Cannot get MH-Z19B CO2 Sensor to Work

cbwatts · August 23, 2020, 7:16pm

Hi there everyone, I'm new this this forum and fairly new to Arduino.

I'm having and issue getting my MH-Z19B CO2 sensor to read in my serial monitor. I've tried looking at

many different examples of code that others have posted in their success stories. When I open my serial

monitor, all I see is that the sensor is preheating. Nothing after that. The sensor needs about 3 minutes

to heat up, and I've waiting for 20 minutes to no avail.

Serial Monitor looks like this:

MHZ 19B
Preheating...

But I don't get anything after that. I just keep seeing more ............... the longer I wait.

I hope that I am not doomed to fail! Please if you can help, with gentle language for simple folk

The MH-Z19B has the following connected to the Arduino Nano:

MH-Z19B Arduino Nano

Vin <=======================> 5V

GND <=======================> GND

PWM <=======================> D10

RX <=======================> TX1

TX <=======================> RX0

Below is the code that I am using.

#include <SoftwareSerial.h>
#include <MHZ.h>

// pin for pwm reading
#define CO2_IN 10

// pin for uart reading
#define MH_Z19_RX 1
#define MH_Z19_TX 0

MHZ co2(MH_Z19_RX, MH_Z19_TX, CO2_IN, MHZ19B);

void setup() {
Serial.begin(9600);
pinMode(CO2_IN, INPUT);
delay(100);
Serial.println("MHZ 19B");

// enable debug to get addition information
//co2.setDebug(true);

if (co2.isPreHeating()) {
Serial.print("Preheating");
while (co2.isPreHeating()) {
Serial.print(".");
delay(5000);
}
Serial.println();
}
}

void loop() {
Serial.print("\n----- Time from start: ");
Serial.print(millis() / 1000);
Serial.println(" s");

int ppm_uart = co2.readCO2UART();
Serial.print("PPMuart: ");

if (ppm_uart > 0) {
Serial.print(ppm_uart);
} else {
Serial.print("n/a");
}

int ppm_pwm = co2.readCO2PWM();
Serial.print(", PPMpwm: ");
Serial.print(ppm_pwm);

int temperature = co2.getLastTemperature();
Serial.print(", Temperature: ");

if (temperature > 0) {
Serial.println(temperature);
} else {
Serial.println("n/a");
}

Serial.println("\n------------------------------");
delay(5000);
}

MH-Z19B.txt (1.2 KB)

wildbill · August 23, 2020, 7:19pm

You're using the hardware serial pins for debugging messages and talking to your sensor.

Put the sensor on some other pins, like in the examples for the library.

cbwatts · August 23, 2020, 7:22pm

Thanks for the quick response!

If I wired the RX/TX from the MH-Z19B to say the D6/D7 pins on the Arduino Nano...is that what you mean?

wildbill · August 23, 2020, 7:25pm

Yes. You will need to make corresponding changes in your code too - here:

// pin for uart reading
#define MH_Z19_RX 1
#define MH_Z19_TX 0

cbwatts · August 23, 2020, 7:37pm

Just reconnected the RX to D7 and the TX to D6 but I still have the same result

I also updated the code there.

// pin for uart reading
#define MH_Z19_RX 7 // D7
#define MH_Z19_TX 6 // D6

wildbill · August 23, 2020, 7:47pm

Post the latest version of your code, in code tags please.

Also, it might be worth uncommenting the stuff that turns debugging on.

cbwatts · August 23, 2020, 7:52pm

#include <SoftwareSerial.h>
#include <MHZ.h>

// pin for pwm reading
#define CO2_IN 10

// pin for uart reading
#define MH_Z19_RX 7  // D7
#define MH_Z19_TX 6  // D6


MHZ co2(MH_Z19_RX, MH_Z19_TX, CO2_IN, MHZ19B);

void setup() {
  Serial.begin(9600);
  pinMode(CO2_IN, INPUT);
  delay(100);
  Serial.println("MHZ 19B");

