Lining up the lazer beam was not difficult with two units. However, one of the units must be the "primary unit" (Label: 001). Lining up three units were a bit more difficult. We eventually got it working, but it did take us close to 30 minutes to align all 3 modules.
We tried setting things up during the day, but it was much easier to set up when the room was dark. The spray from the misting bottle was not helpful in anyway for letting us see the beam, so we didn't rely on it to help with alignment. There is a focusing ring at the front of the emitter (blue arrow). Rotate it to concentrate the beam and aim it at the middle of the receiver.
When the receiver has "locked on" to the beam, the LED (blue arrow) will go on. This particular model also has voice confirmations that says "System Armed"...very cool :-)
Click on the movie on the left to see the arming of the system. Click the movie on the right to hear the tripping of the alarm.
The front of the bookcase is now protected by an invisible beam of light. Anything which interrupts the path of light will trigger the alarm :-)
The Wild Planet Lazer Tripwire works quite well and it's a lot of fun. It will make a perfect gift for the junior spy-in-training or the grown-up spy-wanna-be on your list this holiday season. Go check out this and other fun items at Wild Planet on Amazon.