This device is very simple and can be built on simple prototype board, but its effect can be really annoying. It simple generates a short beep of very annoying frequency at random times (within half an hour to nine hours).

All you need is to hide a device somewhere and see the reaction of your friends. Project source code is written for AVR-GCC compiler and is simple to understand and use. Just be sure to enable CKDIV8 fuse in order to run attiny2313 at 4MHz from internal clock generator.