DIY Smoke Detector for $4

What we’re building

A smoke detector that actually works — green LED when the air is clean, red LED and buzzer alarm when it detects smoke or gas. Total cost: under $4 in components.

Components

Part	Qty
Arduino Uno	1
MQ-2 Gas Sensor	1
Buzzer	1
LED (green)	1
LED (red)	1
220Ω Resistor	2
Breadboard	1
Jumper wires	~10

Wiring

From	To
MQ-2 VCC	Arduino 5V
MQ-2 GND	Arduino GND
MQ-2 AO	Arduino A0
MQ-2 DO	Arduino D7
Green LED + 220Ω	Arduino D9 → GND
Red LED + 220Ω	Arduino D10 → GND
Buzzer	Arduino D11

Code

const int MQ2_AO     = A0;
const int MQ2_DO     = 7;
const int LED_GREEN  = 9;
const int LED_RED    = 10;
const int BUZZER     = 11;


const int SMOKE_THRESHOLD = 400;
const int BUZZER_FREQ     = 2000;
const int READ_INTERVAL   = 500;


bool alarmActive = false;


void setup() {
  Serial.begin(9600);

  pinMode(LED_GREEN, OUTPUT);
  pinMode(LED_RED,   OUTPUT);
  pinMode(BUZZER,    OUTPUT);
  pinMode(MQ2_DO,    INPUT);

  Serial.println("=============================");
  Serial.println("  threeboardslab Smoke Detector v1");
  Serial.println("  Warming up MQ-2 sensor...");
  Serial.println("=============================");

  for (int i = 20; i > 0; i--) {
    digitalWrite(LED_GREEN, HIGH);
    delay(500);
    digitalWrite(LED_GREEN, LOW);
    delay(500);
    Serial.print("Warm-up: ");
    Serial.print(i);
    Serial.println("s remaining...");
  }

  Serial.println("Ready! Monitoring...\n");
  digitalWrite(LED_GREEN, HIGH);
}


void loop() {
  int smokeLevel = analogRead(MQ2_AO);
  int doState    = digitalRead(MQ2_DO);


  Serial.print("Smoke Level: ");
  Serial.print(smokeLevel);
  Serial.print(" / 1023  |  Digital: ");
  Serial.print(doState == LOW ? "TRIGGERED" : "clear");
  Serial.print("  |  Status: ");

  if (smokeLevel > SMOKE_THRESHOLD || doState == LOW) {

    Serial.println("*** SMOKE DETECTED! ***");

    digitalWrite(LED_GREEN, LOW);
    digitalWrite(LED_RED,   HIGH);


    tone(BUZZER, BUZZER_FREQ);
    delay(200);
    noTone(BUZZER);
    delay(100);
    tone(BUZZER, BUZZER_FREQ - 400);
    delay(200);
    noTone(BUZZER);
    delay(100);

    alarmActive = true;

  } else {

    Serial.println("OK");

    if (alarmActive) {
      noTone(BUZZER);
      digitalWrite(LED_RED, LOW);
      alarmActive = false;
    }

    digitalWrite(LED_GREEN, HIGH);
    delay(READ_INTERVAL);
  }
}