by ranjit, published
This simple noisy gadget uses a method known since the 19th century to generate the notes of a scale: the number of teeth on each gear corresponds to the frequency of a note, so when you let something buzz against each gear, you'll hear the corresponding note. Here's a video of the thing in action: http://www.flickr.com/photos/ranjit/5472598312/
This set of gears creates a just intonation diatonic scale - all the pitch ratios are related by simple fractions. The gears have 24, 27, 30, 32, 36, 40, 45 and 48 teeth.
You could use an optical sensor like a photo interrupter to pick up the pitches, or if you cut the gears from steel, you could use magnetic pickups like the Telharmonium did back in 1898! http://www.synthmuseum.com/magazine/0102jw.html
There's two documents here - Gears, intended to be cut from 1/8" wood (or acrylic), and Frame, for 1/4" material.
I found a length of 1/4" steel rod that I used as the axle, and a little DC motor with a little gear to drive the whole thing. You will probably need to adapt the design to fit the axle and motor you have. Note the drive gear in the upper right corner of the Gears document - I set the number of teeth on that gear to mesh with my little motor. There's a 1.5 inch round hole in the two motor support brackets in the Frame document, that's also customized to fit the motor.
To assemble the gear set, stack the gears on your axle with 2 spacers between them, gluing them as you go. Try not to let any glue get onto the axle! The little drive gear goes on last, on top of 3 or 4 spacers.
The three axle support brackets (left side of Frame document) have notches so they can be securely fit into the base, but the motor support brackets are intended to be glued freehand to the base so you can adjust for the motor's gear to press neatly against the drive gear.
Try it at different speeds, and with different scrapers, and see what kind of sounds you get!