I'd like to detect thermostat events (heat, fan, ac etc) with an Arduino. I've done quite a bit of investigation but haven't found a definitive solution. Most projects I see involve controlling the thermostat however I simply want to detect on/off events.
Based on what I've gathered so far, I'd like to use an optocoupler to protect the furnace/thermostat from damage -- protecting the Arduino is important but of lesser concern. I plan to run a separate wire (cat5) from the thermostat to my project so I don't mess with the furnace wiring.
This is the optocoupler I've seen recommended http://www.digikey.com/product-detail/en/HCPL3700/HCPL3700-ND/401373 via this howto https://www.adafruit.com/blog/2012/10/04/ask-an-educator-how-can-i-measure-ac-voltage-with-my-arduino/
But where I'm confused is how to wire it altogether and what values to select for resistance.
Here's the circuit in HCPL3700 datasheet