Building a Carbon Monoxide Detector

Build a circuit that senses carbon monoxide (CO) — the invisible, odourless gas from any incomplete combustion — and raises an alarm when it rises. An MQ-7 sensor gives the Arduino a reading that climbs with CO; the code compares it to a clean-air baseline and triggers a buzzer and red LED. A great way to learn gas sensing — but read the safety note: this is a demonstration, NOT a certified life-safety CO alarm.

中级

1-2 hours

说明

Wire the sensor, buzzer and LED

On the breadboard: MQ-7 VCC to Arduino 5V, GND to GND, and its analog output (AOUT) to Arduino A0. Active buzzer + to pin 8, - to GND. Red LED anode to pin 7 through a 330 Ω resistor, cathode to GND.

此步骤所需材料：

Arduino Uno R31 个

Carbon Monoxide Sensor - MQ-71 个

Active Buzzer Module (5V, 5-Pack)1 个

LED - Basic 5mm1 个

Resistor 330 Ohm 1/6 Watt PTH - 20 pack1 个

Breadboard1 个

Jumper Wires (Male-to-Male)1 包

Connect and open the IDE

Plug the Arduino into your computer with the USB cable, open the Arduino IDE, and select Arduino Uno and its serial port.

此步骤所需材料：

USB-B Cable1 个

所需工具：

Computer with Arduino IDE

Upload the detector sketch

Paste this sketch and upload. It warms the sensor for two minutes, records a clean-air baseline, then sounds the buzzer and lights the LED whenever the reading climbs above that baseline.

co_detector.inoarduino

// Carbon monoxide INDICATOR using an MQ-7 sensor.
// NOT a certified life-safety alarm - see the blueprint's safety note.
// MQ-7 analog output on A0; buzzer on pin 8; red warning LED on pin 7.

const int MQ7_PIN = A0;
const int BUZZER_PIN = 8;
const int LED_PIN = 7;

int cleanAirValue = 0;          // baseline, measured at startup
const int ALARM_MARGIN = 150;   // how far above baseline triggers the alarm

void setup() {
  pinMode(BUZZER_PIN, OUTPUT);
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);

  Serial.println("Warming up the MQ-7 (2 min) in clean air...");
  delay(120000);                // 2-minute warm-up
  cleanAirValue = analogRead(MQ7_PIN);
  Serial.print("Clean-air baseline: ");
  Serial.println(cleanAirValue);
}

void loop() {
  int reading = analogRead(MQ7_PIN);
  Serial.println(reading);

  if (reading > cleanAirValue + ALARM_MARGIN) {
    digitalWrite(LED_PIN, HIGH);      // CO above baseline -> alarm
    digitalWrite(BUZZER_PIN, HIGH);
  } else {
    digitalWrite(LED_PIN, LOW);
    digitalWrite(BUZZER_PIN, LOW);
  }
  delay(500);
}

Warm up, baseline and test

Power it in fresh air and let the 2-minute warm-up set the clean-air baseline (a brand-new MQ-7 also needs a longer 24–48 h burn-in before its readings settle). Then hold a small CO source nearby in a ventilated room — the smoke from a just-blown-out match works — and the buzzer and LED should trigger. Change ALARM_MARGIN to make it more or less sensitive.

Important safety limits

This is a LEARNING project and, at best, a supplementary indicator — it is NOT a certified life-safety device. A hobby MQ-7 gives a relative reading, not a calibrated ppm; it drifts over time and needs regular re-baselining, and it also responds to smoke and other gases. Carbon monoxide is invisible, odourless and can be fatal: for real protection always use a UL- or EN-certified CO alarm. Never rely on this circuit to keep anyone safe.