Little small silly circuit for turning on a lantern's light bulb using a laser pointer:
This circuit must go inside a box for avoiding my room's light:
Now, I'll turn on the laser pointer. I used four photoresistors because the laser must impact any of them for turning on the light bulb my pirate dinosaur called Rigth is holding :D
Circuit diagram and explanation (in spanish) here (The circuit diagram is language independent, just consider that R2 is the light bulb I want to turn on).
If you take off the circuit from the box, the light bulb will turn on while the room's lights are turned on.
The light bulb can be replaced by an alarm :)