Measure Air quality with MQ-135 and ESP8266 Wemos D1 mini

As the MQ135 is not  really suited as a CO2 sensor (See my previous blog) and I still wanted to use it, I will use it as an air quality probe on an ESP8266.

As the title mentioned I will use an ESP8266 connected to my local WiFi router for this to send the data to thingspeak.com.
The ESP8266 I use is the Wemos D1 mini. The Wemos D1 mini has an USB interface and can be programmed with the normal Arduino GUI. You can add the board to it. See this article for how to add this board to the GUI.

I also use an DHT22 (AM2302) in this project to measure temperature and humidity. The DHT22 data pin is connected to pin D5 of the ESP8266 in my project. VCC is connected to the 5V of the ESP, GND is connected to GND.  You can of course any digital pin you want in your project. Just change line “#define DHTPIN D5” with the pin number you want.

For the MQ-135 you can only connect it to pin A0, as this is the only analog pin on the ESP8266.

For the air quality I store the lowest measured value in the EEPROM, so anything worse/higher that this best value is bad air quality. I use variable a1 & a2 & a3 to check if it is an new ESP8266, if these value in EEPROM are different than the stored values I presume that it is a new ESP and then set the lowest value to 510, if the values are the same I do not change the lowest value in EEPROM.

Below the program I use for this project.
I will try to describe any main step of it.

As there are some problems copying the below text I have added download links at the bottom of this page.

// Add the ESP8266 library
#include "ESP8266WiFi.h"

// replace with your channel’s thingspeak API key
String apiKey = "XXXXXXXXXXXXXXXXX";
const char* server = "api.thingspeak.com";

// Add the EEPROM library to store the lowest measured MQ-135 value
#include "EEPROM.h"
int address = 24;
byte value;

// DHT22 setup
#include "DHT.h"
#define DHTPIN D5
#define DHTTYPE DHT22   // DHT 22  (AM2302), AM2321
DHT dht(DHTPIN, DHTTYPE);

float val;
float vall;
int sensorPin = A0;
int sensorValue = 0;
float v;
float h;  // humidity
float tc;  // temperature
float tf;    // temperature
float f = 0.;
float pp = 0.;
// number of analog MQ-135 samples taken each time
int s = 25;
int t;
int x;
int lowest = 500;
int lowest_l;
int lowest_h;
int tel = 0;
int telc = 0;
int id = 1;
int air;
int airt;
int airv;
// change one value below to reset lowest value back to 510
int a1 = 148;
int a2 = 231;
int a3 = 23;
// cl should be >= 3 before writing new low
// value to EEPROM
int cl = 0;

// 5 SSID's possible for when you need to measure
// at different places
const char* ssid1 = "your ssid 1";
const char* password1 = "your ssid 1 password";
const char* ssid2 = "your ssid 2";
const char* password2 = "your ssid 2 password";
const char* ssid3 = "your ssid 3";
const char* password3 = "your ssid 3 password";
const char* ssid4 = "your ssid 4";
const char* password4 = "your ssid 4 password";
const char* ssid5 = "your ssid 5";
const char* password5 = "your ssid 5 password";

// start with ssid1
const char* ssid = ssid1;
const char* password = password1;

WiFiClient client;

void setup() {
  Serial.begin(57600);
  EEPROM.begin(512);
  pinMode(sensorPin, INPUT);
  WiFi.begin(ssid, password);
  Serial.print("Trying ");
  Serial.print (ssid);
  Serial.print(" - ");
  while (WiFi.status() != WL_CONNECTED) {
    telc = telc + 1;
    if ( telc >= 10) {
      if ( id == 1 ) {
        ssid = ssid1;
        password = password1;
        id = 2;
      } else {
        if ( id == 2 ) {
          ssid = ssid2;
          password = password2;
          id = 3;
        } else  {
          if ( id == 3 ) {
            ssid = ssid3;
            password = password3;
            id = 4;
          } else  {
            if ( id == 4 ) {
              ssid = ssid4;
              password = password4;
              id = 5;
            } else {
              if ( id == 5 ) {
                ssid = ssid5;
                password = password5;
                id = 1;
              }
            }
          }
        }
      }
      telc = 0;
      Serial.println(".");
      Serial.print("Trying ");
      Serial.print (ssid);
      Serial.print(" - ");
      WiFi.begin(ssid, password);
    }
    delay(2000);
    Serial.print(".");
  }
  Serial.println("Connected");
  delay(100);
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());

