This DIY Wearable Air Quality Monitor Pendant is a compact, battery-powered device designed to continuously track Total Volatile Organic Compounds (TVOC) in the surrounding air using the SGP40 sensor. Built around an ultra-low-power STM32 microcontroller, the pendant provides real-time feedback through colour-changing LEDs and audio alerts, making air quality status instantly recognisable. The device is ideal for personal health monitoring in homes, offices, or while travelling, and features a sleek 3D-printed enclosure for comfortable everyday wear. The project includes a custom PCB, detailed assembly instructions, and Arduino-compatible code for easy customisation and development.

Motivation & Purpose

Health Awareness: VOCs (volatile organic compounds) are common indoor pollutants (from cleaning products, paints, plastics, etc.) and can adversely affect health. A wearable monitor helps users become more aware of air quality in different places.

Portable Monitoring: Rather than having a fixed air-quality sensor on a desk, this pendant allows you to carry an environmental sensor with you—ideal for commuting, office spaces, or shared environments.

Low Power Design: The device is optimised for battery operation, so it can run for long periods without frequent recharging. Circuit Digest

User-Friendly Feedback: By using intuitive red/green LEDs and audio tones, the device makes air quality information accessible without needing to read numerical data.

Learning & Maker Value: This project is a hands-on way to learn about embedded systems design (STM32), gas sensing (SGP40), PCB design, power management, and wearable technology.

Key Components & Requirements

Microcontroller: STM32U083KCUx — a low-power 32-bit MCU ideal for battery-powered projects.

Gas Sensor: Adafruit SGP40 Air Quality Sensor, which measures TVOC via I²C, offering low power consumption and good stability.

Power Management: MCP73831/2 Li-Po charge controller to safely charge the battery. ADPL44002 (LDO regulator) to produce a stable 2.5 V rail for the sensor and microcontroller.

Feedback: An RGB LED (common cathode) to visually indicate air quality (green = good, yellow = warning, red = critical). A piezo buzzer driven by a transistor to play simple melodies (RTTTL) depending on air quality.

Battery: A small Li-Po cell to make the device portable. PCB: Custom circular PCB (~32.5 mm diameter) designed in KiCad.

3D-Printed Enclosure: Pendant-shaped housing with a hook to wear around your neck; includes a cutout for USB-C charging.

Programming Tools: ST-Link or SWD interface for flashing firmware, plus serial / UART for debugging.

How It Works

Sensing: The SGP40 sensor periodically measures TVOC levels via I²C.

Processing: The STM32 reads the sensor data, evaluates the air quality based on predefined thresholds, and determines a “state” (good, warning, alarm).

Feedback: LEDs fade smoothly between colours depending on the detected air quality.

Buzzer plays different melodies using RTTTL (text-based ringtones) to signal the current air-quality level.

Power Management: The Li-Po battery powers the system. When plugged in via USB-C, the MCP73831 charges the battery, while the LDO ensures a stable 2.5 V for the sensor.

Task Scheduling: Uses a non-blocking scheduler (ptScheduler) so the device can handle sensor readings, LED fading, and audio playback concurrently without lag.

Enclosure: Everything is housed in a 3D-printed pendant case, making the device wearable, portable, and stylish.

Challenges & Considerations

Sensor Warm-Up: The SGP40 requires time to stabilise after being turned on; this is accounted for in the code. Circuit Digest

Battery Life: Since the device is wearable, power consumption must be minimised. Using an ultra-low-power STM32 helps, but battery size and usage patterns will affect how long it runs between charges.

I²C Reliability: Ensuring proper pull-up resistors and correct wiring to the SGP40 is essential for reliable readings. Circuit Digest

User Feedback Calibration: The thresholds for “good,” “warning,” and “alarm” air quality need to be meaningful for real-world conditions; some tuning may be necessary.

Enclosure Design: The 3D-printed case must allow adequate airflow to the sensor, while still protecting the internals and looking good as a wearable.

Learning Outcomes & Impact

By building this project, participants will:

• Gain hands-on experience with embedded firmware development on STM32 (using Arduino IDE or similar).
• Learn to interface gas sensors (SGP40) over I²C, including calibration and data interpretation.
• Understand power management for battery-powered systems (charging, regulation, low-power modes).
• Develop skills in PCB design and 3D casing for wearable electronics.
• Build a real-world health-oriented tool that raises awareness about environmental quality and air pollution.

Conclusion

This wearable air quality monitor is more than just a gadget — it’s a portable environmental sentinel. By combining efficient hardware (STM32), a professional-grade VOC sensor (SGP40), and intuitive feedback (LEDs + sound), this project empowers users to sense and respond to air pollution in their daily lives. It’s ideal for makers, students, or anyone interested in health, IoT, or wearable technology.