add instructions

pi-instructions.md

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+**Structure & Task**
+
+- **Auto-detects** the port (`/dev/serial/by-id/...`, then `ttyACM0` / `ttyUSB0`)
+- **Sends `log on`** on connect (no manual step)
+- **Adds `wall_time`** (UTC ISO timestamp) as the first column — better than `ms` alone for long runs
+- **`scripts/requirements.txt`** — `pip install -r scripts/requirements.txt`
+- **`scripts/dryer-logger.service`** — optional systemd unit for logging at boot
+
+**On the Pi (one-time setup)**
+
+```bash
+git clone <your-repo> ~/voron-filament-dryer   # or copy the repo over
+cd ~/voron-filament-dryer
+pip install -r scripts/requirements.txt
+sudo usermod -aG dialout $USER   # then log out/in
+```
+
+**Manual run**
+
+```bash
+python3 scripts/capture_csv.py
+# logs/dryer_YYYYMMDD_HHMMSS.csv
+```
+
+**Always-on logging**
+
+```bash
+# Edit User/WorkingDirectory paths in scripts/dryer-logger.service if needed
+sudo cp scripts/dryer-logger.service /etc/systemd/system/
+sudo systemctl enable --now dryer-logger.service
+```
+
+No firmware changes needed — the Arduino still streams `csv,...` every 2 s when logging is on. If the Pi and your PC both try to use the same USB cable, only one can hold the serial port at a time.