replacing pushbutton in unknown circuit with optoisolator

I have successfully replaced a push button with an optoisolator for arduino control of an unknown circuit. Everything works fine so far but I wish to know if I might run in to trouble later. I measured the current across the switch of the unknown circuit as 1.6mA. I’m using a Sharp pc817 optoisolator which has a CTR of 50% and a max current of 50mA. on one side of the optocoupler (LED Side) is an arduino output pin and a 150 Ohm resistor to limit current. On the other side (transistor side) I put no resistor at all. is this OK? I assumed that the current will be limited to 1.6mA so there was no need for one but I’m not sure if it is safe.

What you have done sounds OK, provided that the voltage across the pushbutton or optoisolator output when the button is not pressed is low enough. Have you measured that voltage?

Hi dc.
I have, it’s ~12V. The pc817 has the following max ratings on the output:

Collector-emitter voltage, VCEO: 35 V
Emitter-collector voltage, VECO: 6V