Air Quality Sensor MQ-135

MQ-135 Air Quality & Pollution Sensor
The sensor uses a Tin Dioxide (SnO_2) sensing layer. In clean air, the sensor has very low conductivity. As the concentration of pollutants increases, the resistance of the sensor drops, allowing more current to flow. This change is translated into an analog voltage that represents the "freshness" of the air.

Why it’s an Air Quality Essential:
• Multi-Pollutant Detection: It is an all-in-one monitor for common indoor irritants. It reacts strongly to Ammonia (often found in cleaning products), Sulphides, and Benzene (common in paints and tobacco smoke).
• CO2 Correlation: While the MQ-135 is not a dedicated NDIR CO_2 sensor, its readings can be used as a proxy to estimate CO_2 levels in occupied rooms, making it a cost-effective choice for ventilation projects.
• Dual Output Utility:
* Analog (A0): Provides a 0V to 5V signal. This is critical for air quality "scales" (e.g., Good, Fair, Poor).
* Digital (D0): Acts as a threshold trigger. You can use the onboard potentiometer to set a specific pollution limit that instantly turns on an air purifier or opens a window.
• Environmental Versatility: It is robust enough for both simple home DIY projects and more complex industrial air monitoring nodes.

Technical Capabilities:
• Detection Range: 10 ppm to 1000 ppm (parts per million).
• Operating Voltage: 5V (Required to power the internal heater).
• Sensitivity Adjustment: The onboard LM393 comparator allows you to set a digital "alarm" point easily.

Best For:
The MQ-135 is the primary choice for smart air purifiers, indoor pollution monitors, smart greenhouse ventilation, and office air quality trackers.