A simple Arduino MIDI Synthesizer

I had nothing to test my Le Strum so I built a quick and dirty Synthesizer from an Arduino Nano, an optocoupler and a couple of passive parts.
Audio quality is not overwhelming but It works well enough to test the Le Strum.
Circuit diagram is here: http://www.sx2pc.com/arduino_midi_syn...
Software is the MIDI input example inthe MOZZI library:
http://sensorium.github.io/Mozzi/
https://github.com/FortySevenEffects/...