ESP32 Gas Sensor Application Guide

1. Basic Principles and Selection

Gas sensors operate by detecting electrochemical changes (such as resistance changes) caused by target gases. Common types include:

• MQ Series detects specific gases (such as smoke, liquefied gas) and outputs an analog voltage.
• Digital Sensors directly output I²C digital signals (such as BME680 for multi-parameter monitoring).

Practical Selection Guide:

Application Scenario	Recommended Sensor	Features
Fire Alarm	MQ-2	Low-cost smoke detection
Kitchen Safety	MQ-5	High sensitivity liquefied gas detection
Air Quality Monitoring	MQ-135	Detection of pollutants like benzene and ammonia
Indoor Environment Monitoring	BME680	Temperature, humidity, pressure + gas four-in-one

2. Wiring and Configuration

Basic Wiring for MQ Series:

MQ Sensor  →  ESP32
-----------------
VCC      →  3.3V or 5V*
GND      →  GND
AOUT     →  GPIO34 (Analog Input)

* 5V power supply requires a voltage divider circuit

BME680 Digital Sensor Wiring:

BME680   →  ESP32
-----------------
VCC      →  3.3V
GND      →  GND
SCL      →  GPIO22 (I²C Clock)
SDA      →  GPIO21 (I²C Data)

3. Simple Code Examples

Smoke Detection System

// Pin definitions
const int SMOKE_PIN = 34;
const int ALARM_LED = 2;  // Onboard LED

void setup() {
  Serial.begin(115200);
  pinMode(SMOKE_PIN, INPUT);
  pinMode(ALARM_LED, OUTPUT);
}

void loop() {
  int value = analogRead(SMOKE_PIN);

  // Convert raw value (0-4095)
  Serial.print("Smoke Concentration: ");
  Serial.println(value);

  // Simple alarm: value > 1500 triggers
  if(value > 1500) {
    digitalWrite(ALARM_LED, HIGH);
    Serial.println("Warning: Smoke detected!");
  } else {
    digitalWrite(ALARM_LED, LOW);
  }

  delay(1000);  // Check once per second
}

Air Quality Monitoring

#include <Wire.h>
#include "Adafruit_BME680.h"

Adafruit_BME680 bme;

void setup() {
  Serial.begin(115200);

  // Initialize BME680
  if(!bme.begin()) {
    Serial.println("Sensor not found");
    while(1);
  }

  // Configure sensor parameters
  bme.setTemperatureOversampling(BME680_OS_2X);
  bme.setHumidityOversampling(BME680_OS_2X);
  bme.setGasHeater(320, 150); // 320°C heating for 150ms
}

void loop() {
  // Read data
  if(!bme.performReading()) {
    Serial.println("Reading failed");
    return;
  }

  // Print results
  Serial.print("Temperature: "); Serial.print(bme.temperature); Serial.println("°C");
  Serial.print("Humidity: "); Serial.print(bme.humidity); Serial.println("% ");
  Serial.print("Pressure: "); Serial.print(bme.pressure/100.0); Serial.println("hPa");
  Serial.print("Gas Resistance: "); Serial.print(bme.gas_resistance); Serial.println("Ω");
  Serial.println("-----------------");

  delay(3000);  // Check every 3 seconds
}

4. Data Processing Methods

Concentration Conversion Formula

For the MQ-2 sensor:

RS = (3.3 * 4095 / rawADC) - 1  // Calculate sensor resistance
R0 = Resistance value of the sensor in clean air
ratio = RS / R0

// Approximate calculation of smoke concentration
smoke_ppm = 500 * pow(ratio, -1.8)

Practical Alarm Logic

void checkAirQuality(float gasResistance) {
  // Determine air quality based on gas resistance
  if(gasResistance > 200000) {
    Serial.println("Air Quality: Excellent");
  } else if(gasResistance > 100000) {
    Serial.println("Air Quality: Good");
  } else if(gasResistance > 50000) {
    Serial.println("Air Quality: Moderate");
  } else {
    Serial.println("Air Quality: Poor - Ventilation recommended!");
  }
}

5. Application Scenario Implementation

Kitchen Safety System

void monitorKitchen() {
  int gasLevel = analogRead(GAS_SENSOR_PIN);

  // Multi-level response mechanism
  if(gasLevel > 2000) {
    // 1. Sound and light alarm
    activateAlarm();

    // 2. Close gas valve
    digitalWrite(VALVE_CONTROL_PIN, LOW);

    // 3. Start exhaust fan
    digitalWrite(FAN_CONTROL_PIN, HIGH);
  }
  else if(gasLevel > 1500) {
    // Pre-warning level
    blinkWarningLED();
  }
}

Smart Ventilation Control

void autoVentilation() {
  if(needFreshAir()) {
    digitalWrite(FAN_PIN, HIGH);
    Serial.println("Starting ventilation system");
  } else {
    digitalWrite(FAN_PIN, LOW);
  }
}

bool needFreshAir() {
  // Comprehensive assessment of air indicators
  float co2 = readCO2Level();
  float voc = readVOCLevel();

  // Exceedance detection logic
  return (co2 > 800) || (voc > 0.5) || (readSmokeLevel() > 1000);
}

6. Optimization Techniques

1. Preheating Treatment

void preheatSensor() {
  Serial.println("Preheating sensor...");
  for(int i = 0; i < 180; i++) { // 3 minutes preheating
    delay(1000);
  }
  Serial.println("Preheating complete");
}

1. Drift Compensation

float calibrateReading(float rawValue) {
  // Subtract baseline value (to be calibrated periodically)
  static float baseLine = 850.0;
  return rawValue - baseLine;
}

1. Deep Sleep

void deepSleepSavePower() {
  esp_sleep_enable_timer_wakeup(300 * 1000000); // Sleep for 5 minutes
  esp_deep_sleep_start();
}

7. Practical Operation Steps

1. Wiring Check

• Ensure the sensor operates at the appropriate voltage
• Check ground connections
• Confirm data line connections are correct

Calibration Process

1. Power on in clean air
2. Preheat for 30 minutes
3. Record stable value as baseline
4. Repeat calibration quarterly

Troubleshooting

• No change in value: Check wiring
• Value jumps: Add filtering capacitor
• Low readings: Check sensor lifespan
• High readings: Clean sensor surface

With the powerful processing capabilities and rich interfaces of the ESP32, gas sensors can build complete solutions ranging from simple detection to intelligent control.

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