Since I had the Nest already, had become fairly enamored with its great design and liked its ability to learn my schedule, the next step of this project was to figure out how to use it to control my HVAC stepper motors. A 24V AC power supply was an easy find.

Next was turning the AC control signals from the nest into a digital 3.3V signal. In previous projects, I had used optocouplers to isolate a digital control signal from a triac to control an AC signal. But this was the reverse of using an AC signal to drive a digital input. 

As it turns out the HCPL-3700 is great because it can isolate either AC or DC inputs from its logical outputs with a fairly straightforward design of a diode-bridge rectifier powering an optocoupler. The datasheet by fairchild, avago or hp/agilent are very similar, differing only in some of their operating voltages at various temperatures.

All of the datasheets also include an AC threshold-voltage-to-external-resistance chart.  [0033mer]'s youtube video shows a version of this chart, however, which I couldn't find in any of the datasheets. When using an AC input (pins 1 & 4), it recommends using a capacitor on pins 2 & 3 to smooth out the bridge rectifier "output". With this application, the resistance for 24V AC is low and in that range, the capacitance variance is negligible.