How to use electrically isolated weak signal for Arduino interrupt?
@Chris it sounds like the circuit is not complete... do you have a pull up like R2?

This is what I expect to be using in this situation. The opto does not exists in CircuitLab but you get the idea. With only a 4N28 with CTR (Current transfer ratio) of 10% assuming R1 is 80 ohmsish and the diode forward drop is 1.2V, 10mA LED current, we should see 1mA sink capacity on the Output to Micro...

NOTE:
The grounds will be separate in this type of circuit normally.

I like CircuitLab... just wish generic symbols were available by default...







simulate this circuit &ndash; Schematic created using CircuitLab