What to use for a Light Barrier ?

Hello,

because the PIR motion detector is independent of the laser barrier, I already ordered the PIR motion detector. This one is with pins is designed for the use with Arduino.

Can I supply 5V to the 4.5V laser pointer without problems? I tested it with one I have at home, and it worked, will it work again, for longer times?
I read that a reference value for the photoresistor's value should be taken (reference: with normal light (either day or night); and the one the laser lights on). These values can be compared, but how should I compare them? What about: if v1~v2 then alarm, that means if they have circa the same value. I could say "abs(v1-v2 )< 50 then alarm", what about this ? Calibration wouldn't work because the room I am speaking about changes its brightness during the day very much.

Thank you for your help.