SJS-One modular expander

Posted on April 8th, 2013

- Would it not be nice if the SJS-One could be controlled by and control your analog modular synthesizer?

- Yes, yes it would.
- Is it very difficult to do?

- No, it’s not.

Let me show you a couple of mods you can do with the SJS-One Synth!


This post covers the following mods:

  • * Trigger input
    Any short pulse above a few volts will trigger the SJS-One to play a note (selected by CV).
  • * Trigger output
    Fires a pulse every time a note is triggered via MIDI.
  • * CV inputs
    These to connect via schottky diodes and a voltage divider directly to pins A0 and A1 on the Arduino, and can be used to select note to be played when triggered (or you know, whatever).

Basically what needs to be done is to protect the Arduino from ESD and harmful modular voltages. It doesn’t take kindly to ~12V on its inputs (or outputs).
So, we have to add Schottky diodes for super fast clamping to 5V. And for the CV inputs we also have to add a voltage divider, so we don’t loose all those volts above 5V. To keep it simple, we just cut the volts in half, so from a 0-10V CV, we instead get 0-5V. For the trigger output we need to add two transistors to get extra juicy trigger pulses.

If we take a look at the schematics, we see that pins A0, A1, A3, D2, D12, D14 and D15 on the Arduino are unused. Any of these can be configured to do stuf.
But, for now we’ll use A0 and A1 for CV inputs, D2 for trig in, and D12 for trig out.

What I did was to put all the extra components on a stripboard, connect it to the jacks and pots on the front panel, and to the SJS-One. I just soldered wires to the headers between the SJS-One and the Arduino. Since the Arduino has an onboard 5V voltage regulator, I can power the whole thing with my modular PSU using the Vin pin on the Arduino.

Of course we also need to modify the source code for the SJS-One. For this experiment, we modified the Drumstix code. One of the CV inputs is used to select which sound is triggered by the trigger input. Second CV not implemented yet.

Download modified Drumstix code!

Now, to be clear. These mods are just an example. Your imagination and coding skills are the limit!

In a future post, I’ll show how to add external audio input to the MAX261-filter, so you can use the SJS-One as a voltage controlled filter…


Posted on July 16th, 2012

Tech Specs
Sound Demos
Support / Troubleshooting
After-market modifications

SJS-ONE General Informations

SJS-ONE is a software defined 8-bit mono synthesizer with a dual (series) switch-cap filter.

Technically, there is nothing outstanding in the design besides perhaps the very capable MAX261 filter which we are, as far as we know, the only ones to use at this point in a DIY-synthesizer.

But the synthesizer has some interesting history that has certainly colored its development and ultimately its sound. The initial version was developed as an instructional workshop for a synth-event called SyltJam in 2011. The workshop idea was to show people how quickly you can design the basic hardware and software required to produce a platform for versatile audio and music generation. Read more »