  // check if this is new hardware to set EEPROM to 510
  if (EEPROM.read(505) == a1 || EEPROM.read(506) == a2 || EEPROM.read(507) == a3) {
    Serial.println ("EEPROM setup is correct, not changing current values");
  } else {
    // this should be a new start
    EEPROM.write(500, 255);
    EEPROM.write(501, 255);
    EEPROM.write(505, a1);
    EEPROM.write(506, a2);
    EEPROM.write(507, a3);
    delay(50);
    EEPROM.commit();
    Serial.println ("New start for MQ135, startup values set");
  }
  lowest_l = EEPROM.read(500);
  lowest_h = EEPROM.read(501);
  lowest = lowest_l + lowest_h;
  dht.begin();

  // sleep 10 minutes to warm up the MQ135
  // 10 minutes could be to low as warmup can take longer
  Serial.println("Sleep 10 minutes to warm up MQ-135");
  delay(600000);
}

void loop() {
  v = 0;
  t = 0;
  while (t < s) {
    // Read the anolog value s (25) times
    val = (analogRead(sensorPin)) * 1;
    v = v + val;
    t++;
    delay(10);
  }

  vall = v / s;
  Serial.print ("raw = ");
  Serial.println (vall);
  if (vall <= lowest - 1 ) { cl = cl + 1; } else { cl = 0; } // to avoid wrong low value, the value must be 3 times low if (cl >= 3) {
    cl = 0;
    lowest = vall;
    if (lowest >= 255) {
      lowest_l = 255;
      lowest_h = lowest - 255;
    } else {
      lowest_l = lowest;
      lowest_h = 0;
    }
    EEPROM.write(500, lowest_l);
    EEPROM.write(501, lowest_h);
    delay(50);
    EEPROM.commit();
    Serial.println ("New lowest value, saving to EEPROM");
  }

  vall = vall - lowest;
  if (vall <= 0 ) { vall = 0; } airt = airt + vall; airv = airt / (tel + 1); Serial.print ("low: "); Serial.println (lowest); Serial.print ("Bad Air quality : "); Serial.println (vall); Serial.print ("Bad Air quality average: "); Serial.print (airv); Serial.print (" "); Serial.print (tel + 1); Serial.print (" "); Serial.println (airt); delay(4982); tel = tel + 1; if (tel >= 57) {
    tel = 0;
    air = airt / 57;
    ReadDHT();
    if (tc == 1 && h == 1) {
      delay(2000);
      ReadDHT();
    }
    if (tc == 1 && h == 1) {
      delay(2000);
      ReadDHT();
    }
    if (tc == 1 && h == 1) {
      delay(2000);
      ReadDHT();
    }

    ZendData_thingspeak();
    airt = 0;
  }
}

void ZendData_thingspeak() {
  // I am using 4 fields at Thingspeak
  // 1 = temperature
  // 2 = humidity
  // 3 = Air Quality
  // 4 = lowest ever Air Quality
  if (client.connect(server, 80)) { // "184.106.153.149" or api.thingspeak.com
    String postStr = apiKey;
    postStr += "&field1=";
    postStr += String(tc);
    postStr += "&field2=";
    postStr += String(h);
    postStr += "&field3=";
    postStr += String(air);
    postStr += "&field4=";
    postStr += String(lowest);
    postStr += "\r\n\r\n";

    client.print("POST /update HTTP/1.1\n");
    client.print("Host: api.thingspeak.com\n");
    client.print("Connection: close\n");
    client.print("X-THINGSPEAKAPIKEY: " + apiKey + "\n");
    client.print("Content-Type: application/x-www-form-urlencoded\n");
    client.print("Content-Length: ");
    client.print(postStr.length());
    client.print("\n\n");
    client.print(postStr);
    Serial.println("Data send to Thingspeak");
    client.stop();
  }
}

void ReadDHT() {
  // Reading temperature or humidity takes about 250 milliseconds!
  // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
  h = dht.readHumidity();
  // Read temperature as Celsius (the default)
  tc = dht.readTemperature();
  // Read temperature as Fahrenheit (isFahrenheit = true)
  tf = dht.readTemperature(true);

  // Check if any reads failed and exit early (to try again).
  if (isnan(h) || isnan(tc) || isnan(tf)) {
    Serial.println("Failed to read from DHT sensor!");
    h = 1;
    tc = 1;
    return;
  }
  Serial.print("Humidity: ");
  Serial.print(h, 1);
  Serial.println(" %\t");
  Serial.print("Temperature: ");
  Serial.print(tc, 1);
  Serial.println(" *C ");
}

If you only want to use the MQ135 without the DHT22 then use the code below.

