Hi All,
I was wondering if anyone knows of a small, invisible laser that can be attached to the arduino? I plan to have the arduino's code count the number of times that the laser is broken. The laser will be broken by people walking by it (or through it for that matter 
). The laser does not have to have a range of more than 2 - 3 feet.
Search for IR in playground and in forums.
Okay, so I am currently looking at the PNA4602 and here, http://www.solarbotics.com/products/pna4602/ it says that the IR sensor can be hacked to be used as an object detector. How would I do this?
Look at the schematics. Also, they sell an ir emmiter. It should work with it. You're going to need to look at the datasheet. I'd try searching in the playground first and go from there.
Have a look at this:
If you only need a metre or so of range, why does it have to be a laser?
What should I use then? I need something that is very accurate because I will have two of these sensors parallel and 2 cm from each other. The sketch-code should be able to find out which sensor was crossed first, thereby enabling the arduino figure out in which direction the object is traveling.
I have inserted two drawings below from an overhead view.
In the first scenario, sensor 1 breaks first.
In the second scenario, sensor 2 breaks first.
Yes, but why does it have to be a laser?
You could do that with a single fan-beamed emitter.
Do you have any good ones in mind? Something as accurate I have described above?
Let's just hope that if people are the ones to pass through these that they are not flailing their arms around. This may cause inaccurate results. 
I've gotten an ir led to be pretty accurate, i.e. narrow beam, with a thin black tube. Don't know if this simple solution will be accurate enough though.
And also, wouldn't a cheap laser pointer be invisible? Unless there is smoke or dust in the air, you can't really see the beam. And if you can hide a laser pointer and a sensor, you can definitely hide a little red laser dot.
How would I go about using the laser pointer as a beam sensor? It seems that I would have to create the sensor myself? I am not very experienced and your guidance would be much appreciated 
How would I go about using the laser pointer as a beam sensor? It seems that I would have to create the sensor myself?
No - the laser pointer would be your light source; you would use a photo-transistor/photo-diode/LDR as the detector.
Interesting! Thank you so much for your help! I think I've got what I'm looking for. I'll keep all of you up-to-date on my progress!
Here is what I found: http://www.instructables.com/id/Twittering-Laser-Tripwire-with-Webcam-Capture/