Light controlled burglar alarm
This project is a light controlled oscillator. In total darkness it just sits there nice and quiet, but a little light causes the speaker to give out a howl.
The CdS (LDR) cell acts like an open circuit in total darkness, but when some light strikes the cell its resistance decreases enough to allow the feedback of output signal to get through and cause oscillations. Do you want to see if someone is curious?
Hide this circuit in a dresser drawer and then casually mention in someone's hearing that you have hidden something special in your dresser. Now find something to do away from the dresser but within range to hear the alarm. It usually isn't long before the alarm goes off!