The MQ-135 Gas Sensor is a low-cost, semiconductor-based metal oxide sensor designed primarily for monitoring indoor air quality. It utilizes a Tin Dioxide (SnO) sensing element, which exhibits low electrical conductivity in clean air. When exposed to target pollutants, its conductivity increases proportionally to the concentration of the gas.Technical Specifications

• Operating Voltage: 5V 0.1V AC or DC
• Current Consumption: Approximately 150mA
• Heater Resistance: 33 5% at room temperature
• Heating Consumption: 800mW
• Detection Range: 10 to 1000 ppm (Parts Per Million)

Target Gases & SensitivityThe MQ-135 is a broad-spectrum sensor, meaning it reacts to multiple gases rather than isolating just one. It displays its highest sensitivity to:

• Ammonia (NH)
• Benzene (CH) and Toluene
• Sulfide vapors
• Carbon Dioxide (CO)
• Alcohol vapors and Smoke

Module Pinout & Dual OutputsMost commercial versions of the MQ-135 come mounted on a breakout board with four interface pins:

• VCC: Power input (5V)
• GND: Ground connection
• AOUT (Analog Output): Outputs a continuous 0V to 5V signal. The voltage rises alongside increases in gas concentration, making it ideal for microcontrollers with ADCs to log precise trends.
• DOUT (Digital Output): Transmits a TTL-logic level (High/Low). An onboard potentiometer allows you to adjust the threshold limit. If gas levels rise past this limit, the digital pin shifts to a low level and illuminates an onboard LED.

Critical Usage Notes

• Preheating Requirement: The sensor relies on an internal Ni-Cr heating element. For basic daily use, it needs a warm-up period of 5 minutes to stabilize. For first-time setups or precise data logging, a burn-in period of 24 to 48 hours is mandatory according to the technical documentation.
• Cross-Sensitivity & Calibration: Because it responds to many gases simultaneously, it cannot inherently distinguish between CO and alcohol vapor on its own. For accurate Parts-Per-Million (PPM) values, you must calibrate the sensor in clean air (R baseline).