Metsta

Low-Power ESP32 Environmental Telemetry Node

This repository documents the complete hardware and firmware implementation of an ultra-low-power, custom-designed ESP32 environmental sensor node. I designed this system from scratch from KiCad PCB layout to FreeRTOS C firmware as a comprehensive proof-of-concept for deployable, battery-operated IoT telemetry using the connectionless ESP-NOW protocol.

The system continuously monitors temperature, humidity, barometric pressure, estimated CO2 (eCO2), and battery voltage, broadcasting the data to a central gateway before returning to a deep sleep state.

Table of Contents

• Overview
• Hardware Architecture
  • Power Subsystem & Battery Monitoring
  • Environmental Sensing (I2C)
  • USB and Programming Interface
• Firmware Implementation
  • State Machine & Deep Sleep
  • ESP-NOW Telemetry
  • Sensor Drivers & Compensation
• Engineering Challenges & Resolutions
• Future Enhancements

Overview

I designed this wireless station to operate autonomously on battery power for extended periods. It relies on a carefully orchestrated 5-minute wake/sleep cycle. The firmware is written in C using the Espressif IoT Development Framework (ESP-IDF v5+) and FreeRTOS.

The core design philosophy is modularity and power efficiency. The hardware separates the power domain, digital domain, and sensor domain, while the firmware abstracts sensor communication, radio transmission, and sleep management into isolated components.

The complete telemetry payload is structured as follows:

typedef struct __attribute__((packed)) {
    uint8_t  node_id;       // Derived from MAC address
    uint32_t message_id;    // RTC incremented message counter
    float    temperature;   // °C
    float    humidity;      // %RH
    float    pressure;      // hPa
    uint16_t eco2_ppm;      // eCO2 in parts-per-million
    uint16_t battery_mv;    // Calibrated battery voltage (mV)
    uint8_t  status_flags;  // Bitmask for Sensor/Battery errors
} sensor_payload_t;

Hardware Architecture

The custom PCB was designed in KiCad. The design files (schematic, layout, and component libraries) are available in the Hardware/ directory.

Power Subsystem & Battery Monitoring

The board is powered by a lithium-ion battery, regulated to 3.3V by a high-efficiency AP2112K Low Dropout (LDO) regulator. This LDO was chosen for its low quiescent current, which is critical for maximizing battery life during deep sleep.

Battery voltage monitoring is implemented using a simple resistive voltage divider connected to an ADC channel. To ensure accurate readings across the discharge curve, the firmware (battery_adc.c) utilizes the ESP32 internal calibration routines to convert raw ADC readings to precise millivolts.

Environmental Sensing (I2C)

The node features two primary environmental sensors sharing a single 100kHz I2C bus:

• BME280 (Bosch Sensortec): Provides highly accurate temperature, relative humidity, and barometric pressure measurements.
• SGP30 (Sensirion): A metal-oxide (MOX) gas sensor that provides estimated CO2 (eCO2) readings.

The I2C bus utilizes hardware 4.7kΩ pull-up resistors to ensure crisp signal edges, allowing the firmware to disable internal weak pull-ups for improved reliability.

USB and Programming Interface

A modern USB-C receptacle is included for direct flashing and serial debugging. The USB data lines (DP, DN) are routed to a CH340C USB-to-UART bridge, which interfaces with the ESP32 hardware UART0. The auto-reset circuit (utilizing DTR/RTS signals and two NPN transistors) allows seamless firmware uploads via idf.py flash without manual button presses.

Firmware Implementation

The firmware is structured into several modular source components:

State Machine & Deep Sleep

The primary execution flow in app_main() is strictly linear to minimize active time:

1. Wake up from Deep Sleep.
2. Initialize NVS and I2C bus.
3. Initialize sensors and ADC.
4. Read BME280 (Temperature, Humidity, Pressure).
5. Read SGP30 (eCO2).
6. Read Battery Voltage.
7. Assemble sensor_payload_t and set status flags.
8. Initialize Wi-Fi and ESP-NOW radio.
9. Transmit payload to the gateway MAC address.
10. Deinitialize peripherals to prevent current leakage.
11. Enter Deep Sleep for 300 seconds (5 minutes).

ESP-NOW Telemetry

To achieve micro-ampere average power consumption, standard Wi-Fi (connecting to an AP, acquiring DHCP, establishing TCP/MQTT connections) is too slow and power-hungry.

I chose ESP-NOW, a connectionless protocol that allows the node to wake up, broadcast its 18-byte payload in milliseconds, and immediately go back to sleep. The node transmits to a broadcast MAC address (FF:FF:FF:FF:FF:FF), which is received by the central gateway device running central_node.c.

Sensor Drivers & Compensation

I wrote custom, lightweight drivers for both sensors rather than relying on heavy third-party libraries:

• The bme280.c driver includes the full Bosch integer compensation algorithms, reconstructing the calibration matrix from the sensor's NVM to calculate exact physical values.
• The sgp30.c driver implements strict CRC8 verification on every byte received from the sensor to prevent corrupted IAQ readings.

Engineering Challenges & Resolutions

During development, several hardware and firmware interactions required careful resolution:

1. SGP30 MOX Warm-up Delay: The Sensirion SGP30 requires a hot-plate element to reach operating temperature. Immediately after issuing initialization commands, the sensor returns baseline values until warmed up. I structured the firmware sampling loop to ensure valid reading capture before signaling readiness for deep sleep.
2. Isolated Ground Planes (PCB Layout): During routing, thick power traces inadvertently severed the top-layer copper ground pour, resulting in orphaned ground islands and DRC thermal errors. I resolved this by placing stitching vias to bridge top-layer copper islands directly to the continuous bottom-layer ground plane.
3. ESP-NOW Channel Locking: For reliable transmission, ESP-NOW requires the sender and receiver to operate on the same Wi-Fi channel. The firmware explicitly configures the primary channel to ensure packets are not silently dropped by the gateway.