Introduction

In this project, we will go over how to build a smoke sensor circuit with an arduino board.

The smoke sensor we will use is the MQ-2. This is a sensor that is not only sensitive to smoke, but also to flammable gas.

The MQ-2 smoke sensor reports smoke by the voltage level that it outputs. The more smoke there is, the greater the voltage that it outputs. Conversely, the less smoke that it is exposed to, the less voltage it outputs.

Preparations

Hardware

Software

About MQ2 Smoke Sensor

The MQ-2 smoke sensor is sensitive to smoke and to the following flammable gases:

The resistance of the sensor is different depending on the type of the gas.

The smoke sensor has a built-in potentiometer that allows you to adjust the sensor sensitivity according to how accurate you want to detect gas.

Features

1. Wide detecting scope
2. High sensitivity and fast response
3. Long life and stable
4. Simple drive circuit

Due to its fast response time and high sensitivity, measurements can be taken as soon as possible. The sensor sensitivity can be adjusted by using the potentiometer.

Standard Working Condition

Symbol Parameter Name Technical Condition Remarks
VC Circuit voltage 5V±0.1 AC or DC
VH Heating voltage 5V±0.1 AC or DC
RL Load resistance adjustable
RH Heater resistance 33Kohm±5% Room temperature
PH Heating consumption Less than 800mW

Environment Condition

Symbol Parameter Name Technical Condition Remarks
TO Operating Temp. -20°C-50°C
TS Storage Temp. -20°C-70°C
RH Relative Humidity <95%
O2 Oxygen Concentration 21%(standard condition) Oxygen concentration can affect sensitivity Minimum value is 2%

Sensitivity Characteristics

Symbol Parameter Name Technical Condition Remarks
RS Sensor Resistance 3Kohm-30Kohm (1000ppm iso-butane) Detecting concentration scope:
200ppm-5000ppm LPG and propane
300ppm-5000ppm butane
5000ppm-20000ppm methane
300ppm-5000ppm H2
100ppm-2000ppm Alcohol
α (3000ppm/1000ppm iso-butane) Concentration slope rate ≤0.6
Standard detecting Condition Temp.: 20°C±2°C VC: 5V±0.1
Humidity:65%±5% VH:5V±0.1
Preheating Time Over 24 hours

How does it Work?

The MQ2 has an electrochemical sensor, which changes its resistance for different concentrations of varied gasses. The sensor is connected in series with a variable resistor to form a voltage divider circuit , and the variable resistor is used to change sensitivity. When one of the above gaseous elements comes in contact with the sensor after heating, the sensor’s resistance change. The change in the resistance changes the voltage across the sensor, and this voltage can be read by a microcontroller. The voltage value can be used to find the resistance of the sensor by knowing the reference voltage and the other resistor’s resistance. The sensor has different sensitivity for different types of gasses. The sensitivity characteristic curve is shown below for the different type of gasses.

The voltage that the sensor outputs changes accordingly to the smoke/gas level that exists in the atmosphere. The sensor outputs a voltage that is proportional to the concentration of smoke/gas.

In other words, the relationship between voltage and gas concentration is the following:

Working Mechanism

The output can be an analog signal (A0) that can be read with an analog input of the Arduino or a digital output (D0) that can be read with a digital input of the Arduino.

Note


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 detection of certain components in the air usually requires a more precise and costly instrument, which cannot be done with a single gas sensor. If your project is aimed at obtaining the gas concentration at a very precise level, then we don’t recommend this gas sensor.

Gas Detection : Basic Example

In this example, the sensor is connected to A0 pin. The voltage read from the sensor is displayed. This value can be used as a threshold to detect any increase/decrease in gas concentration.

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

void loop() {
    float sensor_volt;
    float sensorValue;

    sensorValue = analogRead(A0);
    sensor_volt = sensorValue/1024*5.0;

    Serial.print("sensor_volt = ");
    Serial.print(sensor_volt);
    Serial.println("V");
    delay(1000);
}

Measurement : Approximation

These examples demonstrate ways to know the approximate concentration of Gas. As per the data-sheet of the MQx sensors, these equations are tested for standard conditions and are not calibrated. It may vary based on change in temperature or humidity.

