Thanks all of you for your help. However, my circuit board has been made like the schematic above for "HI trigger when beam broken". Therefore, I tried Stan's suggestion, but probably didn't make the correct change to the circuit and it did not produce a LOW to operate the relay when beam detected. I tried Stan's simpler approach (w/o transistor) and worked fine. I can remove (unsolder) all the extra parts and go simple, but would like to know what to do to make existing circuit work--and I did change the input as you suggested on the Arduino. That was all I did to the above 'HI trigger' circuit.
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