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Infrared Beam Burglar Alarm Answered

Could someone please tell or show me a fairly inexpensive way to throw an infrared beam across my office and once it's tripped by an intruder an outside alarm or siren (I mean the loud ones-like you can hear several blocks away)would go off. I'm so impressed by the quality of minds on this website. I know someone can probably help. Bless You All.
Thank You So Far!


Or you could make your own, considering you have the time, money and expertise.

You could buy a kit... assuming you consider $19.90 to be inexpensive. Granted, considering that this comes with two PCBs, all components, and instructions, that's a decent price.

This is true too, one could experiment with a few mirrors or reflectors and send the beam all around a doorway or window area.

IR beams are normally used "across" a doorway or something similar; they have the downfall of being able to be stepped over (or under) depending on position, IF the person knows where the beam is. With most frequencies of IR, it would remain pretty invisible even if someone blew smoke into the area. If it is not an entrance you wish to protect, but an area, I believe Guy and Ham0 have the idea. There are IR detection systems for movement, but they are really prone to false positives (I have a few of them at home that I used for a month or so, and got tire of the cars coming up the adjacent street setting them off).

I think there's an instructable on this site that does something with a laser and if the beam is broken it does something. I think for a big room if you want all around you need supersonic system.