Sergegsx:
just saw your message dc42.
let me see if I understand the calculations.V=IR
80 =0,01 * R
R = 8000ohms
so similar to yours.
about watts of resistor, i think i got it wrong, i guess i need small 1/4W resistors
Yes, that's right. You don't need to run the optoisolator anywhere near its maximum current rating. For the 4N35, even 1mA forward current should be enough to pull the digital input pin low, if you are using the internal pullup resistor.