MQ-135 Air Quality

Roughly Evaluate Air Quality

MQ-135 is an air quality sensor in MOS technology and detects a wide range of substances such as ammoina, sulfide, benzene, smoke, and other gases.

One especially important gas to evaluate air quality is not detected: CO2. Even though the datasheet claims that CO2 is part of the detected gases, this sensor type can only indirectly detect it, and sensitivity towards CO2 does not seem to be particularly high.

Overview

MQ-135 is an analog sensor that returns a voltage reading representing the overall gas concentration of all detectable gases.

Parameter Value
Voltage 5V
Heater Current <950mW
Sensitivity (ppm) ammonia 10-300, benzene 10-1000, ethanol 10-300
Preheat Time >24h
Gas sensing SnO2

Pins

The raw sensor has six pins:

Pin Description
H/H apply 5V AC or DC
AA/BB apply up to 24V DC in series with a load resistor, and measure voltage drop

Typically, the sensor is mounted on a breakout board with these pins:

Pin Description
AO analog out: measured sensor voltage
DO digital out: high when threshold is exceeded
GND Ground
VCC 5V

The threshold for the digital output (DO) can be set with a potentiometer.

Materials

MQ-135 Datasheet

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(content created Jul 16, 2024)