Initial commit: ESP32 NextPM sensor reader

Educational version with comprehensive comments for students.
- Reads PM1.0, PM2.5, and PM10 concentrations from NextPM sensor
- Uses UART serial communication with checksum validation
- Non-blocking timing with millis()
- Removed particle counts, kept only mass concentrations
- Extensively commented to help students understand serial protocols

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,91 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+This is a PlatformIO-based ESP32 Arduino project for communicating with a NextPM (NPM) particulate matter sensor over serial connection. The project reads PM1, PM2.5, and PM10 measurements in both µg/m³ and particles per liter (pcs/L).
+
+## Build and Development Commands
+
+**Build the project:**
+```bash
+pio run
+```
+
+**Upload to ESP32:**
+```bash
+pio run --target upload
+```
+
+**Monitor serial output (115200 baud):**
+```bash
+pio device monitor
+```
+
+**Build and upload in one command:**
+```bash
+pio run --target upload && pio device monitor
+```
+
+**Clean build files:**
+```bash
+pio run --target clean
+```
+
+## Hardware Configuration
+
+- **Board:** ESP32 dev board
+- **Serial Configuration:**
+  - Debug serial: `Serial` (USB, 115200 baud)
+  - NPM serial: `Serial1` (UART1, 115200 baud, 8E1 parity)
+  - RX Pin: GPIO 39
+  - TX Pin: GPIO 32
+
+## Architecture
+
+### Serial Communication Protocol
+
+The NextPM sensor uses a binary protocol with checksums. Communication follows a request-response pattern with specific message formats:
+
+**Message Structure:**
+- Header (2 bytes): Command identifier (e.g., `0x81 0x16`)
+- State (1 byte): Device state bits
+- Data (variable): Response payload
+- Checksum (1 byte): Sum of all bytes mod 0x100 must equal 0
+
+**State Machine Enums:**
+The code uses multiple enums (`NPM_waiting_for_4`, `NPM_waiting_for_8`, `NPM_waiting_for_16`) to track parsing state for different response types (4, 8, or 16 bytes total).
+
+### Key Functions
+
+**Initialization (`powerOnTestNPM`):**
+- Waits 15 seconds for sensor startup
+- Queries sensor state
+- Starts sensor if stopped
+- Reads firmware version and temperature/humidity
+
+**Measurement (`fetchSensorNPM_1min`):**
+- Sends concentration command (1-minute averaged readings)
+- Parses 16-byte response containing 6 values:
+  - N1, N2.5, N10 (particle counts as pcs/L)
+  - PM1, PM2.5, PM10 (mass concentrations as µg/m³ × 10)
+- Validates checksum before accepting data
+
+**Loop Behavior:**
+- Reads sensor every 60 seconds (configurable via `interval` constant)
+- Non-blocking timing using `millis()`
+
+### File Organization
+
+- `src/main.cpp`: Main application logic, NPM protocol implementation, sensor state machine
+- `src/utils.h`: Protocol constants, enums, checksum validation prototypes
+- `src/utils.cpp`: Utility functions for checksum validation and NPM command transmission
+- `platformio.ini`: PlatformIO configuration for ESP32
+
+## Important Notes
+
+- The sensor requires a 3-second timeout for serial responses
+- All particle concentration values from the sensor are scaled (PM values by 10, temperature/humidity by 100)
+- The `nextpmconnected` flag prevents infinite loops if sensor disconnects
+- Comments are in French in some sections of the code