Chevy Blazer AC Direction Control

Arduino-based airflow controller for HVAC retrofits. Controls a servo-driven Y-splitter duct door to direct airflow between face vents, defrost, or both simultaneously via a dash-mounted switch.

Overview

Built for a Vintage Air heater retrofit in a 1972 Chevy Blazer -- the new heater unit had no dedicated defrost output. A custom 3D printed Y-splitter duct with an internal servo-driven door solves this, giving three selectable airflow modes via a dash-mounted ON-OFF-ON toggle switch:

Face vents -- all air directed to cabin vents
Defrost -- all air directed to windshield defrost
Both -- air split between vents and defrost

Hardware

Component	Details	Link
Microcontroller	Seeed Studio XIAO SAMD21	Amazon
Actuator	Sub-micro servo	Amazon
Power	12V to 5V buck converter	Amazon
Duct	Y-splitter vent (3" inlet, 2x 2" outlets)	eBay
Input	ON-OFF-ON SPDT center-off toggle switch	Any
Mounting	Custom 3D printed bracket	See 3d-models/

Schematic

See schematics/hvac-3position-controller-schematic.svg for the full wiring diagram.

Note: consider adding a 100uF electrolytic capacitor across the buck converter input for noise filtering, and a 100uF electrolytic + 100nF ceramic in parallel across the 5V output, especially in a vehicle environment.

Wiring

Connection	Details
12V fuse box to buck converter in	Vehicle power
Buck converter 5V out to Arduino VIN	Board power
Switch wire L to Pin 4	INPUT_PULLUP
Switch wire H to Pin 6	INPUT_PULLUP
Servo signal to Pin 7	PWM output

Switch Logic

Switch L	Switch H	Mode	Servo Angle
HIGH	HIGH	Face vents	0 degrees
LOW	HIGH	Face + Defrost	50 degrees
HIGH	LOW	Defrost only	90 degrees

3D Models

The 3d-models/ folder contains the OBJ file for the Y-splitter duct assembly -- air inlet from the Vintage Air unit, two outlets (vents and defrost), and the internal door controlled by the servo.

Flashing

Open src/hvac-3position-controller.ino in Arduino IDE
Select Seeed XIAO SAMD21 as the board
Select your COM port
Upload

License

CC BY-NC 4.0 -- free to use and modify for non-commercial purposes with attribution.
Copyright (c) 2025 Paul Thillier