This is a sound generator (can also be used to create melodies) based on two 555 oscillators.

Each oscillator is also divided down into two sub-octaves using flip flop clock dividers, giving a total of 6 audio sources.

The frequency of each 555 can be controlled with a physical pot, or if the pot is not in-circuit, X9C104 digital pots can be used to control the frequency electronically.

The 6 audio channels go to an op amp mixer and have a single audio output (requires an external power amp for a speaker).

Each audio channel can be switched on and off using 4066 based switches.

The output level of the mixer can be controlled by a digital pot so the level can be adjusted, or tremolo volume swells can be created.

One of the 555's is configurable between free running oscillator or monostable mode. When used in monostable mode, this 555 can be controlled by the first 555, creating the Atari Punk Console circuit.

All digital pots as well as the 4066 configuration switches are controlled by PCF8574 GPIO expanders, and there is an I2C header to allow an external processor to control the board.

The demo projects in the video and on github use an Arduino Uno to create simple melodies and sound effects.

https://github.com/GadgetReboot/GR_555_Controller