Grumpy_Mike:
Close.
Inside the opto isolator is an LED. You drive it like any other LED. An LED is a none linear device which means you can not just put a voltage on it and expect it to work at the proper current. Yes the resistor drops some voltage but only because of the current the LED is drawing. The LED has a specific voltage dropped across it when the rated current is drawn. That resistor should be picked so that it drops the difference between the power supply and the forward voltage of the LED at the required current.Look at LED resistor calculators and get the LED's forward voltage from the data sheet of the opto.
No that is fixed by the Arduino, nothing you can do about that it is just the resistor you need to calculate.
Ok. I think I get it now. The key element for the isolator is the current and the resistor is there to adjust it, and not the voltage. I just need to get an optoisolator and figure out the resistor, THEN Bob's yer uncle 
Thank you everyone for your help.
Carlo.