its very simple.
LDR based monostable multivibrator circuit using IC555
this circuit will give high output pulse when light falling on LDR interrupts. use this pulse to give trigger to ur micro controller.
similar circuit you can build for pressure sensor/switch
you have to detect high to low (or low to high) transition on micro controller pin.
actually you dont require micro controller for such application. alarm can be directly triggered by IC555