I need to couple an external 24v to a Propeller input (tied to 3.3v, via 4.7k). I have a TLP620-4 that I liberated from an old industrial I/O card. The datasheet states a maximum If of 50mA and a recommended 16mA. There is also an indicator LED in series with the optocoupler input. I was looking at using a 1k limiting resistor for ~20mA If but then I saw that the other design uses 2.2k. This means a If of <10mA. The datasheet doesn't specify a minimum If. Furthermore, these fixed 24v power supplies that I work with are unregulated and rarely do I get the full 24v. Which way should I go regarding the limiting resistance?
Edit: Ah, I just found "+/- 8mA" for this 4-channel device. This feels a bit borderline, considering that the nominal 24v can be closer to 20v.