// Add the ESP8266 library
#include "ESP8266WiFi.h"

// replace with your channel’s thingspeak API key
String apiKey = "XXXXXXXXXXXXXXXXX";
const char* server = "api.thingspeak.com";

// Add the EEPROM library to store the lowest measured MQ-135 value
#include "EEPROM.h"
int address = 24;
byte value;

float val;
float vall;
int sensorPin = A0;
int sensorValue = 0;
float v;
float f = 0.;
float pp = 0.;
// number of analog MQ-135 samples taken each time
int s = 25;
int t;
int x;
int lowest = 500;
int lowest_l;
int lowest_h;
int tel = 0;
int telc = 0;
int id = 1;
int air;
int airt;
int airv;
// change one value below to reset lowest value back to 510
int a1 = 148;
int a2 = 231;
int a3 = 23;
// cl should be >= 3 before writing new low
// value to EEPROM
int cl = 0;

// 5 SSID's possible for when you need to measure
// at different places
const char* ssid1 = "your ssid 1";
const char* password1 = "your ssid 1 password";
const char* ssid2 = "your ssid 2";
const char* password2 = "your ssid 2 password";
const char* ssid3 = "your ssid 3";
const char* password3 = "your ssid 3 password";
const char* ssid4 = "your ssid 4";
const char* password4 = "your ssid 4 password";
const char* ssid5 = "your ssid 5";
const char* password5 = "your ssid 5 password";
const char* ssid = ssid1;
const char* password = password1;

WiFiClient client;

void setup() {
  Serial.begin(57600);
  EEPROM.begin(512);
  pinMode(sensorPin, INPUT);
  WiFi.begin(ssid, password);
  Serial.print("Trying ");
  Serial.print (ssid);
  Serial.print(" - ");
  while (WiFi.status() != WL_CONNECTED) {
    telc = telc + 1;
    if ( telc >= 10) {
      if ( id == 1 ) {
        ssid = ssid1;
        password = password1;
        id = 2;
      } else {
        if ( id == 2 ) {
          ssid = ssid2;
          password = password2;
          id = 3;
        } else {
          if ( id == 3 ) {
            ssid = ssid3;
            password = password3;
            id = 4;
          } else {
            if ( id == 4 ) {
              ssid = ssid4;
              password = password4;
              id = 5;
            } else {
              if ( id == 5 ) {
                ssid = ssid5;
                password = password5;
                id = 1;
              }
            }
          }
        }
      }
      telc = 0;
      Serial.println(".");
      Serial.print("Trying ");
      Serial.print (ssid);
      Serial.print(" - ");
      WiFi.begin(ssid, password);
    }
    delay(2000);
    Serial.print(".");
  }
  Serial.println("Connected");
  delay(100);
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());

  // check if this is new hardware to set EEPROM to 510
  if (EEPROM.read(505) == a1 || EEPROM.read(506) == a2 || EEPROM.read(507) == a3) {
    Serial.println ("EEPROM setup is correct, not changing current values");
  } else {
    // this should be a new start
    EEPROM.write(500, 255);
    EEPROM.write(501, 255);
    EEPROM.write(505, a1);
    EEPROM.write(506, a2);
    EEPROM.write(507, a3);
    delay(50);
    EEPROM.commit();
    Serial.println ("New start for MQ135, startup values set");
  }
  lowest_l = EEPROM.read(500);
  lowest_h = EEPROM.read(501);
  lowest = lowest_l + lowest_h;


  // sleep 10 minutes to warm up the MQ135
  // 10 minutes could be to low as warmup can take longer
  Serial.println("Sleep 10 minutes to warm up MQ-135");
  delay(600000);
}

void loop() {
  v = 0;
  t = 0;
  while (t < s) {
    // Read the anolog value s (25) times
    val = (analogRead(sensorPin)) * 1;
    v = v + val;
    t++;
    delay(10);
  }

  vall = v / s;
  Serial.print ("raw = ");
  Serial.println (vall);
  if (vall <= lowest - 1 ) { cl = cl + 1; } else { cl = 0; } // to avoid wrong low value, the value must be 3 times low if (cl >= 3) {
    cl = 0;
    lowest = vall;
    if (lowest >= 255) {
      lowest_l = 255;
      lowest_h = lowest - 255;
    } else {
      lowest_l = lowest;
      lowest_h = 0;
    }
    EEPROM.write(500, lowest_l);
    EEPROM.write(501, lowest_h);
    delay(50);
    EEPROM.commit();
    Serial.println ("New lowest value, saving to EEPROM");
  }

  vall = vall - lowest;
  if (vall <= 0 ) { vall = 0; } airt = airt + vall; airv = airt / (tel + 1); Serial.print ("low: "); Serial.println (lowest); Serial.print ("Bad Air quality : "); Serial.println (vall); Serial.print ("Bad Air quality average: "); Serial.print (airv); Serial.print ("  "); Serial.print (tel + 1); Serial.print ("  "); Serial.println (airt); delay(4982); tel = tel + 1; if (tel >= 57) {
    tel = 0;
    air = airt / 57;
    ZendData_thingspeak();
    airt = 0;
  }
}

