Can I drive an optocoupler input with just 0.5mA (500 microA)?
There is no minimum current but there are some graphs and you've fallen off the end of the curve.




  But when pressing the button, the amount of current that flows is only 0.5 mA. Is this current enough to "turn on" the internal led of an optocoupler, and then it switches the internal transistor?


This suggests to me that you are wiring the opto-coupler in series with the button. (The 0.5 mA suggests a 10 k&Omega; pull-up resistor.) That's a bad idea because the 'low' voltage will now be the voltage across the opto-LED and this may make the original circuit unreliable.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) The original circuit? (b) The proposed opto-isolator modification?

If D1 is inserted as shown in Figure 1b then the voltage at 'A' can no longer be pulled to zero.







simulate this circuit

Figure 2. A fix?

Instead, add an external, strong pull-up in series with the opto-LED and tie the cathode to the switch. Now you can get as much current as you want through the LED. With 330 &Omega; you should get about 10 mA.