Maido Käära's Raspberry Pi Pico MIDI Knob Box Expands a Dirtywave M8 Tracker

Software developer and musician Maido Käära has built a MIDI controller to expand the flexibility of a Dirtywave M8 Tracker — vastly increasing the number of inputs available to its user with a "knob box."

"It's pretty small and doesn’t have a lot of buttons," Käära explains of the M8 in a blog post brought to our attention by Adafruit, "so working with the device needs a lot of key combinations to move through different screens and options. Getting the hang of the workflow is actually quite easy, but the interface is pretty limited if you want to perform or just jam with the device.

"Fortunately it supports MIDI In so if I would have a MIDI controller I could use that instead to control the device. Unfortunately the price point of the grid controllers is pretty steep so as usual I decided to build one myself."

While Käära had initially wanted to build something complex — "something with a screen," he explains, "and a rotary encoder that can be used to implement a menu for the system" — a few rounds of prototyping saw the scope reduced to a knob box, designed in SketchUp and 3D printed for prototyping as well as production of the finished model. "Prototyping with a 3D printer by just printing out the faceplate turned out to be very useful," Käära explains. "You can really get the feel of the layout when you can actually play around with the device instead of just looking at a render."

The finished design uses a Raspberry Pi Pico microcontroller board running a CircuitPython program to send MIDI messages over USB, controlled by a grid of 12 rotary potentiometers connected to a 16-channel analog multiplexer and mapped to undefined MIDI Continuous Controller (CC) messages. These then feed back into the Dirtywave M8 Tracker over a tip-ring-sleeve (TRS) cable — with the size of the box kept compact enough to not dwarf the M8.

The full project write-up, with source code, is available on Käära's website.