Stephen D. Barnes wrote:

>Why not use a standard industrial retro-reflective (IR) sensor with the 
>appropriate reflector?
>See www.banner.com for more info. There are many other manufacturers of 
>these but the Banner mini-beam
>sensors are great.
>

This is exactly what I was going to suggest, but OP should note that 
these will detect anything that breaks the light beam.  People, etc.  
You will also have to align the reflector and sensor every time it is moved.

BTW, that link should be www.bannerengineering.com.

(I am partial to the Mini-Beam SM312.)

Aaron
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