The tones produced by my littleBits tone sequencer are too basic. So, I decided to add a littleBits filter module and envelope module to spice things up. I built the Arduino part of the project on one mounting board and built the synthy part of the project on a separate board. Three wire modules connect the two subsystems together as shown in the picture below.

Of course, since the whole thing is Arduino-based, it makes sense to drive filter modulation and envelope trigger (gate) from the Arduino. The trigger signal is turned on at the beginning of a note and is turned off at the end of a note. Nothing could be simpler.

The filter modulation signal is more fun. The dimmers connected to the Arduino control the attack and release time and the sustain level. Here is a simple diagram showing the shape of the filter modulation signal.

One dimmer controls both the attack time and the release time. Close enough for rock and roll. I suppose that I could have added a third dimmer and controlled these times separately. A project for you perhaps?

Per standard operating procedure, I posted the design and code. The code is explained in detail. I also posted this project to the littleBits project site. The littleBits page has the source code, too, and has simple directions for building the project.

Have fun and keep on experimenting!