MQ-135 Air Quality Sensor

The MQ-135 Air Quality Sensor is a low-cost semiconductor gas sensor for general air-quality and pollution-gas detection. It is suitable for hobby electronics, Arduino projects, ESP32 projects, air-quality experiments, gas alarm prototypes, and educational sensor demonstrations.

The sensor element uses tin dioxide (SnO2) as the gas-sensitive material. In clean air, the material has relatively low conductivity. When pollution gases are present, the conductivity of the sensor increases as the gas concentration rises. With a simple external circuit, this change can be converted into an analog voltage signal that can be read by a microcontroller or comparator circuit.

The MQ-135 Air Quality Sensor is especially sensitive to ammonia gas, sulfides, and benzene-series vapours. It can also respond to smoke and other toxic gases, making it useful as a general-purpose air-quality sensor for experiments and monitoring projects.

Download MQ-135 Datasheet (PDF)

Typical applications:

• Arduino, ESP32, Raspberry Pi, and other microcontroller air-quality projects
• Indoor air-quality experiments
• Gas alarm prototypes
• Smoke and pollution-gas detection experiments
• Educational gas sensor demonstrations
• Portable gas detector prototypes
• Analog sensor experiments and comparator-based alarm circuits

Helpful information:

• The MQ-135 is not a digital sensor. The sensing element changes resistance depending on the surrounding gas concentration.
• A load resistor is required to convert the sensor resistance change into a measurable voltage.
• The heater draws significant current. Make sure your 5V supply can provide enough power.
• The sensor requires warm-up time. Readings directly after power-up may drift and should not be treated as stable measurements.
• For more repeatable measurements, allow the sensor to stabilize before taking readings.
• Temperature, humidity, airflow, oxygen concentration, sensor age, and previous gas exposure can affect readings.
• The MQ-135 Air Quality Sensor responds to several gases, so it cannot reliably identify one specific gas by itself.
• For accurate concentration readings, calibration with a known reference gas or known test setup is required.

Basic operating principle:

• The heater brings the sensing material to its required operating temperature.
• The sensing resistance changes when pollution gases are present.
• The external load resistor converts this resistance change into a voltage.
• The voltage can be connected to an analog input of a microcontroller or to a comparator circuit.

Recommended handling and operating precautions:

• Avoid exposure to silicone vapours, silicone adhesives, silicone rubber, putty, or plastics containing silicone. Silicone contamination can permanently reduce sensor sensitivity.
• Avoid highly corrosive gases such as H2S, SOx, Cl2, and HCl.
• Avoid contamination from alkali materials, alkali metal salts, salt spray, halogens, or similar harsh chemicals.
• Do not expose the sensor to water, condensation, freezing conditions, or direct liquid contact.
• Do not apply voltages above the rated values.
• Avoid long-term storage in polluted, humid, or high-temperature environments.
• Avoid strong vibration, shock, or dropping the sensor.
• Do not test the sensor by directly spraying lighter gas or highly concentrated vapours onto it. Very high gas concentrations can damage or degrade the sensor.

Important safety note:

This sensor is intended for electronics projects, prototyping, education, and experimental air-quality or pollution-gas detection. It is not a ready-made certified gas alarm, workplace safety device, environmental compliance instrument, or life-safety detector. For safety-critical, legal, workplace, or certified air-quality applications, use properly certified equipment.