Well, I can offer this:
There are three types of infrared "alarm" devices (that I can think of):
1. Passive or PIR sensor. A thermal pile coupled with a special multi-faceted infrared lens that detects changes in the ambient thermal background (i.e. it watches an area and if anything changes the thermal pattern, such as someone walking through that area, it toggles a voltage on it's output.) There are varying fields of view and these devices are VERY sensitive. I worked for a guy, back in the mid 70's, that owned a security firm that used these outdoors. I was sent to troubleshoot one installation that kept going off 24/7. When one of these devices would detect something, a very loud relay would click. This allowed me to sit under the thing and watch for what was setting it off. About a mile away there was a freeway that it was pointing at. I noticed that every time a large truck went by it would trip. It was _that_ sensitive! In another case, there was one pointing at a wall that got sun part of the day. During that time, whenever the wind would blow and cool the wall down, the thing would go off!
2. Infrared beam with sender and receiver. When someone walks through the beam the receiver toggles a voltage or a set of contacts (such as on a relay).
3. Infrared laser with reflector. In this case there are, typically, twin beams -- one going to the reflector, and one returning -- interrupt either beam and, again, a voltage toggles or a set of contacts change. The advantage of this over #2 is there is only one active unit, and thus power doesn't need to be run to two places.
Try Googling PIR.
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