MQ135 Air Quality Gas Sensor Module

The MQ135 Air Quality Gas Sensor Module is a ready-to-use gas sensing board built around the MQ-135 metal-oxide semiconductor gas sensor. It is designed for detecting air-quality changes caused by gases such as ammonia gas, sulfide, benzene series vapour, smoke and other harmful gases. The module is widely used in Arduino air quality monitors, IoT pollution detection systems, classroom experiments and basic indoor ventilation alarms. Inside the MQ135 sensor, a heated tin dioxide sensing layer changes resistance when exposed to target gases. The module converts this change into a usable analog voltage on the AO pin, allowing a microcontroller ADC to track relative gas concentration. It also includes an LM393 comparator circuit and an onboard potentiometer, so the DO pin can be set to switch HIGH/LOW at a chosen threshold for simple alarm applications without complex code. Onboard power and output LEDs make testing easier during prototyping. This module operates from 5V and uses a 4-pin interface: VCC, GND, AO and DO. For accurate readings, MQ sensors require warm-up time; first-time use and calibrated applications need longer burn-in and calibration against known conditions. The MQ135 is excellent for relative air-quality monitoring and trigger-based detection, but it is not a laboratory-grade selective gas analyser. Sensitivity varies with temperature, humidity and calibration method. Typical projects include Arduino air quality meters, smoke or pollution warning systems, smart exhaust fan control, greenhouse air monitoring, GSM-based gas alert systems, and LCD/OLED ppm display prototypes. Package includes one MQ135 Air Quality Gas Sensor Module with pre-mounted sensor, comparator circuit, trimpot and 4-pin header interface.