Electronics HELP!!!?

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Kevin

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I am building a laser burglar alarm that includes the following systems. I have a laser that is emitted from a project box and hits a photocell on another project box. I have no problem with the laser, but I have a problem with the photocell box. The photocell is wired in the schematic that is at the bottom. Supposedly, when the laser beam that is hitting the photocell is broken, the alarm will sound, but in my case, I have to rid the photocell of light in order to let the alarm stop.
Follow this URL to see the guide that I have followed in order to make this project (ignore the schematic that has been posted on the website and look at mine, because I have emailed the publisher of the guide and he has given me a corrected version that doesn't actually work)-----> http://www.instructables.com/id/Protect-Your-Home-with-Laserbeams/

The schematic is at this website> http://i304.photobucket.com/albums/nn193/haloofshadows/laseralarmrevised.jpg
 
First thought that comes to mind is, modulate the laser beam with a tone or some kind of electronic code. At the box with the photocell, interface some kind of tone or code detector. So, if the laser beam and demodulated tone / code is present, X happens. If the laser beam and the tone / code is not present, then Y happens.

That way, it does not matter if ambient light is present at the photocell after the alarm has been activated. Maybe after some timer delay, the alarm can reset on its own.
 
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