This is my design. The optocoupler is: VO619A-4 (http://www.vishay.com/docs/83430/vo617a.pdf) and ULN2803A (http://www.ti.com/lit/ds/symlink/uln2803a.pdf)
At the moment the parameters are: voltage from mc 3,3V, therefore the current should be 3,3/680= about 5mA. If I understood the optocoupler correctly then minimum CTR is 160%, normalizedCTR at 5mA is 0,7. therefore output current is 1,6*0,7*5=5,5mA. If I use a 10kohm resistor then I get current to ULN: 12:10000=1,2mA. ULN needs 0,93 to 1,63 mA to switch to high?(I think I got this rigth)
Q1: Is my understanding correct and would this work?
Q2: Could I also lower the input current to the optocoupler. What would be the min. current so that it still works?
Maybe some additional info: the ULN drives a relay.