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

myoung84 6 years ago in Browsers Tablets Phones updated by Morgan Young 3 years ago 15

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!

I have been trying to figure this out for awhile. Can you help me understand exactly how you hooked up the converter(s) to the thermostat wiring?

Looks great and thank you so much in advance!

Depending on your thermostat wiring, you should have a power and neutral wire at the thermostat. Some installations only have a 4 wire system and lacks the needed neutral wire. If you have 5 wires you should be good. 

Wire the power wires (typically red and white) to the input of the AC/DC converter. Wire the output to a DC buck converter, then plug in the output of that to your tablet. You can find them on Amazon with micro USB. 


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!

I don't recall where I bought the last one, it's not in my Amazon history. But, this one below will work. Your thermostat should be 24V AC for the input of the converter. Then and 12VDC to 5VDC buck converter will work for the tablet charger input. I haven't touched my setup since I posted this 2 years ago. Still running strong. 


This is the inside of my Honeywell thermostat. Are you saying to plug this red and white wire into the converter?  I ordered these earlier. Can you confirm these will work?

Those will work. Red is your 24V, blue is common. 

I had hobby lobby make a custom frame a while back and it looks a lot nicer now. Alse just went back to actiontiles for hubitat. Like it so much better than sharptools that I was using.