Air Quality Monitor

Overview

An air quality monitor is an indoor environmental sensor that tracks the primary pollutants affecting human health and comfort: suspended particulates (dust, pollen, smoke), carbon dioxide (CO2), and volatile organic compounds (VOCs). Unlike a single-sensor device, this is a multifunctional instrument combining four measurement channels into one networked display.

The Housing Assembly contains a PM Sensor Module that continuously samples room air through the Intake Port, a CO2 Sensor Module that measures CO2 by infrared absorption, a VOC Sensor that detects odours and off-gassing, and a Environmental Sensor sensor for context. All four sensors feed the Main Board SoC, which fuses the data, computes an Air Quality Index (AQI), and displays the result on the Display Module. The data is also uploaded to a cloud service (Home Assistant, Prometheus, InfluxDB) or a mobile app via Wireless Module for remote monitoring and trend analysis.

The device runs continuously on USB power, operating in the background with minimal visual or acoustic signature. Color-coded LEDs or display modes indicate air quality at a glance: green for good, yellow for moderate, red for unhealthy.

How it works

The PM Sensor Module is a laser particle counter. The Pump Motor draws ambient air through an optical chamber where a Laser Diode (660 nm red laser) illuminates particles. The Photodiode detects light scattered by each particle; larger particles scatter more light. The Main Board MCU counts pulse heights and bins them into size ranges (0.3–0.5 µm, 0.5–1.0 µm, 1.0–2.5 µm, 2.5–10 µm), then reports PM1.0 (mass of particles <1 µm), PM2.5 (<2.5 µm), and PM10 (<10 µm). This is accurate to ±10 µg/m³ and is the primary metric used to compute public health AQI.

The CO2 Sensor Module employs non-dispersive infrared (NDIR) spectroscopy. A CO2 IR LED emits infrared light at 4.26 micrometers, the strong absorption band of CO2. The light passes through the sample air chamber and is absorbed in proportion to CO2 concentration. A CO2 Detector (infrared photodiode) measures the transmitted intensity; the ratio of transmitted-to-emitted power is converted to ppm via a calibration curve stored in the MCU. Typical indoor CO2 is 400–600 ppm; poorly ventilated spaces exceed 1000 ppm.

The VOC Sensor is a metal oxide (MOX) or PID (photoionization detector) element that responds to airborne organic vapors (formaldehyde, toluene, xylene, etc.). The MCU reads the sensor resistance and converts it to an index score (0–500) without identifying specific compounds. This is qualitative but useful for detecting off-gassing from new furniture, cleaning products, or inadequate ventilation.

The Environmental Sensor sensor provides context: humidity above 60% encourages mold growth, below 30% causes dry skin and respiratory irritation. Temperature is logged for trend analysis.

All four measurements are combined by the Main Board into a fused AQI, typically weighted heavily toward PM2.5 (the most health-critical). The result is displayed on the Display Module as a number (0–500) or color code. Simultaneously, the Wireless Module WiFi/Bluetooth module uploads data to a cloud service (Home Assistant, Prometheus, InfluxDB) for home automation triggers (e.g., turn on ventilation fan if AQI exceeds 100) or a mobile app for remote viewing.

The Power System accepts USB-C 5 V input and generates isolated low-noise rails (3.3 V for MCU and sensors, dual-tracked rails for laser and CO2 LED to prevent cross-talk). The entire device draws 3–6 W during active sampling, dominated by the Pump Motor motor and CO2 IR LED during their duty cycles. An idle mode (display off, sensors paused) reduces consumption to <0.5 W, useful for battery-powered portable models.