An ATTiny1604 can be programmed to control two 555 oscillators to generate melodies or sound effects/noises.

There are also headers for a UART or I2C interface so the ATTiny can also be programmed to accept commands from serial inputs.

Each 555 circuit has two digital pots controlling them - one controlling the main oscillator frequency and another controlling voltage on the CV pin for another way to manipulate the audio tones.

The output of each 555 goes into two D-Flip Flops connected in series, dividing each tone frequency by 2, resulting in another tone one and two octaves below the main tone on each 555.

In total there are 6 audio channels: two separate 555 tones, and two sub octave tones for each 555.

Each of the audio channels can be turned on and off by controlling a 4066 switch IC.

All enabled audio signals are summed with an op amp mixer before being sent to the audio output header.

There is another digital pot controlling the final output audio signal level between minimum (ground) and maximum.

This can be controlled by the sketch to mute/change volume level, including sliding the level up and down to create a tremolo effect.