IMakeProjects.com made sonic grenades from cheap personal alarms – “Pull a pin, and after a few seconds these “grenades” emit a very loud and annoying alarm – instead of shrapnel. This can be used just for fun (toss one into a sleeping roommate’s bedroom, for example) or used on the honor-system to enhance any wargames you might play – like laser tag, Airsoft, Nerf, or even Paintball. (If one “goes off” next to you, you’re out.) I modified some personal alarms to make these, and if you’re somewhat experienced with electronics you can make them too – I’ll describe what I did below. “ – [via] Link.