The story of ADHS-Tron

The mission: My friend Lena's Korg Monotron should become controllable via MIDI from a DAW.

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Starting Point: Monotron, a Perfboard and an Arduino Uno
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pic 3 First goal reached: Arduino receives MIDI notes through a MIDI input circuit

Arduino receives MIDI through input circuit and triggers Gate and Pitch CV inputs on Monotron. As you can hear it's not quite a major or chromatic scale. Arduino Uno's 8bit DAC can't output precise enough voltages for Pitch CV.

The code necessary for above functionality:

At this point it’s about time to tell you that the snippets you see on this page are reconstructed with the help of my git history and I can’t guarantee that the program was completely working/bugfree at that particular commit. If you find something odd please leave a comment and I’m happy to help, correct it, whatever. On the bottom of this page you find a link to the final code that worked for me.

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pic 4.5

The solution to the Pitch Control Voltage problem with Arduino Uno: Microcontroller "Teensy 3.2", equipped with a 12 bit DAC, precise enough to generate the needed voltages.

Arduino compatible code needs to be changed slightly to run on a Teensy (different serial ports and obviously pin numbers):

At that time I was not quite sure how I could possibly get the pitches right, I just did not know what the volts/octave definition is for the Monotron. After hours of trial and error I came up with this formula that sounded correctly over almost a 3 octave range (the lowest 3 notes always are a bit too low if you tune your Monotron to about 10:00 o’clock):

Weeks later I found a little mark on the freely available Monotron schematic that probably would have helped a little! ;-)

whole ribbon Vbe offset

pic 5 Added two new features: Filter cutoff controllable via MIDI CC or MIDI velocity, MIDI control can be switched off to keep original onboard filter control intact (see bottom of post for code)

pic 6 The first cardboard prototype of Tron"s new housing.

Filter controlled via MIDI CC (pink line), playing around with the resonance is always fun

pic 7 To fit everything in the case, all components on the breadboard (white) are to be soldered on a perfboard (brown). The original Monotron printed circuit board should sit right next to it.

pic 8 Designing and soldering the perfboard

pic 9 The almost finished perfboard

pic 10 Perfboard flipside

Features presented in order of appearance (If unpatient skip to 2:30 for some acidish sounds)

  • Filter cutoff and resonance controlled locally
  • Filter cutoff controlled via MIDI CC (pink line), resonance locally
  • LFO modulating filter cutoff, amount and rate controlled locally

additional features not shown here

  • Filter cutoff controlled via MIDI velocity
  • LFO controlled via MIDI CC

pic 11 Happy outdoor hacking

pic 12 Designing the final layout of the control elements

pic 13 The final cardboard prototype

pic 14 Inside the prototype, trying to fit the cables in the case…

pic 15 Drilling the case

pic 16 Mounting the control elements

pic 17 ADHS-Tron ready to roll!

Get the final code to this project here: Feel free to fork, send issues and so on!

This article was first published on Jan 31, 2018 and last updated on Feb 7, 2018. I built/coded/did this project around July 2017. jump to top