I used the same method for controlling the vacuum that I described earlier. That is, solenoid valves were controlled by the Arduino Due. When an individual valve was opened it would apply a partial vacuum to a particular pneumatic.
In the picture to the right you can see the plastic pipe that connects the vacuum pump to the solenoid valves, and thence to the pneumatics. Connected to each of the pneumatics is a wire. When the vacuum is applied to a particular pneumatic it pulls on the wire.
Here is the attachment of the wire to the pneumatic. As you may be able to discern this is done through a turnbuckle that I 3D printed. The reason that I went to this trouble is that the length of the wire is critical as you can see in the next picture.
These are the "fingers" that actually press on the strings. The wires from the pneumatics attach to the top of the fingers. Pulling on the wire causes the lower end of the finger to push the string against the fret. As I said, the length of the wire is critical. That is because when the pneumatic is not activated the bottom of the finger must be well clear of string so as not to buzz or damp the string if it is plucked. On the other hand, it must be close enough to the string so that when the pneumatic is activated it presses firmly on the string.
And here is what it looks like overall. The silver wires from the twenty-three pneumatics lead to the compressed space where the corresponding fingers hover about the fret board. Thus, twenty-three fretted strings plus six open strings gives twenty-nine notes or about two and one-half octaves.
And once again here is the link to a quick video that might help to understand how it all goes together in case my explanation is lacking.
