To sense the String vibration, I will use an IR refelctive sensor as the HSDL-9100-021 which is analog with the good sensing range (3-20mm) and easy to source.

with a PNP transistor as photodiode amplifier and a 15kHz low-pass filter as polarization tail and 100Hz high-pass filter wich I hope will cancel the 50Hz-60Hz hum.

Now we got an analog signal from the string, we have to square it up. For that I choose to use an guitar effect FUZZ distortion.

A FUZZ distortion is an hard clipping distortion with high gain stage, in this project I will do it with logic gate use in linear mode.

The diodes are here to transforme the last logic gate as a schmitt-trigger. 

Here are the coplete schematic for the first try of chiptune analog sound :

all the files can be found on my github.

To be faster I prefered go step by step by choosing a maximum of  already created board to keep the soul of the Game boy creator Gunpei Yokoi and his philosophy of "Lateral Thinking of Withered Technology".

with this mantra in mind i will use a gameboy zero AIO board

add the adafuit PAM8302A audio amplifier and the board I design

And solder wire in place of the raspberry pi to power up the board and send audio to the power amplifier 

I am looking for a better camera to have a good sound to recording!