Hello. There's a device installed in many stores that consist of a IR emitter, a reflector, and a detector that detects when the beam from the emitter is interrupted, for instance when somebody walks through the beam.
Is it possible to get a device like that to work outdoors (with the IR radiation from the sun and everything), over, say 3-5 meters? What is this kind of device called?
Is it possible to get a device like that to work outdoors
Possibly but normally the sun would saturate the receiver so that even modulating the beam would be no good.
To google try:-
ir interrupting circuit
Google "ir beam alarm outdoor long range" and see plenty. Commercial outdoor units are not cheap.
Add some circuit terms like "modulate LED volts" and you can find DIY stuff like:
http://www.electronics-project-design.com/DoorMonitor.html
Basically the commercial RX and TX work like TV remotes, they are modulated by a carrier such as 38kHz. For alarm you add another modulation to pulse the carrier on an off at an audio-like rate of 10s or 100s of Hz. The cheap IR RX module demodulates the 38 kHz and then you detect your own audio rate to hold off the alarm. The good commercial units use 2 beams at 2 unique freqs for diversity and thus reduce false alarms.
For just a coupla meters you can locate the TX and RX together and use a retroreflector.
Has anyone tried using a laser pointer for something like this?
An alternative idea: Set up some kind of photo-detector with an analog output, long with the "masking tube" suggested by Richard, and monitor the output, but looking for sudden changes in the light seen, rather than the simpler "presence/absence" test? Of course, without an effective tube, a day with fast moving, solid clouds may cause problems. Can a tube be effective enough?....
To google try:-
ir interrupting circuit
Yep, that's the word 
If I wanted to DIY one myself, I would use a laser diode as a transmitter...
That's probably what I'm going to try, something like http://palebluedot.nl/jml/arduino/16-flash-trigger-with-arduino.html
Commercial outdoor units are not cheap.
One more reason to make my own