat that haha. plus it has extra midi thru’s and bus control etc. I dunno. hopefully that project will be up soon!! but if your not aware of it check out “midimuso”
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This one?
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yep! been chatting to rob about it the person who designed it. and worked on a circuit that works nice in modular. waiting to see if its all come together nicely!
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That’s a little more featured than what I could have suggested with the optoisolator and the DAC. Just looking at the board and not the schematic, it looks like he’s using the 324s as the DACs. I experimented with that once and got about the same iffy results I got just using the PWM last week, but that’s probably down to my build.
@popflier I’d recommend you wait for Sam’s build or try to get one of the Midimuso kits. If you can’t though, I can still give you the basics of how to pull the elements together (which would probably be very frustrating and then very rewarding and then very frustrating again when you grow into needing the six lines you can get from Sam’s version against the maximum of 2 I could help you with on this build.)
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From a very quick peek at the schematics, each channel goes through a level shifter, a first-order lowpass filter (100k/10nF) and a voltage follower. The reference voltage is provided by an LM317. And the opto coupler is a 4N35, which is even worse than the one I complained about above. Definitely a slight “are you sure this is good enough” factor 
but Sam’s tested it, so I guess it does work 
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There are also some simpler versions available, like this one: https://github.com/elkayem/midi2cv
I made a pcb for that, which I can share, but I have not yet build it and it is therefore untested.
It only has 1 channel, but you could change the software and convert the outputs for velocity, control and pitchbend into V/oct. My plan was to make a ‘polyphonic’ code version, which I can connect to my 4 VCO’s to play chords! I forgot to connect a fourth pin as an additional GATE, but I can still just solder it directly to the arduino pin…
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If you want to go truly minimal, there’s https://www.hackster.io/janost/diy-good-ol-midi-to-cv-d0e2bf (haven’t built it, but if midimuso’s cv-12 works I don’t see why that shouldn’t work 
)
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Ah-now I see the schematic and I wouldn’t have read it right. I’ve never seen a schematic with that many non-right angles. I hope it works!
I’ve got quite a lot of ATtiny85s including some on Digispark development boards. Another chip to watch out for is ATtiny2313 which comes with a built-in UART and a 16-bit timer, though it’s much smaller in flash and RAM. All these chips share the same basic architecture and are compatible with Arduino.
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The 85 “jellybeans” have a million uses and are easy as hell to program, but also limited. I don’t know if they’d have the pins to drive a single ADC with a pin for gate and MIDI in, or if that would require you to dedicate two pins to an external crystal (something I’ve tried and I’m not sure if it worked or not!) If it does work though you’re probably looking at a $6-7 project even after sourcing out a PCB, so it would definitely be an appealing build!
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It would also be neat to have a module that enabled poly mode by taking midi notes, buffering and distributing them across voices via CV at some point. I’ll have to think about it.
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I think that’s pretty much what Sam (and @fredrik ?) came up with for the GBMM, so you’re probably in luck!
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I assume you mean a DAC. The idea is you use fast PWM mode on the ATtiny85, using the duty cycle of the PWM output to vary the effective CV.
ATtiny85 is so versatile that somebody even designed and sells a complete 8-bit modular kit based on identical modules that behave differently depending on their software. It would make a great present for a child interested in learning electronics and playing with synths.
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Yup, yup I did. Still not in focus I guess. I’ll have to check out that minimonosynth-I’m fascinated to see how they handled the 1v/oct oscillators if it’s standard Moog levels. Even though I sort of tried to tell myself to give up on the idea of the “budget west-coast” plans after my build this weekend.
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It won’t be Moog levels. The modules apparently run on those little coin cells, and the ATtiny85 processor typically runs at 2.7 to 5.5V. This is a toy synth, really.
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what do you mean? I have been thinking about poly modules recently. as the midimuso can be a 6 channel poly converter. the only thing is the more I think the quicker it just becomes a poly synth voice. however I’m trying to figure out either an autotune circuit or maybe even going for DCO’s
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Well, im not coming from a modular background here, but i was thinking about how I have seen some poly synths designed from what i remember.
Lets say you have 3 voices. Then you would have a “brain” like an arduino that tracks the voices and throws voltage to output CVs connected to voices. I am thinking of something similar to how round robin load-balancing works. Essentially, i press key1 and that routes to voice1, press key2 and routes to voice2, press key 3 routes to voice3. If i press another key, either no routing occurs, or it overrides the previous key1’s value.
So you would need to take midi input, and then route the key presses to different output voltages in that manner. If this is unclear (prolly is) ill try and do a diagram later.
I will have to dig into and actually implement midi via arduino to get a feel for it. Hope im not underestimating this lol. I learned quite a bit about serial stuff with my eeprom projects.
Oh, latency will probably be the main concern.
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Yeah, as @Maxhirez says, that’s basically the GAMEBOY MEGAMACHINE’s midisplitter (https://github.com/juhakivekas/midisplitter_2000) (by juha, not me) but with CV output. Demo in episode #7:
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