Electrical Gizmo Project

OK, for reasons I won't go into, I would like to make a device that powers a little alarm/siren/light/whatever for perhaps 1-2 seconds upon a motion sensor being tripped.  Along the lines of those annoying lights your neighbor probably has mounted at the corners of his garage, the ones that light up when you walk down the opposite side of the street at night, robbing you of all night vision.  You know the ones.

Except smaller and shorter in duration.  I just want to provide power to a 12V DC device for a second or two, then power it down.  I figure it should involve a relay, as that's simple to close/open.  The alerting element should be triggered for a couple seconds and then turned off (until/unless the sensor again closes the relay).  Don't worry about power sufficient for the alerting element -- I can take care of that elsewhere.  Mostly, I need to kow how to make the alerting element turn off after a second or two -- I don't want it actuated for as long as your standard exterior security floodlight.

Thoughts?  Come on, don't you guys know everything?

I made one in college, only I called it a burglar alarm - made a piercing sound when an infrared beam was interrupted.  Had a manual switch to shut it off, but setting a timer would be easy enough.

Ready for a do-it-yourself project?  etch your own board, solder your own parts, etc. (If I even still have the design - was 1989 ffs)

Really long cables.