> I have a need to detect an object passing by a phototransistor, 2" 
> to 6" away.  Indoors is easy: I have an IR LED reflecting off the 
> object.  (I can not have the object break a light beam from the LED. 
> Everything must be contained in a single, small box.)  When the 
> object passes by, I get an increase in voltage.  Outdoors, I expect 
> the object
/>

You are more liable to get reliable results by sending a signal with 
chosen characteristics and detecting its reflection.

A simple method is to apply a constant frequency modulation to 
transmitted  IR and look for the reflected signal of the same 
frequency. Filtering for the desired signal allows you to do this with 
good discrimination. More on this if interested


        Russell 

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