Kenneth Marut made a very cool, battery powered, hand held, digital synthesizer – Moon Germs.
This compact synth uses a combination of buttons and triggers to produce different waveforms and effects. An 8×8 LED matrix shows information while use. It also uses an IR proximity sensor to modulate frequency. In recent updates to the project he synth engine was restructured to include a Low Pass Filter and Low Frequency Oscillator (LFO).
As a lifelong musician Kenneth wanted to explore digital synthesis and experiment with unique ways of interacting with a synth using minimal buttons and knobs. He hadn’t really explored digital synthesis before and decided dive in using a Teensy 3.2 and audio shield.
Be sure to check out the HackaDay project page, it has a lot of great information.
Code for the project is available on GitHub