The MQ2 gas sensor is a versatile metal-oxide semiconductor (MOS) chemiresistor designed to detect smoke and combustible gases in the air at concentrations ranging from 200 to 10,000 parts per million (ppm). It functions by altering its internal electrical resistance when it comes into contact with target gas molecules, allowing connected microcontrollers to measure changing output voltages.Target Gases & SensitivityThe sensor has a wide detection profile but cannot differentiate between individual gases. It has a particularly high sensitivity to:

• LPG (Liquefied Petroleum Gas)
• Propane and Butane
• Smoke
• Hydrogen
• Methane, Alcohol, and Carbon Monoxide (at lower sensitivities)

Module Pinout ConfigurationThe sensor is commonly sold on a breakout board featuring four physical pins:

1. VCC: Connects to the +5V DC power source.
2. GND: Connects to the system ground.
3. AO (Analog Output): Provides a continuous voltage signal (typically 0.1V to 4.0V+) that rises proportionally with gas concentration.
4. DO (Digital Output): Outputs a clean High (5V) or Low (0V) signal handled by an onboard LM393 comparator.

Operational Characteristics

• Internal Heating Element: The core utilizes a nickel-chromium heater coil. It is completely normal for the sensor module to feel warm or hot during standard operation.
• Preheating Period: The sensor requires a “warm-up” period when first powered on. It takes around 20 seconds for basic stabilization, but requires up to 510 minutes before readings are fully precise.
• Sensitivity Adjustment: The board features an onboard potentiometer (trimmer). Rotating it changes the threshold voltage for the DO pin, letting you set exactly what concentration triggers an immediate hardware alarm.