Turning a Dumb AC Unit Smart (Without Losing my Security Deposit) :: prilik's blog

Turning a Dumb AC Unit Smart (Without Losing my Security Deposit)

2026-07-20

- 15 minute read

#c++

#arduino

#homeassistant

#esp32

TL;DR: DIY home automation is ezpz with nothing more than a stepper motor, an esp32, and a high tolerance for Jank.

My rental apartment’s AC unit can only be controlled using these retro-looking analog knob-based controls, mounted right onto the unit. No separate wall-mounted thermostat, no remote control… nothin’ fancy whatsoever.

These knobs work… but having to constantly stand up and fiddle with them gets annoying pretty quick.

Fortunately, there’s an ‘ol Prilik family saying that goes something like this: “remember son - the hardest problems in life can usually be solved with nothing more than a stepper motor, an esp32, and a dream”1

And sure enough, after dropping ~$15 on parts, waiting for things to arrive from China, and spending a few hours iterating on the hardware assembly and firmware… I hacked together this beautiful mess:

What you’re looking at here is a jerry-rigged esp32-controlled stepper motor , coupled to one of my AC unit’s knobs using a shaft coupler , all affixed to the AC’s back-plane using a cheap L-bracket and a binder clip (with some industrial-grade cardboard padding for good measure).

This whole MacGyver’d up contraption talks to my Home Assistant instance over MQTT , which turns the AC unit on/off based on the state of a temperature sensor located in the same room.

Et voila 🪄 ✨

Just like that - I’ve managed to free myself from the shackles of having to get up off the couch just to tweak my AC!

For all you professional embedded and mechatronics folks out there: I strongly suggest you stop reading here.

The rest of this blog post is a walkthrough of a software engineer’s approach to home automation and building custom hardware, and let me tell you: both the final product, and the journey to get there, are hella jank.

That said… if you’re not afraid of a bit of jank: read onwards, and join me on this fun foray into how I managed to hack together some totally bespoke home-automation hardware with almost no budget, or experience!

🗽 Setting the stage⌗

In April 2025, I moved to New York City. After a brief apartment hunt, I managed to find a place I’m pretty happy with: the location is convenient, the building is fairly modern, and the rent is an absolute steal. There’s not much to complain about!

…well, except for the AC situation.

See, for whatever reason, developers in NYC really like using these loud, power-hungry, wall-mounted PTAC units in their apartment buildings. Some buildings might spend a bit extra in order to wire these units up to wall-mounted thermostats… but oftentimes, they’ll just go with the cheapest option: completely analog, unit-mounted knobs.

Here’s a picture of what I’m talking about:

Yeah - guess which option my building went with 🙃

🤔 What are my options here?⌗

Like any good renter, the first thing I did was gently ask my landlord if there was any way to “upgrade” the controls to something a bit more… modern. As expected, the response was roughly along the lines of “lol no, why would we do that?”, which, to be fair, was basically the response I expected.

Well, no matter, like any good engineer - surely I can hack my way out of this pickle?

After a bit of finessing, I managed to pop the cover off the unit, and expose its soft underbelly inner workings. Much to my surprise, not only did I find some info about the unit, but even a whole wiring diagram!

Now, I’m no expert when it comes to wiring diagrams, but by following the wiring, it certainly seems like this entire circuit is operating at line voltage, with nary a low-voltage digital signal I can hook into in sight.

⏺️ Smart Relays?⌗

Ok, maybe I can just splice in a smart relay somewhere? I’m no electrician, but it’s probably not that hard, right?

Well, maybe?

But honestly, I didn’t think this was gonna be a viable route for me.

Setting aside the fact that working with line voltage and HVAC equipment is a bit “spooky” for someone with zero electrical wiring experience (and that my landlord probably wouldn’t be thrilled with me messing about with these sorts of things), the bigger issue was that all the juicy wires I’d be interested in intercepting are stuffed deep inside the AC unit.

As far as I could tell, the only way to access those wires would be to yank the whole unit out of the wall… something that I wasn’t particularly interested in doing. This thread from ~2017 reinforced my impressions that this would be far more trouble than its worth.

So… what now?

🔌 Smart Plugs?⌗

Ok, here’s an idea: what if I just cut power to the unit using a smart plug?

Note: The internet...