So I've got this old passive volume pedal lying around. I haven't used it in many years. Today I decided to use it as an expression pedal, which is something that I am far more likely to use.
The pedal itself uses a very basic setup - a trim pot and the main pot which is controlled by the tilt of the foot. The audio signal usually goes through the pots which adjust the volume.
I made a cable that connects my Arduino board to two 6.5mm mono jacks. The shield of the blue connector goes to ground, the tip of the blue connector goes to 5V, the shield of the red connector goes to ground and the tip of the red connector goes to Analog Input Pin 5.
The red connector is connected to the output of the pedal. The blue connector is connected to the input. The idea is that the volume put acts as a voltage divider. The output is then a voltage between just over 0V and 5V, depending on the tilt of the pedal. This voltage is then read by the Arduino
The Arduino has a very simple sketch, which checks for a change in voltage on its analog voltage, and then sends the data to a Max/MSP patch.
The Max/MSP patch takes the value, which is between 0 and 255, and divides it by two. This value is sent as a MIDI CC value to other programs or devices, such as Ableton Live
I plan on housing the microcontroller etc in the pedal itself, and running actual MIDI hardware from the pedal, so that it doesn't need the Max/MSP patch.
Below you can see a quick demo video of the pedal in action.