This weekend I finished my black box, which is a 5V utility box (input voltage 6.6V and higher) to be used in combination with Eurorack. It has a number of functions, the first of which is the ability to provide random notes on a minor scale.
It uses a PIC10F206, with the GP3 the clock input, and GP0, GP1 and GP2 the random note output. In order to produce the voltage it alternates between these pins, and uses a very high frequency PWM signal. Between switches the port is set to input. An MCP6274 OPAMP is used to ensure that the voltage doesn't drop, and a two-pole passive low pass filter is used to ensure that PWM is smoothed out. The disadvantage of the passive low pass filter is a slowness between tones, but for the minor chords I'm hoping to generate this is alright.
The source code is available here. The circuit is relatively straight forward. The output is between 0-5V initially, and there are trimmers to lower this to a 1V/octave range.
