For those unfamiliar, Dub sirens are simple, triggerable, electronic sound effects devices commonly used in Dub Reggae music for synthy accentuation and atmosphere.
GetLoFi shares instructions on how to build your own dub siren using an inexpensive sound effects keychain and a few other simple parts with a total cost around 20 dollars or less. Great tutorial - demonstrates how much potential lies in the inexpensive electro-gadgetry so ubiquitous nowadays. MAKE SOME NOOOOOOIIIIISSSE! - How to Make an Easy Dub Siren...
http://blog.makezine.com/archive/2008/06/diy_dub_siren_for_cheap.html