The MQ-135 is an air quality gas sensor that can detect a wide range of pollutant gases in the environment. It is especially sensitive to ammonia (NH₃), nitrogen oxides (NOₓ), benzene, smoke, CO₂, and other harmful gases, making it useful for air quality monitoring systems. 🔹 Details and Explanation 1. Main Features Type: Air quality sensor Gases Detected: Ammonia (NH₃), Nitrogen oxides (NOₓ), Benzene, Smoke, CO₂, CO, Alcohol vapors, and other harmful gases Operating Voltage: 5V DC Current Consumption: ~150 mA Output: Analog (AO): Continuous voltage proportional to gas concentration Digital (DO): HIGH/LOW signal depending on threshold set via onboard potentiometer Detection Range: 10 ppm – 1000 ppm (varies by gas type) 2. Working Principle The sensor has a heating element and a metal oxide semiconductor layer (SnO₂). In clean air → SnO₂ has high resistance. In polluted air → target gases reduce resistance → current increases. This resistance change is converted into a voltage signal. AO pin gives proportional readings (for ppm calculations). DO pin gives digital HIGH/LOW (gas above or below set level). 3. Pin Configuration (MQ-135 Module) VCC → +5V supply GND → Ground DO → Digital output (1 = gas detected, 0 = no gas) AO → Analog output (gas concentration level) 4. Applications Indoor/outdoor air quality monitoring Pollution detection systems Smart home air purifiers Industrial safety monitoring IoT-based environmental monitoring 5. Important Notes Needs burn-in time (24–48 hours) before stable readings. Best performance after preheating for a few minutes during each use. Should be calibrated with known gas concentrations for accurate ppm values. It is a general-purpose air quality sensor, not a precision medical device.