Gas Sensor: MQ-2

Working Principle

The MQ-2 uses a heated tin dioxide (SnO₂) semiconductor. In clean air, oxygen adsorbs on the surface, creating a high-resistance depletion layer. When combustible gases (methane, propane, hydrogen, smoke) contact the surface, they react with adsorbed oxygen, releasing electrons and lowering resistance. The resistance change is proportional to gas concentration.

Analogy: Imagine a sponge (SnO₂) that soaks up oxygen and swells (high resistance). Gas molecules squeeze the oxygen out, making the sponge shrink (low resistance).

Electrical Characteristics

Parameter	Value
Heater Voltage	5 V ± 0.1 V
Heater Current	~150 mA
Detectable Gases	LPG, methane, propane, hydrogen, smoke
PPM Range	200 – 10,000 ppm
Warm-up Time	> 24 hours (full), > 2 min (functional)
Output	Analog (voltage divider) + digital (comparator threshold)

Calibration & PPM Calculation

The key calibration value is R₀ — the sensor resistance in clean air. Measure it after the sensor has warmed up for 24+ hours:

R_s = R_load * (V_cc - V_out) / V_out

R_0 = R_s / 9.8 (clean air ratio from datasheet)

Code Example

/*
 * MQ-2 Gas Sensor — Arduino
 * Wiring: A0 → Analog out, VCC → 5V, GND → GND
 * IMPORTANT: Allow 2+ min warm-up before trusting readings
 */
#define MQ2_PIN   A0
#define R_LOAD    10000.0   /* 10kΩ load resistor on module */
#define R0_CLEAN  9.8       /* Rs/R0 ratio in clean air */

float R0 = 10000.0;  /* Will be calibrated */

float readRs() {
    int adc = analogRead(MQ2_PIN);
    float voltage = adc * (5.0 / 1023.0);
    if (voltage < 0.01) voltage = 0.01;  /* Prevent divide-by-zero */
    return R_LOAD * (5.0 - voltage) / voltage;
}

void calibrate() {
    Serial.println("Calibrating in clean air...");
    float rsSum = 0;
    for (int i = 0; i < 50; i++) {
        rsSum += readRs();
        delay(100);
    }
    R0 = (rsSum / 50.0) / R0_CLEAN;
    Serial.print("R0 = "); Serial.println(R0);
}

void setup() {
    Serial.begin(9600);
    delay(3000);   /* Warm-up delay */
    calibrate();
}

void loop() {
    float rs = readRs();
    float ratio = rs / R0;

    /* Approximate LPG ppm from datasheet curve (log-log) */
    float ppm = pow(10, ((log10(ratio) - 0.26) / -0.47) + 2.3);

    Serial.print("Rs/R0: "); Serial.print(ratio, 2);
    Serial.print(" | ~PPM: "); Serial.println(ppm, 0);

    if (ratio < 2.0) {
        Serial.println("⚠ WARNING: Elevated gas level detected!");
    }
    delay(1000);
}

MQ-Series Gas Sensor Family

The MQ series shares the same SnO&sub2; MOX architecture but with different doping and heater profiles tuned for specific gases. All use the same load-resistor + heater circuit:

Model	Target Gas	Range	Use Case
MQ-2	LPG, Methane, Smoke	300–10,000 ppm	Kitchen gas leak alarm
MQ-3	Alcohol (ethanol)	25–500 ppm	Breathalyser / BAC
MQ-4	Methane (CH&sub4;)	200–10,000 ppm	Natural gas pipelines
MQ-5	LPG, Natural gas	200–10,000 ppm	Gas stove leak detection
MQ-6	LPG, Butane	200–10,000 ppm	Butane torch/LPG cylinders
MQ-7	Carbon monoxide (CO)	20–2,000 ppm	CO safety alarm
MQ-8	Hydrogen (H&sub2;)	100–10,000 ppm	Fuel cell / battery labs
MQ-9	CO + Combustible gas	10–10,000 ppm	Multi-gas alarm
MQ-135	NH&sub3;, Benzene, CO&sub2;, VOCs	10–1,000 ppm	Indoor air quality

Limitations

• Cross-sensitivity: Cannot distinguish between gases; LPG and methane produce similar responses. Use dedicated sensors (MQ-7 for CO, MQ-3 for alcohol) to reduce ambiguity.
• Power hungry: 150 mA heater makes it unsuitable for battery-powered devices.
• Long burn-in: 24+ hours needed for stable R₀; casual use gives wildly inaccurate PPM values.
• Not calibrated: All MQ sensors ship uncalibrated; each unit needs individual R₀ measurement in clean air.