MQ-3 - Gas Sensor (Ethanol)

The MQ-3 is an alcohol sensor designed to detect ethanol (C₂H₅OH) vapors in the air. It uses a tin dioxide (SnO₂) sensing material that changes resistance with alcohol concentration. Operating at 5V, it provides both analog and digital outputs. The MQ-3 is highly sensitive to alcohol, making it ideal for breathalyzer devices, vehicle alcohol detection systems, and safety applications requiring alcohol monitoring.

Technical Specifications

📄 MQ-3 datasheet (56 kB)

Important notes

Please keep the following notes in mind before using/connecting the MQ-3:

• The sensor value only reflects the approximated trend of gas concentration in a permissible error range, it DOES NOT represent the exact gas concentration!
• The MQ-3 sensor module requires a lot of power, so you must supply 5V externally (e.g. from a Powerbank). Powering them via USB alone or via a pin of a microcontroller is not sufficient and could cause damages!
• Liquid alcohol must never be applied directly to the sensor, as this could damage or destroy it!
• Do not expose this sensor to water and frost!
• Applying a voltage higher than 5V or applying the voltage to the wrong pins may damage the sensor!
• Exposing the sensor to a high concentration of gases for a long time may have a negative effect on its performance!
• Shaking or vibrating the sensor may decrease its accuracy!

Connections

Digital Output DO

Using the potentiometer on the back of the board, you can set the threshold at which the DO output switches from HIGH to LOW. The DO pin can be connected directly to a microcontroller or a development kit.

Analog Output AO

The sensor also has an AO analog output, which should be connected to the A/D converter (e.g.: analog input on Arduino). This allows for more accurate determination of the alcohol content by measuring the proportional voltage signal.

Used Components

Programming

Direct indicator

Through the DO output, it is possible to operate a signal LED directly on the MQ-3 module using TTL logic. The following circuit diagram shows the corresponding setup. The LED lights up as soon as a certain amount of alcohol is detected and turns off once the value drops below that threshold again. The threshold for this transition can be adjusted using the built-in trim potentiometer.

Arduino UNO

Note that the Arduino UNO is connected via USB to the PC but has ONLY a ground (0V) connection to the rest of the circuit.

#define PIN_AO  A0
#define PIN_DO  7
#define PIN_LED 8

void setup() {
    Serial.begin(9600);
    pinMode(PIN_LED, OUTPUT);
    pinMode(PIN_AO, INPUT);
    pinMode(PIN_DO, INPUT);
}

void loop() {
    float sensorValue, sensorVolt;

    sensorValue = analogRead(PIN_AO);
    sensorVolt = sensorValue / 1024 * 5.0;

    Serial.print("Sensor: " + String(sensorValue) + " (" + String(sensorVolt) + "V)");

    if (digitalRead(PIN_DO) == LOW) {
        digitalWrite(PIN_LED, HIGH);
        Serial.print(" Ethanol present");
    } else {
        digitalWrite(PIN_LED, LOW);
    }

    Serial.println("");
    delay(1000);
}

The test for the MQ-3module was conducted with a Glen Garioch Single Malt Scotch Whisky. After a small sip, the sensor was breathed on, and the values were observed in the serial console.

To display the measured alcohol levels over time as a graph, the following code is used:

#define PIN_AO  A0

void setup() {
    Serial.begin(9600);
    pinMode(PIN_AO, INPUT);
}

void loop() {
    Serial.println(analogRead(PIN_AO));
    delay(1000);
}

ESP32

Since the ESP32 is generally operated at 3.3V and its input pins are not 5V-tolerant, the AO output of the MQ-3 must be conditioned using a simple voltage divider.
Note that the ESP32 is connected via USB to the PC but has ONLY a ground (0V) connection to the rest of the circuit.

To display the measured alcohol levels over time as a graph, the following code is used:

#define PIN_AO 13

void setup() {
    Serial.begin(9600);
    pinMode(PIN_AO, INPUT);
}

void loop() {
    Serial.println(analogRead(PIN_AO));
    delay(1000);
}