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Need Help With Laser Trip-Wire

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Ok so i saw this video on Youtube (YouTube - Protect Your Home with Lasers!) on how to build a laser trip-wire alarm system. A simple project where if you cross the path of the laser an alarm will sound.
The parts it requires are:
1- 5kΩ Variable Resistor
1- 9V Battery Connector and battery
1- 2N3904 Transistor
1- 12V Siren
1- Photocell

Here is the schematic that is given in the video (sorry these are hand drawn and scanned onto my computer)

<a href="http://s126.photobucket.com/albums/p117/greenchix/?action=view&current=origional.jpg" target="_blank"><img src="http://i126.photobucket.com/albums/p117/greenchix/origional.jpg" border="0" alt="Schematic"></a>

Here is the schematic i made from this (i think it is the same as the top)

<a href="http://s126.photobucket.com/albums/p117/greenchix/?action=view&current=untitled.jpg" target="_blank"><img src="http://i126.photobucket.com/albums/p117/greenchix/untitled.jpg" border="0" alt="other"></a>

When i breadboard this though the siren ends up sounding no matter what. I have a few questions about this circuit

1. How exactly does this circuit work?

2. If the Resistance is lower on the photocell with the laser on , why is it that the siren sounds when the laser is off

3. Any ideas to fixing my problem

I will be very grateful to any ideas to help me fix my problem. Thanks!
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