MQ135 sensor is a SNO2 with a lower conductivity of clean air. The MQ series of gas sensors uses a small heater inside with an electrochemical sensor. These sensors are sensitive to a range of gases used at room temperature. When the target explosive gas exists, the conductivity of the sensor increases more and the gas concentration increases. Using simple electronic circuitry, it converts the charge of conductivity into the output signal of gas concentration. The MQ135 gas sensor has a high sensitivity to ammonia, sulfide, benzene steam, smoke and other harmful gases. It is inexpensive and suitable for a variety of applications. There are different types of gas sensors such as MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, etc.


MQ-135 Gas Sensor

The MQ-135 gas sensor detects gases such as ammonia, nitrogen, oxygen, alcohols, aromatics, sulfide and smoke. The boost converter of the chip MQ-3 gas sensor is PT1301. The operating voltage of this gas sensor is from 2.5V to 5.0V. The MQ-3 gas sensor has a lower conductivity to clean the air as a gas detection material. In the atmosphere we can find polluting gases, but the conductivity of the gas sensor increases as the concentration of polluting gas increases. The MQ-135 gas sensor can be implemented to detect smoke, benzene, steam and other harmful gases. It has the potential to detect various harmful gases. The MQ-135 gas sensor is cheap to buy. The base image of the MQ-135 sensor is shown in the image below.

Working Principle And Circuit Diagram

The MQ-135 gas sensor consists of a tin dioxide (SnO2), a perspective layer in aluminum oxide microtubes (measuring electrodes) and a heating element in a tubular housing. The end of the sensor is enclosed in a stainless steel net and the back contains the connection terminals. Ethyl alcohol present in the breath is oxidized to acetic acid which passes through the heating element. With the ethyl alcohol cascade on the tin dioxide detection layer, the resistance decreases. By using the external load resistor, the resistance variation is converted into a suitable voltage variation. The circuit diagram and connection arrangement of an MQ 135 is shown below.

MQ–135 Air Quality Sensor

The air quality sensor is also an MQ-135 sensor for detecting toxic gases present in the air in homes and offices. The gas sensor layer of the sensor unit is made of tin dioxide (SnO2); it has a lower conductivity compared to clean air and air pollution increases its conductivity. The air quality sensor detects ammonia, nitrogen oxide, smoke, CO2 and other harmful gases. The air quality sensor has a small potentiometer that can be used to adjust the load resistance of the sensor circuit. The 5V power supply is used for the air quality sensor.

The air quality sensor is an instruction for a signal output indicator. It has two outputs: analog output and TTL output. The TTL output is a low signal light that can be accessed through the IO ports on the microcontroller. The analog output is a concentration, i.e. increasing voltage is directly proportional to increasing concentration. This sensor also has a long life and reliable stability.

Applications Of MQ 135 Gas Sensor

The following are the applications of the MQ 135 gas sensor:

  • Air quality monitor
  • Detection of harmful gases
  • Domestic air pollution detection
  • Industrial pollution detection
  • Portable air pollution detection

Characteristics Of MQ 135

  • Good sensitivity to harmful gases in a wide range.
  • It has a long life and low cost.
  • Possesses high sensitivity to ammonia, benzene, sulfide gases.
  • It is a simple drive circuit

This is about the circuit diagram of the MQ135 Gas sensor and its operation and applications. We hope that the information provided in the article will assist you in gaining a good knowledge and understanding of future projects.


Olimex, SNS-MQ135 (2019), SNS-MQ135 datasheet , Elprocus, MQ135 (2019), MQ135 Alcohol Sensor Circuit And Working,

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