want to make a SPECIAL TYPE SPEAKER

[kartik nighania]

im a newbie.. so sorry for non technical terms I am going to use..
I had made a circuit that gets opened for a very short time and then gets closed again as it was before. (this happens in less than half second)
but I want to make an alarm that will run when the circuit gets opened and will continue to ring even after the circuit is closed...
actually I had made a circuit using an LDR light sensor when the circuit gets opened and closed I want an alarm to work for minimum of 15 sec...
PLS HELP ITS FOR MY SCIENCE EXHIBITION.......