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

void loop() {
    float sensor_volt;
    float RS_air; //  Get the value of RS via in a clear air
    float R0;  // Get the value of R0 via in H2
    float sensorValue;

    /*--- Get a average data by testing 100 times ---*/
    for(int x = 0 ; x < 100 ; x++)
    {
        sensorValue = sensorValue + analogRead(A0);
    }
    sensorValue = sensorValue/100.0;
    /*-----------------------------------------------*/

    sensor_volt = sensorValue/1024*5.0;
    RS_air = (5.0-sensor_volt)/sensor_volt; // omit *RL
    R0 = RS_air/9.8; // The ratio of RS/R0 is 9.8 in a clear air from Graph (Found using WebPlotDigitizer)

    Serial.print("sensor_volt = ");
    Serial.print(sensor_volt);
    Serial.println("V");

    Serial.print("R0 = ");
    Serial.println(R0);
    delay(1000);

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

void loop() {

    float sensor_volt;
    float RS_gas; // Get value of RS in a GAS
    float ratio; // Get ratio RS_GAS/RS_air
    int sensorValue = analogRead(A0);
    sensor_volt=(float)sensorValue/1024*5.0;
    RS_gas = (5.0-sensor_volt)/sensor_volt; // omit *RL

          /*-Replace the name "R0" with the value of R0 in the demo of First Test -*/
    ratio = RS_gas/R0;  // ratio = RS/R0
          /*-----------------------------------------------------------------------*/

    Serial.print("sensor_volt = ");
    Serial.println(sensor_volt);
    Serial.print("RS_ratio = ");
    Serial.println(RS_gas);
    Serial.print("Rs/R0 = ");
    Serial.println(ratio);

    Serial.print("\n\n");

    delay(1000);

}



Arduino MQ-2 Smoke Alarm

The circuit we will build is shown below.

Arduino MQ-2 smoke sensor circuit

So to power the smoke sensor, we connect pin 2 of the smoke sensor to the 5V terminal of the arduino and terminal 3 to the GND terminal of the arduino. This gives the smoke sensor the 5 volts it needs to be powered.

The output of the sensor goes into analog pin A0 of the arduino. Through this connection, the arduino can read the analog voltage output from the sensor. The arduino board has a built-in analog-to-digital converter, so it is able to read analog values without any external ADC chip.

Depending on the value that the arduino reads determines the action that will occur with the circuit. We will make it in our code that if the sensor outputs a voltage above a certain threshold, the buzzer will go off, alerting a user that smoke has been detected.

These are all the physical connections in order for our circuit to work.

Code for the Arduino MQ-2 Smoke Sensor Circuit

Being that we've just gone over the circuit schematic for the smoke sensor circuit, all we need know is the code necessary to upload to the arduino for this smoke alarm cicrcuit to work.

The code that we need to upload is shown below.

/*Code for MQ-2 Smoke Sensor Circuit Built with an Arduino Board*/

const int sensorPin= 0;
const int buzzerPin= 13;
int smoke_level;

void setup() {
Serial.begin(115200); //sets the baud rate for data transfer in bits/second
pinMode(sensorPin, INPUT);//the smoke sensor will be an input to the arduino
pinMode(buzzerPin, OUTPUT);//the buzzer serves an output in the circuit
}

void loop() {
smoke_level= analogRead(sensorPin); //arduino reads the value from the smoke sensor
Serial.println(smoke_level);//prints just for debugging purposes, to see what values the sensor is picking up
if(smoke_level > 200){ //if smoke level is greater than 200, the buzzer will go off
digitalWrite(buzzerPin, HIGH);
}
else{
digitalWrite(buzzerPin, LOW);
}
}

The first block of code declares and initializes 3 variables. The sensorPin represents the smoke sensor. It is initialized to 0, because it will be connected to analog pin A0 of the arduino board. The next variable, buzzerPin, represents the pin that the anode of the buzzer will be connected to; it is initialized to 12 because it will be connected to digital pin D12 of the arduino board. And the variable, smoke_level, represents the amount of smoke that the smoke sensor picks up.

The next block of code defines the baud rate and the input and output of the circuit. The sensorPin, which is the smoke sensor pin, serves as the input of the circuit. This sensor is input into the arduino so that the arduino can read and process the value. The buzzerPin serves as the output. If the smoke level is above a certain threshold, the output of the circuit, the buzzer, will go off.

The next block of code uses the analogRead() function to read the value from the sensorPin (the smoke sensor). This will be a numerical value from 0 to 1023. 0 represents no smoke, while 1023 represents smoke at the absolute maximum highest level. So the variable, smoke_level, represents the smoke level that can range from 0 to 1023. We put a line to print this value just for debugging purposes, so that you can see what values are being returned from this function. In our code, we make it so that if the smoke level rises above 200, we will trigger the buzzer to sound by sending the digital pin D12 high. So 200 is our threshold level. If the smoke level is below this value, then the buzzer does not go off.

This last block of code was the loop() function. This is the part of code that repeats over and over in an infinite loop. This means that our code is always checking to see what the smoke_level is, so that it can know whether to trigger the buzzer or not.

And this is how a smoke sensor works with