Published November 1, 2019 © CC BY-NC-SA

Getting Started with the MH-Z14A CO2 Detector

This is a short tutorial for connecting and setting up the MH-Z14A carbon dioxide sensor in Arduino w/OTA updates and Blynk.

BeginnerFull instructions provided1 hour11,539

Getting Started with the MH-Z14A CO2 Detector

Things used in this project

Hardware components

SparkFun ESP8266 Thing - Dev Board

MH-Z14A CO Sensor

Software apps and online services

Blynk

Story

I'm going to show you how to hookup and use the MH-Z14A carbon dioxide gas sensor. I received this sensor by accident when I bought a different one so I figured I would see how well it works. The MH-Z14A according to the datasheet:

MH-Z14A NDIR Infrared gas module is a common type, small size sensor, using non-dispersive infrared (NDIR) principle to detect the existence of CO2 in the air, with good selectivity, non-oxygen dependent and long life. Built-in temperature compensation; and it has digital output, analog voltage output, and PWM output.

Here are the specs:

Input Power: 4.5V - 5.5V DC @ 150ma
Logic: 3.3v (5v compatible w/ analog out)
Preheat Time: 3 min
0-5000ppm +/-50ppm
Outputs: UART(which we will use here), PWM, and Analog 0.4-2v or 0-2.5v DC

Here is a diagram of the pinouts/pads on the sensor. It is also labeled with abbreviations on the front of the sensor.

Sensor Pins

For this project I decided to go with using UART for the sensor interface with arduino rather than PWM or analog out. I feel why not stay digital for the best accuracy right? You can use those others but I won't be covering that here. So with UART there are 2 commands we are going to use. One to get the CO2 concentration and the other to manually calibrate the sensor. Each command & response contains 9 bytes. Below is a list of the commands and syntax:

Serial commands

So basically to initiate a read, we will send 0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79 to the sensor. You want to setup your baud rate in your program to 9600 so it can talk correctly. For the physical hookups we will use four wires: power, ground, RX, & TX and those are pads 19, 18, 17, & 16 but many are redundant. Connect as follows to your board:

5v DC -----> Pad 17
Ground ---> Pad 16
TX ----------> Pad 18
RX ----------> Pad 19

I selected for my board a Sparkfun ESP8266 Thing Dev but you can use any ESP8266 with this sketch or you can use any other board (if its 3.3v logic) by taking out some code. I went with a ESP8266 because it has built in wifi for Blynk and can do OTA updates.

I setup the program so that it polls the sensor every 15 seconds and then uploads the data for the Blynk app. The whole code is listed below. A couple important things to note about the sketch. Look at the notes in the sketch to see the various parts and how to easily modify the code to suit your needs such as taking out the OTA update functionality. The reason I use Blynk is because the ESP8266 only has one hardware serial port. Soooooo... you can't read the values off the USB hookup because the sensor is using the hardware serial. You would need to use software serial if you wanted it to read to a computer via serial. I just avoid that whole thing and write my values to Blynk.

With regard to calibrating, the sensor self calibrates but you can initiate a manual calibration by sending the calibration command. The sensor must be outside in (400ppm) CO2 for at least 30 min to be safe. I find it isn't needed for most use.

Hopefully this helped someone out! Leave a comment if you have issues or questions. Thanks

Code

CO2 Monitor

#include <Arduino.h>
#include <ESP8266WiFi.h>  // for OTA, Blynk, & restart command
#include <BlynkSimpleEsp8266.h>  // for blynk
#include <ESP8266mDNS.h>  // for OTA
#include <WiFiUdp.h>  // for OTA
#include <ArduinoOTA.h>  // for OTA

char auth[] = "token";
char ssid[] = "IoT";
char pass[] = "password";

byte cmd[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};  // get gas command
byte cmdCal[9] = {0xFF, 0x01, 0x87, 0x00, 0x00, 0x00, 0x00, 0x00, 0x78};  // calibrate command
char response[9];  // holds the recieved data

int CO2ppm = 0;

unsigned long warmingTimer = 0;
unsigned long timer1 = 0;

void setup()
{
	Serial.begin(9600);  // Setup a serial connection with the sensor

	Blynk.begin(auth, ssid, pass);

	wifi_station_set_hostname("ESP-CO2 Monitor");  // network device hostname
	ArduinoOTA.setHostname("ESP-CO2 Monitor");  // for OTA
	ArduinoOTA.begin();  // for OTA

	warmingTimer = millis();  // initilize warmup timer
}

void loop()
{
	Blynk.run();
	ArduinoOTA.handle(); // for OTA

	while (millis() - warmingTimer < 310000)
	{
		Blynk.run();
		ArduinoOTA.handle();  // for OTA

		if (millis() - timer1 > 1000)
		{
			Blynk.virtualWrite(V2, (310000 - (millis() - warmingTimer)) / 1000);  // counts down till the sensor is ready
			timer1 = millis();
		}
	}

	if (millis() - timer1 > 15000)  // runs every 15 sec
	{
		getReadings();
		maint();
		timer1 = millis();
	}

} // loop

void getReadings()
{
	while (Serial.available())  // this clears out any garbage in the RX buffer
	{
		int garbage = Serial.read();
	}

	Serial.write(cmd, 9);  // Sent out read command to the sensor
	Serial.flush();  // this pauses the sketch and waits for the TX buffer to send all its data to the sensor

	while (!Serial.available())  // this pauses the sketch and waiting for the sensor responce
	{
		delay(0);
	}

	Serial.readBytes(response, 9);  // once data is avilable, it reads it to a variable
	int responseHigh = (int)response[2];
	int responseLow = (int)response[3];
	CO2ppm = (256 * responseHigh) + responseLow;

	Blynk.virtualWrite(V0, CO2ppm);
}

void calibrate()
{
	Serial.write(cmdCal, 9);
	delay(3000);
}

void maint()
{
	int rssi = wifi_station_get_rssi();
	Blynk.virtualWrite(V12, rssi);
}

BLYNK_WRITE(V10)
{
	byte cali = param.asInt();

	if (cali)
	{
		calibrate();
	}
}

BLYNK_WRITE(V11) // restart chip
{
	byte reset = param.asInt();

	if (reset)
	{
		ESP.restart();
	}
}