ADHS-Tron - MIDI controlled Korg Monotron

The mission: My friend L'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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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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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

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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)
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The first cardboard prototype of Tron"s new housing.

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

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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
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Designing and soldering the perfboard
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The almost finished perfboard
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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
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Happy outdoor hacking
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Designing the final layout of the control elements
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The final cardboard prototype
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Inside the prototype, trying to fit the cables in the case...
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Drilling the case
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Mounting the control elements
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ADHS-Tron ready to roll!
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After a couple of gigs, Tron still rolling!

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

Article published Jan 31, 2018 and last updated Apr 30, 2022 , Project time around July 2017. jump to top