Daemon that reads CPU temperature and toggles a fan on GPIO pin 17. Uses hysteresis to avoid rapid on/off cycling.

Polls /sys/class/thermal/thermal_zone0/temp every 1.5 seconds. Fan turns on when temperature crosses threshold + variance, turns off when it drops below threshold - variance. With defaults (55°C threshold, 5°C variance), that means on at 60°C, off at 50°C.

GPIO pin 17 → relay or MOSFET → fan. Nothing else special.

Requires libgpiod. Build with cargo:

cargo build --release

For NixOS there's a service.nix that wires everything up. Drop it into your configuration and enable the rpi-fan-control systemd service.

Copy example_configuration.toml to /var/fan_control/config.toml and adjust:

threshold = 55.0    # °C
variance = 5.0      # fan on at threshold+variance, off at threshold-variance
interval_ms = 1500
gpio_line = 17
gpio_chip = "/dev/gpiochip0"
log_path = "/var/log/fan_control.log"

All fields are optional, the defaults worked fine for me.

There's an archived-c branch with the original implementation. It's a single C file that runs as a one-shot binary via systemd timer (fires every minute), persisting fan state to /var/log/fanstatus between invocations. It uses pigpio instead of libgpiod. Same hysteresis logic, same defaults. It works, but I decided to rewrite it in Rust because it's blazingly faster. /s