MIDI, Part 4

My two modules have been assembled, and are ready for testing. No time to test tonight, but i'm eager to hit it tomorrow.

Top: My current MIDI2CV module, inspired by Jan Ostman's article, and really implemented with Kevin's code. There's no DAC, so there are limitations to accuracy. However, in practice, it is VERY usable.

Bottom right: The 1:5 MIDI Thru. I used 6N138 for the optocoupler and vamped the circuit that Kevin used. Ideally, i should see the same midi data on all 5 of the pin headers at the top. I just bodged a crappy DIN5 connector on there for now, i'll snip that off when it's panel time. I'll use the old MIDI2CV module to test this new 1:5 MIDI Thru.

Bottom Left: My prototype of a Mutable Instruments CVPal with Serial MIDI In. This is not something that is supported by the current CVPal firmware, and i'm not 100% confident that what i want to do is possible. My schematic has removed all of the USB connectivity. Serial MIDI input appears on Pin12 of the ATTiny84. My theory is that the firmware can be modified to use a SoftwareSerial input instead of the USB MIDI library. The most desirable parts of the existing firmware are related to the DAC implementation. Much more hacking ahead.

If the CVPal hax work as planned, i'll build 2 more of that board and tie it all together on a single panel with the MIDI Thru. That should give 6 cv/gate, and render the old MIDI2CV module obsolete ... which just means i can flash a new custom firmware to it and use it as a MIDI drum interface - i think i should be able to get 4 outputs...

If the CVPal can't parse Serial MIDI, i'll have to work on using Kevin's code to interface with the MCP4822.

ATTiny MIDI to CV, Part 3

I finished a layout of the previously mentioned 1:5 MIDI Thru. I don't think the community needs my voice on this particular submodule, so i'll spare you the details until it all comes together.

Today, i'll be working on a compact layout for the Mutable Instruments CVPal. The schematic is pretty un-complicated, essentially just power and clock for the microcontroller(s), a couple lines between them, and then the usual crew of jacks and pots.

dreams: Midi2cv redux

The building blocks:

1. Mutable Instruments CVPal
2. 1:5 MIDI Thru

The mods:

1. Modify the CVPal firmware to accomodate Serial MIDI instead of USB midi. This allows it to work with my existing Alesis MIDI keyboard.
2. Modify the CVPal Firmware to accept MIDI Program Change events. This replaces the Channel-based behavior of the stock firmware.
3. Modify the CVPal Firmware to use a hardware channel setting / indication. Either dedicated buttons and displays, or a documented key combination and indication on the GATE output.
4. Modify the 1:5 MIDI thru to include 3x of the CVPal and 1x MIDI Thru output
5. The unused slot on the MIDI Thru bus can be occupied by a custom microcontroller to facilitate the newly supported MIDI Program Changes. This could further be expanded to support a MIDI file player or other MIDI controls.

I guess i need to go parts shopping because i don't have any of hex inverters in this schematic. I also need some 20Mhz xtals for the ATTiny84 in the CVPal.

Module #007: Thomas Henry's VCO-555, Stripboard Build

I put the whole thing on a single small stripboard because that's where i'm at in my life right now.

I like stripboard because i can do the whole thing without asking for help from a PCB shop in China. Granted, i asked those Chinese shops to make me all the OTHER electronic bits in my kit, but i'm doing the best i can. I would brew silicon transistors in my kitchen if i could.

I have respect for SynthDIY projects that look super polished, like something out of NASA's skunkworks. But the ones that speak to my heart look like they were assembled in a post-apocalyptic wasteland. With my builds, i'm trying to walk the line between MadMax and some kind of weird Antiques Road Show folk art.

I almost wanted to call this the "Pepe Silvia Edition" because of all the flying jumpers. There are a lot of places in this layout where i've "broken the rules" for tidy stripboards. My advice to you if you attempt to build this:

1. Build from one side of the board to the other, soldering 2-5 components at a time to build up a cluster of components at a time. I have read about folks doing all the jumpers, then all the resistors and diodes, and then all the caps, etc. I don't recommend that for a build like this. Some of the jumpers need to fly over the top of existing components sooo...
2. Leave room for the flying bus wires that go from one side of the board to the other. This means you may want to "loft" certain resistors to avoid covering the perfboard holes beneath them.
3. Use scraps of wire insulation on the "at-risk" resistors to avoid accidental shorts. I know it looks like they don't touch right now, but when you get it in the case and drive it to the studio, things will shift. R14 next R32 is a disaster waiting to happen, don't let it get you down. Worst case, hit it with the Hot Snot™ (AKA hot glue, aka hot melt) to keep things from shifting. We aren't proud people, but we do need to troubleshoot so don't get carried away.
4. KNIFE. THE. TRACE. Always. Just run your craft knife or box cutter between all the copper traces on the board. Once you've knife'd all the rows, hold your board up to a lamp with the solder side towards you. This will help you spot solder whiskers, which you may have made, even if you're a badass synth wizard.

Without further ado:

And my build:

Module #006 Dual Yusynth ADSR pt. 2

Completed! The board didn't initially work because I had made some errors, and thus, there were some challenges:

1. 
   My initial layout had omitted the link from R3 to 15v. This was easy to fix by visual inspection
   starting at the switch. Fixed with a new jumper and addressed in the layout.
2. I included the BS170 FET in my layout but had to substitute it with a 2N7000. You can substitute the BS170 with a 2N7000, but you must remember that the pinouts change. I had installed mine backwards. You can also omit this, but the module may "misbehave" if you cycle the gate before the DECAY cap has drained. 
3. I ran out of knobs. I think the cheapest plastic ones from Tayda are gross looking, so I tried to fabricate my own:
1. On the left, I have some 3D printed knobs from Thingiverse. They worked OK but my filament color is horrendous for this task. So i kind of hate them, easy as they are to print.
2. On the right, i made some knobs out of 5/8" dowel rod trimmed to half-inch length and drilled out. There is a tedious procedure one can do with a drill press to center the shaft hole, but my equipment was not compatible with it, and thus my shafts are off center. Still, they look OK and feel surprisingly good for being off-center.
4. The blue LED in my build is noticeably brighter than the green LED in the OFF state. My DMM shows ~2.7 volts across the blue LED terminals, and only 1.4v on the green. I spent several hours reviewing the build looking for errors. I finally put it on the oscilloscope, and, comparing the outputs from both boards, the ADSR envelope is identical for a given setting. If there is an error in my build, it is benign. I also suspect the blue LED might simply be brighter than the green one, but that seems flimsy...

Here is the layout. If you build it, I'd appreciate it if you shared a photo of your build in the comments. 

module #006 Dual Yusynth ADSR pt. 1

module #005: Thomas Henry VCO-1, Second Build

On stripboard, using the layout i shared with electro-music.com.

I haven't tuned it up yet, there's always a nice high that comes with finishing the last solder joint. Of course, if it doesn't work, that's not the last solder joint after all...

I spent 3 hours working on a stripboard layout for the Thomas Henry 555 VCO, only to have DIYLC crash and lose the work. That's ok, it was starting to look like black MIDI anyway. Stripboard layouts always take multiple attempts to get right.

I'll be splitting my time between working on the 555 VCO stripboard layout, and wiring up another stripboard version of the Yusynth ADSR. The next milestone for me is 3 oscillators and 1 white noise, controlled via MIDI, for a nice follow up to that Super Mario theme video I previously shared.