This thing has moved to Youmagine https://www.youmagine.com/users/jensa so I don't also get sued by Stratasys...
Want to scare some scarers for Halloween? Use Adafruit's Music & sound add-on pack, an Arduino, an IR-sensor & a 9V battery and mount it in this box.
Just print the box on a well calibrated printer and all the parts (http://www.adafruit.com/products/175) will snap into this box. Place the Wave shield on the Arduino and snap it in. Power it with the 9V battery and connect the IR sensor and speaker by pressing them into their sockets.
Just position the box wherever you want to sense bypassers and play sound. My 9V battery lasted about 3 hours when used with this (http://www.adafruit.com/products/1031) IR Sensor. Attached to this thing is also the Arduino source I used to run the 5 boxes I made as well as a un-booleaned Rhino file in case someone want's to modify things.