ESP32 MQ-2 Gas/Smoke Detector Alarm
In this project, combustible gas (LPG, methane, propane) and smoke detection is performed using an MQ-2 gas sensor module, and a siren-like alarm is played through a passive buzzer upon detection. The sensor's digital threshold output (DO) is connected to GPIO 4 on the ESP32; when gas or smoke exceeds the threshold, DO goes LOW and the alarm is triggered. Sensitivity is adjusted via the module's potentiometer. The passive buzzer is driven using `tone()` to produce a two-frequency siren effect.
Video
Circuit Diagram
Source Code
1// ─────────────────────────────────────────
2// BlueGrays · bluegrays.com
3// ESP32 MQ-2 Gas/Smoke Detector Alarm
4// ─────────────────────────────────────────
5// Reads the digital threshold output (DO) of the MQ-2 module. When gas or
6// smoke passes the threshold, the warning LED turns on and the passive
7// buzzer plays a siren switching between 800 and 1200 Hz every 0.2 s. Set
8// the threshold with the trimpot on the module (Arduino Core 3.0+).
9//
10// Pins:
11// GPIO 4 -> MQ-2 DO (digital threshold output)
12// GPIO 18 -> Buzzer + leg
13// GPIO 5 -> Red LED (with a 220 ohm resistor)
14// MQ-2 VCC -> MQ-2 VCC -> 5V (VIN), GND -> GND
15
16// Pin definitions
17const int MQ2_DO_PIN = 4; // MQ-2 D0 (digital threshold out)
18const int BUZZER_PIN = 18; // Passive buzzer
19const int LED_PIN = 5; // Warning LED + 220R
20
21// Warm-up time (ms): the MQ-2 heater needs minutes to warm up. In Wokwi 2000 is enough.
22const unsigned long WARMUP_MS = 60000;
23bool warmedUp = false; // Is the warm-up over
24
25void setup() {
26 pinMode(MQ2_DO_PIN, INPUT);
27 pinMode(BUZZER_PIN, OUTPUT);
28 pinMode(LED_PIN, OUTPUT);
29 digitalWrite(BUZZER_PIN, LOW);
30 digitalWrite(LED_PIN, LOW);
31}
32
33void loop() {
34 // Warm-up: the DO reading is ignored, the LED blinks slowly without blocking
35 if (!warmedUp) {
36 if (millis() < WARMUP_MS) {
37 digitalWrite(LED_PIN, (millis() / 500) % 2); // 0.5 s on, 0.5 s off
38 return;
39 }
40 warmedUp = true;
41 }
42
43 // MQ-2 D0 is normally HIGH and goes LOW above the threshold
44 int gasState = digitalRead(MQ2_DO_PIN);
45
46 if (gasState == LOW) {
47 // Alarm: the LED turns on, the buzzer plays a two-tone siren
48 digitalWrite(LED_PIN, HIGH);
49 tone(BUZZER_PIN, 800); // Low tone
50 delay(200);
51 tone(BUZZER_PIN, 1200); // High tone
52 delay(200);
53 } else {
54 // Idle: LED and buzzer off
55 digitalWrite(LED_PIN, LOW);
56 noTone(BUZZER_PIN);
57 delay(50);
58 }
59}
60FAQ
What gases does the MQ-2 detect?▾
The MQ-2 is sensitive to LPG, propane, methane, hydrogen, alcohol vapor, and smoke. It cannot distinguish between different gases; it only detects an overall increase in density.
Can the MQ-2's analog output be connected directly to the ESP32?▾
The MQ-2 is powered by 5 V, and its analog output can reach up to 5 V. Since the ESP32’s ADC input accepts a maximum of 3.3 V, a voltage divider (e.g., 10 kΩ + 20 kΩ) must be used.
The sensor shows a high reading when it's first turned on. Why?▾
It takes a few minutes for the MQ sensors' heater to stabilize; during this time, the readings will be high. Disabling the alarm for a few minutes after startup prevents false alarms.
How is a siren effect created?▾
The passive buzzer is driven using `tone()` to rapidly oscillate between two different frequencies (e.g., 800 Hz and 1200 Hz). This oscillation mimics the classic alarm sound.