It's always great to see Arduino being used in creative, musical ways. These piano stairs are a great example of musical creativity. As you put your foot on a step, a different piano note plays, so as you ascend the stairs, you will hear a lovely piano scale. It's a clever take on the giant flat pianos you often see in tech museums and art installations.
The piano stairs were made for the HackPrinceton event by Erica Portnoy, Vincent Castaneda and Instructables user Bonniee. As so many people have asked here to share the code and schematics, she has decided to make an Instructable.
The project uses a basic light "tripwire" set up on each step. A photo-resistor measures the light shining on it from a light source on the opposite side of the step. As a foot interrupts the light hitting the LDR, a note on the piano is triggered.
The whole project is pretty simple, using 6 sets of LDRs, resistors and light sources, hooked up to a speaker, an Arduino and a Raspberry Pi. The Arduino is used to measure the LDRs, and sends the data to the Raspberry Pi, which makes the notes.
The instructable is an excellent demonstration of how to use photo-resistors, and interfacing between an Arduino and a Raspberry Pi. You can read the whole tutorial here.