i have a 5v logic signal and wish to trigger an opto-isolator with it . i have tried an on-line calculator for working out a resistor, but need expert help to be sure.
Opto-isolator is a: dual HCPL2731 DIP8
ratings from the data sheet
Input Forward Voltage 1.4v(typical) - 1.7v(max)
i think it says 12ma input max? ( so i've gone for 8ma)
Your math is ok and 8 ma is probably ok if your logic signal can supply that much. You don't say how much current you need from the output but with 8 ma drive you can get at least 20 ma of drive from the opto.
The logic signal is from an arduino which can output 40ma max per output pin , i think it is very close to 5v , i shall check the actual voltage tomorrow when i am set up again.
i not 50 percent sure about the actual 'ma' for the opto-isolator now, i can't make out the schematic very well. The Internet shop i'm buying from is saying 20ma max on their web page , but the schematic says otherwise i think .5 min and 12 max so i thought about 8ma would be o.k.