This is a 14 pin PIC, mono synth controller.
It uses cheap external DACs and has the following features:
MIDI In, Channel 15 or 16 (switchable)
5 Octave output
DAC 1: 1v/octave
DAC 1: Tune output
DAC 2: Velocity
DAC 2: Mod Wheel
The tune output is set at 1 volt, the pitch control raises and lowers this voltage accordingly.
You can use only the main DAC if you do not requrie the velocity or mod wheel outputs.
DAC 1: dual channel MCP4922 12 bit
DAC 2: dual channel MCP4902 8 bit
Above: Standard schematic of the MIDI to CV chip.
DAC U3 is required for basic operation, DAC U4 can be left out if not required and pins on the microcontroller left unconnected.
The PBEND output sets at 1 volt, sume this input along with the VCO-CV into the VCO before tuning, then the pitch bend will adjust the output frequency. Note the Pitch Bend output is not really tuned, it will just adjust the voltage to control the frequency.