>Hmmm. Looks like it's either an IR sig, modulated @ 38-40kHz, to a 
>reflector
>and back or a magnetic sensor, Honeywell probly.
>The reflector set up would work, I'd say, but needs to be aligned + I'm
>thinking about battery capacity if you're flashing the light non stop all 
>day.
>The battery wouldn't have to be particularly small, however.

The alignment at the range you need is really not as difficult as it sounds; 
the beam is fairly divergent, both ways, transmitting and reflecting. Its 
the shielding to prevent direct reception you will have to be careful about.

Take a look at http://www.rentron.com/PicBasic/infrared_object_detection.htm

This is for PBP users, but you'll get the idea.

Regards,

Anand

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