Gas Sensor

written by hek

There are quite a few different gas sensors you can use together with MySensors. Detect alcohol, methane (fart-sensor?), fire etc. We list a few in the buying guide below.

The example provided here uses the MQ2 sensor to detect air quality.

Wiring Things Up

Start by connecting the radio module.

Sensor Arduino Comment
VCC +5V -
A-out A0 -


Last updated by tekka007, 14 Jul 2019, "Fix issues after CPPcheck 1.88 update (#1308)"

 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 * Created by Henrik Ekblad <[email protected]>
 * Copyright (C) 2013-2018 Sensnology AB
 * Full contributor list:
 * Documentation:
 * Support Forum:
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 * Connect the MQ2 sensor as follows :
 *   A H A   >>> 5V
 *   B       >>> A0
 *   H       >>> GND
 *   B       >>> 10K ohm >>> GND
 * Contribution: epierre
 * Based on
 * License: Attribution-NonCommercial-ShareAlike 3.0 Unported (CC BY-NC-SA 3.0)
 * Modified by HEK to work in 1.4

// Enable debug prints to serial monitor
#define MY_DEBUG

// Enable and select radio type attached
#define MY_RADIO_RF24
//#define MY_RADIO_NRF5_ESB
//#define MY_RADIO_RFM69
//#define MY_RADIO_RFM95

#include <MySensors.h>

#define     CHILD_ID_MQ                   0
/************************Hardware Related Macros************************************/
#define     MQ_SENSOR_ANALOG_PIN         (0)  //define which analog input channel you are going to use
#define         RL_VALUE                     (5)     //define the load resistance on the board, in kilo ohms
#define         RO_CLEAN_AIR_FACTOR          (9.83)  //RO_CLEAR_AIR_FACTOR=(Sensor resistance in clean air)/RO,
//which is derived from the chart in datasheet
/***********************Software Related Macros************************************/
#define         CALIBARAION_SAMPLE_TIMES     (50)    //define how many samples you are going to take in the calibration phase
#define         CALIBRATION_SAMPLE_INTERVAL  (500)   //define the time interval(in milliseconds) between each samples in the
//calibration phase
#define         READ_SAMPLE_INTERVAL         (50)    //define how many samples you are going to take in normal operation
#define         READ_SAMPLE_TIMES            (5)     //define the time interval(in milliseconds) between each samples in
//normal operation
/**********************Application Related Macros**********************************/
#define         GAS_LPG                      (0)
#define         GAS_CO                       (1)
#define         GAS_SMOKE                    (2)
uint32_t SLEEP_TIME = 30000; // Sleep time between reads (in milliseconds)
float Ro = 10000.0;    // this has to be tuned 10K Ohm
int val = 0;           // variable to store the value coming from the sensor
float valMQ =0.0;
float lastMQ =0.0;
float           LPGCurve[3]  =  {2.3,0.21,-0.47};   //two points are taken from the curve.
//with these two points, a line is formed which is "approximately equivalent"
//to the original curve.
//data format:{ x, y, slope}; point1: (lg200, 0.21), point2: (lg10000, -0.59)
float           COCurve[3]  =  {2.3,0.72,-0.34};    //two points are taken from the curve.
//with these two points, a line is formed which is "approximately equivalent"
//to the original curve.
//data format:{ x, y, slope}; point1: (lg200, 0.72), point2: (lg10000,  0.15)
float           SmokeCurve[3] = {2.3,0.53,-0.44};   //two points are taken from the curve.
//with these two points, a line is formed which is "approximately equivalent"
//to the original curve.
//data format:{ x, y, slope}; point1: (lg200, 0.53), point2:(lg10000,-0.22)

MyMessage msg(CHILD_ID_MQ, V_LEVEL);

void setup()
    Ro = MQCalibration(
             MQ_SENSOR_ANALOG_PIN);         //Calibrating the sensor. Please make sure the sensor is in clean air

void presentation()
    // Send the sketch version information to the gateway and Controller
    sendSketchInfo("Air Quality Sensor", "1.0");

    // Register all sensors to gateway (they will be created as child devices)
    present(CHILD_ID_MQ, S_AIR_QUALITY);

void loop()
    uint16_t valMQ = MQGetGasPercentage(MQRead(MQ_SENSOR_ANALOG_PIN)/Ro,GAS_CO);

    Serial.print(MQGetGasPercentage(MQRead(MQ_SENSOR_ANALOG_PIN)/Ro,GAS_LPG) );
    Serial.print( "ppm" );
    Serial.print("    ");
    Serial.print(MQGetGasPercentage(MQRead(MQ_SENSOR_ANALOG_PIN)/Ro,GAS_CO) );
    Serial.print( "ppm" );
    Serial.print("    ");
    Serial.print(MQGetGasPercentage(MQRead(MQ_SENSOR_ANALOG_PIN)/Ro,GAS_SMOKE) );
    Serial.print( "ppm" );

    if (valMQ != lastMQ) {
        lastMQ = ceil(valMQ);

    sleep(SLEEP_TIME); //sleep for: sleepTime

/****************** MQResistanceCalculation ****************************************
Input:   raw_adc - raw value read from adc, which represents the voltage
Output:  the calculated sensor resistance
Remarks: The sensor and the load resistor forms a voltage divider. Given the voltage
         across the load resistor and its resistance, the resistance of the sensor
         could be derived.
float MQResistanceCalculation(int raw_adc)
    return ( ((float)RL_VALUE*(1023-raw_adc)/raw_adc));

