LEGO Piano mechanism
There is a wonderful site listed in my LEGO links that details a fully working harpsicord made out of LEGO. It's the only working LEGO instrument that I'm aware of, although many years ago I saw a LEGO guitar in the pages of Bricks 'n' Pieces and somewhere I have a photo of a LEGO ukulele that I built.
Unfortunately, the string tension required in a piano makes building a working replica difficult, not to mention the number of bricks that would be needed! My plan is to build an octave or as near to an octave as I can get before running out of bricks.
I have already prototyped the mechanism and pictures can be found below. As it can be seen it's fairly rough and ready. The model is based on the mechanism found in a traditional grand piano.
Key to the operation is the Wippen assembly. This allows the hammer to rest back slightly without going all the way back its original position, whilst the key is held. This makes repetition of the note much faster since the pianist doesn't have to completely release the key before strinking the note again.
There are only four photos of my prototype here, so scroll on down and have a look.
The Wippen Assembly
The Wippen Assembly - close up
The hammer in the rest position and the lower end of the Wippen assembly
The whole mechanism