I have a window air conditioner. It works well but is a bit old
— new enough to be electronic, have a remote and a digital
display, but not new enough to turn on from my phone or do anything
smart with its thermostat.
I'm dipping my big toe into electronics by hooking up a Nest smart
thermostat to this air conditioner, and it's proving to be an
*The air conditioner's electronics run on 16V DC. A 68k
microcontroller switches the negative sides of some SJ-S-112DM relays
via a darlington array on an IC to control the fan and compressor.
I'd like to bypass everything except the relays (but leave the rest
functional if I want to go back to basics).
*The Nest wants 20-30V AC (24V is standard for residential
HVAC). It switches the hot side of the HVAC's relays. It can power
itself by running a small amount of current through the relays (but
not enough to activate them), or you can connect a dedicated neutral
So, not so compatible. Anyway, I'm a computer programmer by trade,
and this is my first electronics project since high school. I'll take
an elegant solution over an easy one — especially if I can
learn a few things along the way.
I assume that the Nest uses s-state relays to pass voltage out to
the air conditioner's compressor and fan. I further assume that that
path is agnostic to DC vs. AC. I even further assume that some kind
of rectifier is used to power the Nest's guts, and that it will be
equally happy on DC.
I've partially tested the last assumption by hooking the Nest up
to the 16V out from the air conditioner. I thought that maybe it
would just work — that 16V was close enough. Alas, it refuses
to operate the system, giving me a "power wire not working" (E24)
error. But it didn't blow up! And it even shows the correct voltage
on its debug screen. Encouraging. DC might be fine.
Here's my current plan:
*Buy a small DC/DC buck converter to boost the 16V coming
out of the air conditioner to 24V. Connect this to the Nest. Its
outputs will be 24V, too, though, so…
*Buy two (or 200, turns out they're super cheap) 2N2904
transistors. Connect the Nest's outputs to the bases of the
*Connect one 2N2904(www.kynix.com/.../2N2904.html)
to the compressor relay, and the other to the fan relay.
So the Nest gets the 24V it wants (but DC instead of AC), and its
24V outputs drive the transistors. The transistors switch the
negative sides of the relays.
Here's a schematic. It's my first so be harsh with your
The questions I could use help with are are:
*Is it safe to drive the Nest, which expects 24V AC, with
24V DC? If not, why not?
*If the Nest expects to switch AC, will it switch DC
*The relays don't seem to be specced to use much current.
Is it okay to switch them with transistors like this?
If you see any other big holes in my story, I'd much appreciate
you pointing them out too.
I also work as a programmer, and I was surprised at how your plan is painted. You are a genius in all areas. I couldn't do that! I hope that my air conditioner will not break down because I looked at the current prices for a/c repairs, and I admit I was a little shocked. No, dry cleaning costs very little, but if you need to change some details, you already have to pay big money to specialists. I hope that nothing will happen to my air conditioner because I constantly clean it and charge it, so everything should be fine. But I admire your project, good luck to you!