Disposable camera with flash £2.99
Carefully open camera with insulated screwdriver, find the cap on the board (without touching the board), short out the cap with said screwdriver (hold the insulated bit) this will cause a big spark, and possibly eat into your screwdriver. once thats done, cut off the cap with wire snips. remove battery, cut off the bulb too, now wire from the bulb to wires on the doorhandle (not shorted out, you may need to be creative with selotape to keep the wire bare to a hand but not shorting through the handle. short out the flash switch on the board and short out the trigger switch on the board.
insert battery, touch door handle (if you cant handle it yourself - dont do it to others)
you will get a zap, uncomfortable but not gonna kill you (although i take no responsibility for anyone building this - use at your own risk)
DO NOT do this with the cap attached as you could kill someone, and you dont need it!