/***************************** MQCalibration ****************************************
Input:   mq_pin - analog channel
Output:  Ro of the sensor
Remarks: This function assumes that the sensor is in clean air. It use
         MQResistanceCalculation to calculates the sensor resistance in clean air
         and then divides it with RO_CLEAN_AIR_FACTOR. RO_CLEAN_AIR_FACTOR is about
         10, which differs slightly between different sensors.
float MQCalibration(int mq_pin)
    int i;
    float val=0;

    for (i=0; i<CALIBARAION_SAMPLE_TIMES; i++) {          //take multiple samples
        val += MQResistanceCalculation(analogRead(mq_pin));
    val = val/CALIBARAION_SAMPLE_TIMES;                   //calculate the average value

    val = val/RO_CLEAN_AIR_FACTOR;                        //divided by RO_CLEAN_AIR_FACTOR yields the Ro
    //according to the chart in the datasheet

    return val;
/*****************************  MQRead *********************************************
Input:   mq_pin - analog channel
Output:  Rs of the sensor
Remarks: This function use MQResistanceCalculation to calculate the sensor resistance (Rs).
         The Rs changes as the sensor is in the different concentration of the target
         gas. The sample times and the time interval between samples could be configured
         by changing the definition of the macros.
float MQRead(int mq_pin)
    int i;
    float rs=0;

    for (i=0; i<READ_SAMPLE_TIMES; i++) {
        rs += MQResistanceCalculation(analogRead(mq_pin));

    rs = rs/READ_SAMPLE_TIMES;

    return rs;

/*****************************  MQGetGasPercentage **********************************
Input:   rs_ro_ratio - Rs divided by Ro
         gas_id      - target gas type
Output:  ppm of the target gas
Remarks: This function passes different curves to the MQGetPercentage function which
         calculates the ppm (parts per million) of the target gas.
int MQGetGasPercentage(float rs_ro_ratio, int gas_id)
    if ( gas_id == GAS_LPG ) {
        return MQGetPercentage(rs_ro_ratio,LPGCurve);
    } else if ( gas_id == GAS_CO ) {
        return MQGetPercentage(rs_ro_ratio,COCurve);
    } else if ( gas_id == GAS_SMOKE ) {
        return MQGetPercentage(rs_ro_ratio,SmokeCurve);

    return 0;

/*****************************  MQGetPercentage **********************************
Input:   rs_ro_ratio - Rs divided by Ro
         pcurve      - pointer to the curve of the target gas
Output:  ppm of the target gas
Remarks: By using the slope and a point of the line. The x(logarithmic value of ppm)
         of the line could be derived if y(rs_ro_ratio) is provided. As it is a
         logarithmic coordinate, power of 10 is used to convert the result to non-logarithmic
int  MQGetPercentage(float rs_ro_ratio, float *pcurve)
    return (pow(10,( ((log(rs_ro_ratio)-pcurve[1])/pcurve[2]) + pcurve[0])));

Gas Sensor Comparison

Sensor Detects Heater Voltage
MQ-2 Methane, Butane, LPG, smoke 5V
MQ-3 Alcohol, Ethanol, smoke 5V
MQ-4 Methane, CNG Gas 5V
MQ-5 Natural gas, LPG 5V
MQ-6 LPG, butane gas 5V
MQ-7 Carbon Monoxide Alternating 5V and 1.4V
MQ-8 Hydrogen Gas 5V
MQ-9 Carbon Monoxide, flammable gasses. Alternating 5V and 1.4V
MQ131 Ozone 6V
MQ135 Air Quality (Benzene, Alcohol, smoke) 5V
MQ136 Hydrogen Sulfide gas 5V
MQ137 Ammonia 5V
MQ138 Benzene, Toluene, Alcohol, Acetone, Propane, Formaldehyde gas, Hydrogen 5V
MQ214 Methane, Natural gas 6V
MQ216 Natural gas, Coal gas 5V
MQ303A Alcohol, Ethanol, smoke 0.9V
MQ306A LPG, butane gas 0.9V
MQ307A Carbon Monoxide Alternating 0.2V and 0.9V.
MQ309A Carbon Monoxide, flammable gasses Alternating 0.2V and 0.9V
MG811 Carbon Dioxide (CO2) 6V
AQ-104 Air quality -


NameSize# Downloads
SNS-MQ135.pdf144.97 kB2659
MQ2.pdf179.82 kB4018
MQ-3.pdf54.95 kB1337
MQ-4.pdf52.71 kB1378
MQ5.pdf59.55 kB1382
MQ-6.pdf53.88 kB1159
MQ-7.pdf52.19 kB2301
MG811.pdf95.92 kB1549
ESP3SAQ201.pdf165.46 kB2519
MQ307A.pdf151.27 kB1937
MQ303A.pdf152.2 kB751
1341.pdf39.66 kB2160
MQ-138.pdf59.92 kB5632
NH3.pdf143.21 kB826
MQ-136.pdf142.92 kB2589
Ozone.pdf154.03 kB925
MQ9.pdf176.2 kB1516
MQ-8 Ver1.3 - Manual.pdf451.29 kB1052

Shopping Guide

MQ-135 Air Quality Sensor
Sensitive for Benzene, Alcohol, smoke.
951 available - $1.69   Buy
In stock - $2.08   Buy
MQ-2 Gas Sensor
Sensitive for Methane, Butane, LPG, smoke.
2519 available - $1.25   Buy
In stock - $0.90   Buy
MQ-3 Alcohol Sensor
Build a breath analyzer. Sensitive for Alcohol, Ethanol, smoke.
60 available - $1.35   Buy
In stock - $0.94   Buy
MQ-4 Gas Sensor Module
Sensitive for Methane, CNG Gas
802 available - $1.54   Buy
Stock unknown   Buy


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