A $16 stepper motor and an ESP32 make a rental's analog AC smart

Faced with a New York rental whose PTAC air conditioner offers only unit-mounted analog knobs and a landlord unwilling to upgrade, the author built a non-invasive automation rig for about $16. After ruling out riskier approaches — splicing in a smart relay meant working with inaccessible line-voltage wiring behind the unit, and a smart plug was blocked by the AC’s uncommon NEMA 5-20P plug plus the danger of hard-cycling a running compressor — they settled on physically turning the dials with a motor.

The build couples a low-torque, low-power stepper motor driven by an ESP32 to the AC’s loose temperature knob via a shaft coupler. Rather than fight the stiff six-position mode dial, the author leaves mode on Lo-Cool and swings the thermostat between extremes to effectively toggle cooling on and off. To avoid drilling into the chassis and forfeiting the security deposit, the motor is held in place with scavenged IKEA L-brackets, VESA screws, a binder clip, and cardboard padding — the brackets brace against the control cavity wall so the motor’s torque transfers into the shaft instead of spinning the whole assembly.

The controller reports to a Home Assistant instance over MQTT and switches the unit based on a room temperature sensor. It’s a deliberately janky, second-order hack — controlling power indirectly through target temperature — but it demonstrates how far cheap microcontrollers, open-source home automation, and improvised mechanical mounting can go for renters who can’t modify their hardware.