If someone steps in the lasers path I have gotten it to set off a L.E.D., but you can make it set off other things.
You are going to need:
2. Photocell, perferably a big one.
3. PNP switching transistor
4. 2 nine volts
*DONT POINT LASERS INTO PEOPLES OR ANIMALS EYES*
Step 1: The Laser
Step 2: The Circuit
The PNP transistor:
If you dont know which sides are what on the PNP transistor. Put the flat side North and the far left one emitter, middle is the base, and the far right one is collector.
To make it work:
Put the potentiometer on low so the L.E.D is totally off. Shine the laser directly on the photocell. When the laser is on the photocell turn the potentiometer slowly till it turns on, once its on turn it until it has just gone off, so it should be only a little to the opposite direction. Then to test it works put your hand through the laser and if the L.E.D turns on(should be brightly) then you've finished the project. If it doesnt make sure when the laser is on it the L.E.D is only barely off. Try to experiment with buzzers, relays, and other stuff.
If anyone knows Circuit and breadboard design software so I can make it easier to visualize the curcuit comment me. And if you have any questions or problems comment me.