Well, the wavelength thing sort of makes this tough. But, having faced
similar challenges before, I can tell you that using a number of gel type
filters from the Rosco sample book is a good place to start. Keep in mind that
it will be nowhere near as good as a nice narrow band optical filter, it can
still get a lot of the ambient light out of the way.
Start by looking in the sample book at the spectral response curves and pick
about three filters that pass your longer wave red well while blocking most
everything else.
The #19 (Fire) filter is a good, sharp cutoff type to start playing with.
Overlap that on a #93 (Blue-green) and it will cut out a huge portion of the
visible spectrum while letting roughly 50% of the laser pointer light through.
Overlapped once again, this time with a #84 (Zephyr blue) and it will cut nearly
everything out but allow red laser light to get through pretty well.
Of course, this is no substitute for a real laser filter, which can
sometimes be found for a buck or so at good surplus outlets, but if you spend a
little time experimenting with the gel filter books and their passbands, you can
get a pretty fair filter together.
You know, modulating a laser pointer is really very simple and would greatly
enhance the sensitivity of your circuit. Check my web page and you can find a
section about laser pointers there which might be helpful.
Finally, if you go just for light and no modulation, almost any cheap
phototransistor can be used as your sensor and then any low current relay (or
even a solid state relay) driven from it. But I would really rather use a
modulated pointer and a "tone decoder" or PLL to do that.
Cheers!
Chip Shults
My robotics, space and CGI web page -
http://home.cfl.rr.com/aichip