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MQ-135 Air Quality Sensor

Sensor

General air quality sensor sensitive to gases such as ammonia, nitrogen oxides, benzene, smoke and CO₂; used in indoor air and ventilation projects. It does not tell gases apart; it shows how polluted the air is, and gives no calibrated CO₂ reading. Preheat it for 24 hours before first use.

Specifications

Detected gases

Ammonia (NH₃), nitrogen oxides, benzene, smoke, CO₂ (indirect)

Detection range

10 - 1000 ppm (NH₃, benzene)

Supply voltage

5V DC

Heater power

≤ 800 mW

Preheating

24 hours (first use)

Output

Analog (AO) + digital (DO)

Pins

VCC, GND, DO, AO

Shop prices

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Prices read from the shops' pages on 22 October 2026. Prices and stock may have changed since; check at the shop before you buy.

Projects with this part

• Air Quality IndicatorBeginner30 minAn indicator that shows a room's air like a traffic light: the MQ-135 sensor reacts to pollutants such as smoke, ammonia, benzene and the CO₂ that builds up from breathing; green lights while the air is clean, yellow as it starts to get stale and red when it is bad, and the Serial Monitor tells you to air the room. At start the sensor warms up and the clean-air value is measured; every reading after that is judged against it, and if the air gets even cleaner the value is updated. It teaches reading an analog sensor against its own clean-air value, reducing jitter by averaging several readings and leaving a margin at the thresholds so the LEDs do not flicker at the edge. No extra library is needed. The MQ-135 does not tell gases apart and does not give a calibrated CO₂ reading; it shows roughly how polluted the air is.Open project