How to sense 24V AC (Thermostat) with a 5V Microcontroller

Question

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

Answer 1

The schematic for use of the HPCL3700 "AC/DC To Logic Interface Optocoupler" is provided in the HP Application Note AN 1004, Figure 6:

The same section of the application note provides the calculations for determining \$R_X\$ and thus the values of the two resistors on the input side, \$R_{X/2}\$ each, based on the desired trigger threshold (see Example 2). A graph to directly obtain \$R_X\$ values is provided in Figure 7.

For monitoring a 24 V AC line and with some margin for low voltage conditions, referring to Figure 7 and considering high and low thresholds of around 25 and 15 Volts yields \$R_X = 20\ k\Omega\$: use 10 kΩ 2 Watts resistors to be on the safe side, one on each leg i.e. pin 1 and 4.

If you also plan to monitor 110 V mains voltages, the values used would be 18 kΩ 2 Watts each. Ensure that the resistor used is rated for AC mains operation, not the common or garden variety which is not safe for mains voltage applications.