The Sparkfun folks have a sweet recipe for building an Arduino-based, accelerometer-triggered Tardis sound-effects box into the ceiling of an elevator, noting that care must be taken not to freak out riders and precipitate a bomb-squad visit.
As it stands, the contraption works well enough (much like the TARDIS itself). But for those looking for perfection, there could be a few improvements. These are left as exercises for the reader:
Improve battery life using sleep mode. Right now the Arduino is on all the time, using about 20mA of current continuously. We’re got a pretty big battery attached to it (6000mAh), which gives it a lifetime of about a week, but the battery could last for months if the project went to sleep between playings. The Arduino can indeed be programmed to go to sleep, waiting for an interrupt signal (a pin changing state) to wake it up. And the ADXL345 accelerometer can be configured so that it sends an interrupt when an acceleration threshold is reached, so this shouldn't be difficult to do. In practice you can’t get down to microamps with a full Arduino board, since it will always be burning some current in the voltage regulator, power LED, etc. But with a bit of software and hardware hacking, sleep mode would definitely improve the lifetime situation.
Stop playing when the elevator stops. Currently, the TARDIS MP3 was edited so that it is approximately the length of an elevator ride, and the code plays the MP3 to the end before listening for further accelerometer bumps. But you could also end a longer MP3 when you detect that the elevator stops (or, since the MP3 chip has a volume control command, you could even fade it out!)
Bigger and better! Bigger speakers, disco lights and music, black lights... you can really go in any direction. Make us proud. But remember...