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README.md

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+# rpi-fan-control
+
+Daemon that reads CPU temperature and toggles a fan on GPIO pin 17. Uses hysteresis to avoid rapid on/off cycling.
+
+## How it works
+
+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.
+
+## Hardware
+
+GPIO pin 17 → relay or MOSFET → fan. Nothing else special.
+
+## Install
+
+Requires `libgpiod`. Build with cargo:
+
+```sh
+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.
+
+## Config
+
+Copy `example_configuration.toml` to `/var/fan_control/config.toml` and adjust:
+
+```toml
+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.
+
+## Legacy C version
+
+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