I actually just finished coding a laser trip wire system using the Arduino UNO that uses two beams, motion sensors, and an internet connection to do the following:
When the level of resistance for both beams is below a certain amount, the alarm triggers and the software starts looking for motion on one of the four motion sensors around my house. It then reports back, telling me which side of the property has been breached. In addition, this system send me a text message when the lasers are tripped. I am very pleased with this solution. The major reason why IR lasers would not work for me: impossible to reflect them around the house...
To take care of the lasers at each reflection post, I will build a secondary tube around the column and put a small tree in front of it. this will allow the laser to go into the outer tube, hit the small mirror and reflect the beam to the next point without being seen.
Drawbacks:
FOG, RAIN, SNOW -->
Luckily I live in Florida so snow is no issue.
Although, rain is a major issue. (I plan on adding rain sensors to this system, U will program the system to ignore certain types of chatter when it is raining only.)
As for fog... If I was driving through a town looking for a place to steal something, it wouldn't be from the people who has a laser trip wire system...
Anyway, if anyone is interested I will be happy to share my code.