Laser pointers were on sale at Office Depot ($8.99 a pop, as I recall) so I
thought I'd get a couple and make my RCX respond to them. Turns out it's really
difficult -- your laser dot has to be absolutely spot on the photoreceptor
of a Mindstorms light sensor to get a reaction and it's almost impossible to get the laser dot in the right
place when you're, say, the other side of the room. This was driving me nuts,
because I wanted to use the laser pointers for controlling the RCX, playing RCX
laser tag, and so on. I was trying lenses, translucent Lego bricks, and everything
I could think of to make the target larger.
...this little assembly allows the light sensor to respond to the laser pointer -
I can hit it across the room every time, and the only part you'll need apart from
the RIS is a piece of paper (I suppose you could always tear a piece out of the
manual, if you want to be 100% Lego parts).
Just for the record, you need to take care with hand-held laser pointers -
repeated exposure could damage your eyes so don't fool about with them: use
them in a controlled way in environments where there's little chance of stray
reflections (don't monkey with them near windows, for instance).
Also, although this works well indoors, it doesn't work well in outdoor
sunlight (just so you don't waste time trying it).