For my senior capstone project, I designed an automatic transmission for a pedal bicycle. A monster servo is mounted in the rear derailleur, replacing the current cable system. An encoder wheel was placed on the front sprocket so the users pedal speed can be measured using an infrared reflectivity sensor.
Depended on the riders pedal speed and rider-adjusted thresholds, the bike shifts accordingly.
The brain behind the bike is a Basic Stamp. A 2x16 character LCD and a few buttons were installed so a user interface could be implemented.