  // enable debug to get addition information
  
  co2.setDebug(true);

  if (co2.isPreHeating()) {
    Serial.print("Preheating");
    while (co2.isPreHeating()) {
      Serial.print(".");
      delay(5000);
    }
    Serial.println();
  }
}

void loop() {
  Serial.print("\n----- Time from start: ");
  Serial.print(millis() / 1000);
  Serial.println(" s");

  int ppm_uart = co2.readCO2UART();
  Serial.print("PPMuart: ");

  if (ppm_uart > 0) {
    Serial.print(ppm_uart);
  } else {
    Serial.print("n/a");
  }

  int ppm_pwm = co2.readCO2PWM();
  Serial.print(", PPMpwm: ");
  Serial.print(ppm_pwm);

  int temperature = co2.getLastTemperature();
  Serial.print(", Temperature: ");

  if (temperature > 0) {
    Serial.println(temperature);
  } else {
    Serial.println("n/a");
  }

  Serial.println("\n------------------------------");
  delay(5000);
}

I still only see this in the Serial Monitor...

MHZ 19B
MHZ: debug mode ENABLED
Preheating...........................

wildbill · August 23, 2020, 8:01pm

If that is literally what you saw, you didn't wait long enough. Also, I don't know anything about that particular sensor, but many with a burn in period need far longer than that.

cbwatts · August 23, 2020, 8:04pm

I've been letting it run for 15 minutes at this point...and yes above is all I see on the serial monitor except now it's:

MHZ 19B
MHZ: debug mode ENABLED
Preheating...

wildbill · August 23, 2020, 8:18pm

Where did you get the library?

The version I'm looking at just waits three minutes based on millis. I had assumed it was checking something on the sensor, but no.

cbwatts · August 23, 2020, 8:23pm

I downloaded the latest one, version 1.2.0

MH_Z_CO2_Sensors-1.2.0.zip (6.47 KB)

wildbill · August 23, 2020, 8:55pm

Looks like the same one. I can't see why it never ends preheat.

I would try commenting out all the code that checks for preheating and replace it with three minutes of delay.

cbwatts · August 23, 2020, 9:49pm

So I took out the preheating part...and added in a 3 minute delay. It definitely does something...although only PWM seems to work and if it is indeed working I should be dying right now...this is what I see:

----- Time from start: 310 s
PPMuart: n/a-- reading CO2 from pwm ...

PPM PWM: 2175

, PPMpwm: 2175, Temperature: n/a

----- Time from start: 317 s
PPMuart: n/a-- reading CO2 from pwm ...

PPM PWM: 2145

, PPMpwm: 2145, Temperature: n/a

Here is the updated code:

#include <SoftwareSerial.h>
#include <MHZ.h>

// pin for pwm reading
#define CO2_IN 10   //D10

// pin for uart reading
#define MH_Z19_RX 7  // D7
#define MH_Z19_TX 6  // D6


MHZ co2(MH_Z19_RX, MH_Z19_TX, CO2_IN, MHZ19B);

void setup() {
  Serial.begin(9600);
  pinMode(CO2_IN, INPUT);
  delay(100);
  Serial.println("MHZ 19B");
  

  // enable debug to get addition information
  
   co2.setDebug(true);

 delay(180000); // 3 minute delay for warmup
}

void loop() {
  Serial.print("\n----- Time from start: ");
  Serial.print(millis() / 1000);
  Serial.println(" s");

  int ppm_uart = co2.readCO2UART();
  Serial.print("PPMuart: ");

  if (ppm_uart > 0) {
    Serial.print(ppm_uart);
  } else {
    Serial.print("n/a");
  }

  int ppm_pwm = co2.readCO2PWM();
  Serial.print(", PPMpwm: ");
  Serial.print(ppm_pwm);

  int temperature = co2.getLastTemperature();
  Serial.print(", Temperature: ");

  if (temperature > 0) {
    Serial.println(temperature);
  } else {
    Serial.println("n/a");
  }

  Serial.println("\n------------------------------");
  delay(5000);
}

Any thoughts to why I can't see the uart readings?

wildbill · August 23, 2020, 10:38pm

Don't know. Bad sensor? Rx and Tx reversed? Other wiring issues? Tricky thing to debug. It's time for dinner here, I'll take another look at the library tomorrow.

wildbill · August 24, 2020, 12:41pm

Print what you get from the readCO2UART call. It can return several different negative status numbers to indicate why it didn't get a reading.

Also, I'm still baffled as to why the isPreHeating function never returned false. Can you post the copy from your library?

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Cannot get MH-Z19B CO2 Sensor to Work

PPM PWM: 2175

PPM PWM: 2145

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