How about a complete battery-powered MIDI monosynth in CircuitPython synthio? Three oscillator w/ 2-pole resonant low-pass filter. Great for basslines! Both USB & Serial MIDI. Responds to MIDI velocity & CCs to adjust loudness, filter, vibrato, and release time.
https://www.youtube.com/watch?v=S1-TDjxE3Qs
#CircuitPython #SynthDIY #MIDI #synthio