midi2cv

This repository contains the code and schematic for a DIY MIDI to CV converter. I installed this converter into a home-built analog synthesizer, allowing me to play the synthesizer with my Yamaha CP50 keyboard over MIDI.

The MIDI to CV converter includes the following outputs:

• Note CV output (88 keys, 1V/octave) using a 12-bit DAC
• Note priority (highest note, lowest note, or last note) selectable with jumper or 3-way switch
• Pitch bend CV output (0.5 +/-0.5V)
• Velocity CV output (0 to 4V)
• Control Change CV outout (0 to 4V)
• Trigger output (5V, 20 msec pulse for each new key played)
• Gate output (5V when any key depressed)
• Clock output (1 clock per quarter note, 20 msec 5V pulses)

Parts

• Arduino Nano
• Optocoupler (I used a Vishay SFH618A, but there are plenty of alternatives out there)
• 2x MCP4822 12-bit DACs
• LM324N Quad Op Amp
• Diode (e.g., 1N917)
• 220, 500, 3x1K, 7.7K (3K+4.7K), 10K Ohm resistors
• 3x 0.1 uF ceramic capacitors
• 5 pin MIDI jack
• 7x 4mm banana plug jacks
• 3-pin header and jumper or 3-way switch

The Arduino code uses the standard MIDI and SPI libraries, which can be found in the Arduino Library Manager.

The schematic is illustrated at the bottom of this page (Eagle file included). Input power (VIN) is 9-12V. This is required for the Note CV op amp, used for the 0-7.3V note output. 1% metal film resistors are recommended for the 7.7K and 10K resistors, for a constant op-amp gain that does not change with temperature. Note that 7.7K is not a standard resistor value. I used a 3K and a 4.7K resistor in series, which are much more common values. If precise tuning is desired, a trim pot can be added or the constant NOTE_SF can be adjusted in the code. I opted for the latter.

Note priority is selected using a jumper attached to the three-pin header labelled NP_SEL in the schematic. This header connects to the Arduino pins A0 and A2, with the center pin attached to ground. Alternatively, a 3-way switch can be attached to this header.

The note priority options and jumper configuration are as follows:

• Highest Note: When multiple notes are sounded simultaneously, the highest note being held will be sounded. When the highest note is released, the next highest note will be played, and so on. Remove the NP_SEL jumper to select this configuration.
• Lowest Note: Analagous to highest note, except the lowest note being held will be sounded. Connect the NP_SEL jumper to the A0 pin and center pin (ground) to select this configuration.
• Last Note: The most recent note played will be sounded. When that note is released, the next most recent note still being held will be sounded. Connect the NP_SEL jumper to the A2 pin and center pin (ground) to select this configuration. The illustration below shows the jumper configuration for the Last Note setting.