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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.
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