Monitoring Thermostat w/ Arduino

[knoxjl]

Hi folks. I'm a software engineer who's been learning electrical engineering via Arduino projects. I'd like to step things up from the fairly basic examples I've found around to do something more interesting. We just moved into a new house and keeping our energy bills down is a priority. We've noticed that the A/C in our bonus room seems to run way more than it should based on the temp we've set the thermostat (the room temp is consistently 4-5 degrees below the thermostat's setting). I've built a simple shield to measure the temperature and log it via the serial port to my raspberry pi, but now I want to step up to monitoring the A/C itself. Here's what I've learned about thermostats: the heat, cooling, and fan are activated by connecting 24 VAC to one (or more) of 3 colored wires. If I wanted to control the unit myself I know I could put a relay between the 24VAC and the 3 wires to connect them, but what I'd really like to do is simply monitor when the existing thermostat is activating the unit. This leads me to two electrical engineering questions:

1) How can I signal the arduino with 24VAC. It seems like I need the inverse of a relay: something that can take in a high voltage and trigger 5 VDC when enabled.
2) How can I hook this into the system without disrupting the current setup? I don't want to replace the thermostat or control anything, only passively monitor when it's turning on the HVAC unit. I've thought about making the arduino automatically trigger a relay when it gets the signal that the A/C should run, but then I need to worry about something going wrong with the arduino causing it to not change the state and either leave the A/C stuck on or off. If I could leave the existing wires in place and just run a few additional wires down to the arduino that would seem ideal, but I'd think that'd constitute wiring in parallel, which may have undesirable effects on the system.

Thanks to anyone who responds.

[Franklin97355]

Where in the system do you have your best access to the three wires? The most room and out of the way (probably at the AC unit) This unit might be adaptable but you will need to add a fullwave bridge and filtering. https://www.sparkfun.com/products/11005

[knoxjl]

Where in the system do you have your best access to the three wires? The most room and out of the way (probably at the AC unit) This unit might be adaptable but you will need to add a fullwave bridge and filtering. https://www.sparkfun.com/products/11005

Thanks for the suggestion. The non-invasive clip is probably going to be too large, since the wires are generally 18 gauge, but just by pointing out the current sensor you may have put me on the right path. I can't seem to find how many amps the standard HVAC setup puts out, but maybe I can find a current sensing breakout that can handle these conditions. I only really care about high or low, but that'd be simple enough to deal with in software.

On the other hand, maybe I can just hook the clip to the HVAC unit's wall plug and just sense when it's running. That's what I really care about, not necessarily whether it's running the compressor, heater, and/or fan. This particular unit is just behind a door on that same floor, so that may be an option.

Thanks for the suggestion!

[Franklin97355]

Before you decide be aware that the current sensor has to go around only one of the wires in the cord not the whole cord. Is there a light that you may be able to monitor? That might work too. It might help if you took a picture of the wiring connections at the HVAC end of things.

[wethaguy]

I would go the simple route and duct tape an accelerometer to the unit if this is a window unit. Otherwise, it would be easy enough to add another temp probe to your current setup and put it at the duct. The air entering the room will be 10-30 degrees F cooler than the room temp while the unit is operating. This won't be exact, but you can account for the warm-up and cool-down times.

[knoxjl]

The second temperature probe would make things really simple and probably precise enough for my needs. Thanks for the suggestion! =)