This instructable will teach you how to make a laser trip mine.
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
Attach the laser to the switch and the 9 volt. Make sure everything is compact so that later if you want you can put it in a box. Also to make sure the laser doesnt get burned use a resistor.
Step 2: The Circuit
First, you connect the negative side of the 9 volt to the emitter of the PNP transistor. Next, put the negative side of the L.E.D to the collector side of the PNP transistor. Then, attach one side of photocell to the base of the PNP transistor and the other side to positive side of the L.E.D. Finally, attach the positive side of the 9 volt to the left side of the potentiometer and the middle part of the potentiometer to the positive side of the L.E.D and the photocell. Make sure the potentiometer is set on low so you dont burn out the L.E.D.
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.