Twittering Laser Tripwire with Webcam Capture

Step 2 Connecting the Laser Pointer

You can find these really cheap at some retail stores, I know that mine was under $3.

Unscrew the end and remove the batteries.

Use a hacksaw or sharp seraded knife to cut off 1 to 1/2 inch of the laser pointer, so that you can access the spring. You may not need to do this depending of what kind of laser pointer you found.

Connect and solder a wire to the spring ( I bent mine out to make it easier ) then rough up a small part of the outside of the laserpointer with a knife/sandpaper/pliers ect so that you can solder it to the outer casing. Then tape the button that activates the laser.

On my laser pointer the spring is + and the casing is - yours might be reversed. The best way to check is to take note of what position the batteries go in, the coin cells will have + marked on one side, if that side touches the spring then the spring is +.

Connect your + wire (Casing) to digital pin 13 on your arduino and you - wire (spring) to GND. 

Now upload the arduino sketch and ensure that you are getting readings from the LDR and that the laser is on.

To accomplish this you must uncomment this line:
//Serial.println(ldrVal);

and comment out this one:
Serial.print("1");

If all is well you can position your Laser and LDR in a  fixed position, bounce the laser to the LDR with a mirror to see that its registering. When the laser is detected it should print a "1".  You may need to adjust the threshold values depending on your LDR.