The MIDI Synthesizer gives the possibility to connect your Paper Bits to a Midi Keyboard like theMIDI Keyboard. It is polyphonic up to six notes and has three different waveforms: square, triangle and sine.
It's basically an Arduino clone running a small program based on the wonderful Mozzi library.

For burning the bootloader look here:
AVR & Bootloader
For uploading the program one needs an USB to Serial adapter or an Arduino:
The Arduino as a Serial Adapter

pb702_midi_synth_22.zip

• Resistors:
• 1x 220 Ohm
• 1x 330 Ohm
• 2x 470 Ohm
• 2x 1 KOhm
• 2x 1.2 KOhm
• 1x 2 KOhm
• 1x 3.3 KOhm
• 1x 3.9 KOhm
• 2x 8.2 KOhm
• 1x 10 KOhm
• 2x 47 KOhm
• 1x 100 KOhm
• 1x 220 KOhm
• 1x 1 MOhm
• Capacitors:
• 2x 22 pF
• 2x 1 nF
• 1x 4.7 nF
• 3x 100 nF
• 1x 150 nF
• Electrolytic Capacitors:
• 1x 1 µF
• 2x 10 µF
• Semiconductors:
• Atmega 328P-PU
• OpAmp MCP 6002
• Optocoupler 6N138
• Cristal
• 1x 16 MHz
• Diodes
• 1x 1N4148
• 1x LED
• Mechanical:
• 1x IC socket, 28 pin
• 2x IC socket, 8 pin
• 1x pin strip 1x14pin
• 2x pin header, female 90° 1×3, 1×2
• 2x pin header, male 90° 1×3, 1×2
• 2x push button
• 3x jumpers

The synthesizer program is based on the Mozzi Library, have a look at the Mozzi Dokumentation to learn more about it.
For the installation download the library: https://sensorium.github.io/Mozzi/download/
And install ZIP library like described here: https://www.arduino.cc/en/Guide/Libraries#toc4

Unfortunately there is a glitch in Mozzi. Arduino change he compiler so the uploaded code is smaller now, but also a timing problem appeared. To fix the glitch follow these steps:
In: arduino-x.x.x\hardware\arduino\avr\platform.txt change -O2 to -Os.
https://sensorium.github.io/Mozzi/fixes/updates/howto-for-arduino-1-dot-6-11/
Here is an explanation about the problem. https://www.instructables.com/id/Arduino-IDE-16x-compiler-optimisations-faster-code/

Code:
mozzi_midi_poli_12_1sec_limit.zip