Steal 5vDC from 24vAC thermostat for wall mount Tablet?

myoung84 2 years ago in Browsers Tablets Phones updated by Lane 2 years ago 9

I just started using AT, love it so far. I'm considering mounting a tablet to start experimenting with. I'm building a house next year, currently finalizing the design. I'll be doing all the low voltage installation so I need to know pretty soon exactly what I need for smart house items so I can run cabling as needed. I'd always wanted to do a wall mount tablet in a few locations and I think AT will fit the bill.

So for now in the current house, I don't care to dig into sheet rock to run cables for power then have to repair before we sell the house, but I still want to play, mostly to get the wife on-board and so I'm not having to experiment on the new place.

I have a perfect spot downstairs right above or next to my nest thermostat. I do have 5 wires to the nest so I have 24VAC with common available.  I was thinking of a AC to 5VDC buck converter mounted inside a picture frame with the tablet. I could poke a small hole in the wall from the nest to behind tablet. Has anyone ever done this? It shouldn't pull more than a half amp at 24VAC...

I'm not sure what size AC transformer is in my HVAC system, it's a Lennox system about 2 years old. Also not sure how much power the nest requires.


Seems like a convenient idea... and I'm somewhat optimistic. I'm guessing the 24vAC "C-line" from the furnace isn't likely to be stingy ... but a Nest or Ecobee could draw more or less than relatively dumb old thermostats.

Ummm... Do you have a multimeter (and/or do a bunch of Googling) to determine the used and available "amps"?

I do have a meter, I'll measure this weekend to see what the nest is pulling. I can't imagine it's very much. I found a 24VAC to 12VDC converter in my desk, just need to pick up a 5V buck converter to test with.

I also found a 7" 5th gen fire in the pile of old electronics as well as an iPad with a slightly cracked screen (both were the kids tablets that got upgraded). Both would be good for testing.

Love it already, just stuck it on the wall with some command strips. Really like my Blue Iris camera integration. Now just to figure out how to power it without looking terrible. 


Power works from the thermostat wiring. These are parts I had laying around, AC to 12VDC converter and a 5V buck converter. Now just need to frame the tablet and hide the wiring.

Not sure why my pics are rotating... 

The image rotation is probably a User-Echo Forum Bug. You can possibly try pre-rotating the image by right-clicking it in Windows Explorer or using a photo editor (Google photos, etc). We could try some clever CSS injection, but I'm not sure it would "stick" either.

Great to see the project coming along so well!


Everything is buttoned up and working great. I may poke a small hole in the wall to hide the wires to the thermostat, but for now it's fine. My AC/DC converter and buck converter are both mounted behind the tablet. I bought a cheap sign from Hobby Lobby, did a little modifications with the router and cut out thin piece of board for a frame, also Hobby Lobby. I had the frame painted nicer but it's next to a rustic window frame picture collage and looked too nice, so I roughed it up a little.

Have you had any issues with furnace/t-stat/tablet operations? A standard furnace transformer is 40 volt amps.. I'm in the same boat using a fire 10, ecobee 3 with no local power supply...


Hi myoung84, could you tell us what specific AC/DC converter and buck converter you used? I want to do the same thing!