void ZendData_thingspeak() {
  // I am using 2 fields at Thingspeak
  // 1 = Air Quality
  // 2 = lowest ever Air Quality
  if (client.connect(server, 80)) { // "184.106.153.149" or api.thingspeak.com
    String postStr = apiKey;
    postStr += "&field1=";
    postStr += String(air);
    postStr += "&field2=";
    postStr += String(lowest);
    postStr += "\r\n\r\n";

    client.print("POST /update HTTP/1.1\n");
    client.print("Host: api.thingspeak.com\n");
    client.print("Connection: close\n");
    client.print("X-THINGSPEAKAPIKEY: " + apiKey + "\n");
    client.print("Content-Type: application/x-www-form-urlencoded\n");
    client.print("Content-Length: ");
    client.print(postStr.length());
    client.print("\n\n");
    client.print(postStr);
    Serial.println("Data send to Thingspeak");
    client.stop();
  }
}
 

Download links.

Microsoft Explorer and Microsoft Edge seems to put this on one unusable line.
But Firefox and Google Chrome are both working.

MQ135_DHT22_ESP8266.ino
MQ135_ESP8266.ino

Measure CO2 with MQ-135 and Arduino

I had bought 3 MQ-135 gas sensors on AliExpress to test if it is possible to measure CO2 with them.

First I started with a very simple analog read to check the values in my computer/hobby room with a CO2 ppm around 650.

// select the input pin for the MQ-135 sensor
int sensorPin = A6; 
// variable to store the value coming from the sensor.
int val = 0; 

void setup() {
  Serial.begin(9600);
}
void loop() {
  // read the value from the sensor
  val = analogRead(sensorPin);
  Serial.println (val);
  delay(1000);
}

 

24 hours Burn-In

I connected the 5V power to the sensors and let them alone for 24 hours to burn in.

After these 24 hours I checked the values measured with the above little test program.
The values were 13 – 32 – 55
Breathing on them gave very little difference, as the values were only doubled to 28 – 61 – 103

With these values you can say that the first two are useless to measure CO2 as the difference is to little.
13 – 28 for CO2 ppm of about 500 – 2000 gives a resolution of about 100ppm/value measurement
32 – 61 gives about 52ppm/value
55 – 103 gives the best resolution of about 31ppm/value

These resolutions are valid if the scale is lineair, but the scale is logirithmic.
So in the lower ppm part the resolution is much better, but above the 1000ppm the resolution will be very low.

Wrong hardware

Found out that the resistor towards ground was just 1K ohm, after replacing the resister with one 22K ohm the results were getting much better.

In my room s the real ppm was about 770 ppm according to my NETATMO.

The raw value measured with the arduino was now 241.
Using MQ135-master from G.Krocker site and modifying MQ135.h with the correct RLOAD resistor value of 22K and a RZERO of 879.13

and using the below Arduino code

// The load resistance on the board
#define RLOAD 22.0
// Calibration resistance at atmospheric CO2 level
#define RZERO 879.13 
#include "MQ135.h" 
MQ135 gasSensor = MQ135(A6); 
int val; 
int sensorPin = A6; 
int sensorValue = 0; 
void setup() { 
  Serial.begin(9600);
  pinMode(sensorPin, INPUT); 
} 

void loop() { 
  val = analogRead(A6); 
  Serial.print ("raw = "); 
  Serial.println (val); 
  float zero = gasSensor.getRZero(); 
  Serial.print ("rzero: "); 
  Serial.println (zero); 
  float ppm = gasSensor.getPPM(); 
  Serial.print ("ppm: "); 
  Serial.println (ppm); 
  delay(5000); 
} 

The Arduino sends out the following output to the serial port.

raw = 241
rzero: 691.60
ppm: 777.87

With these values I am very close to the values of the NETATMO
I will of course also run the calibration outside where it should show about 400ppm, and eventually adjust the RZERO in MQ135.h

Next day

The next morning ppm was down to 500 according to the NETATMO, but the arduino showed a ppm of 600. I change the RZERO to 819 and the arduino also showed 500.

This is no good of course, so something must go wrong in the calculations in the MQ135 library. Or I am using a MQ-135 sensor with a bad response curve.

The next day I have tested the same with another MQ-135, but the results were about the same.

Wrong formula in the MQ135.h library

The formula to calculate the resistance of the sensor is wrong.
It should read:

float MQ135::getResistance() {
  int val = analogRead(_pin);
  return ((1023. / (float)val) - 1.) * RLOAD;
}

MQ135 not suitable for CO2

I think that I will stop my attempts for using the MQ-135 as a CO2 meter. With 1 sensor I measured different voltages on the analog port with same amount of CO2. It probably is to responsive to other gases in my surroundings. (Airport and highway).
One evening while my wife was one floor lower and took some perfume the ppm went sky high from 640ppm to 5570ppm, and then slowly (30 minutes) went down again.

I think that I will connect it to an ESP8266E to measure the outside air quality and sent the data over WiFi to thingspeak.com .