I agree with John. This thing is made to interface directly. No external resistor needed. Just one wire.Wire the NPN current-sinking output directly to an Arduino input pin, with the internal pull-up on.
Looks like the NPN (current sinking/Open Collector) output of the light sensor (White wire) is TTL compatible. Just turn on the internal pull-up resistor and connect the Open Collector output of the light sensor directly to the Arduino pin. Connect the - side of the power (Blue wire) to Arduino ground.
Does, "(NPN output) less than 200 millivolts at 10 mA and less than 1 volt at 150 mA" mean that should my device draw 150mA (which the Arduino cannot do)
I figured that the 222mA should be more than enough to both power the Arduino and the ethernet shield, but what do you guys/girls think?