I use the MCT6 opto-coupler a lot in 5vdc circuits.

Resistor is easy, 330R like most LED.


Very interesting read here on optos:
http://www.jaycar.com.au/images_uploaded/optocoup.pdf

Didn't know about using a reverse diode on input side as added protection, nice:


...
In some circuits, there may be a chance that at times the driving voltage fed to the input LED could have reversed polarity (due to a swapped cable connection, for example).

This can cause damage to the device, because optocoupler LEDs tend to have quite a low reverse voltage rating: typically only 3 - 5V. So if this is a possibility, a reversed polarity diode should be connected directly across the LED...