Documentation

// MH-Z19C CO2 Sensor - Arduino Example (raw UART protocol)
// Sensor Tx -> D2, Sensor Rx -> D3 (via divider), Vin -> 5V, GND -> GND

#include <SoftwareSerial.h>

SoftwareSerial co2Serial(2, 3);  // RX = D2 (from sensor Tx), TX = D3

// Command: read CO2 concentration
const byte readCmd[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};

void setup() {
  Serial.begin(9600);
  co2Serial.begin(9600);   // MH-Z19C fixed baud rate
  Serial.println("Warming up (about 60 s after power-on)...");
}

void loop() {
  byte response[9];

  co2Serial.write(readCmd, 9);
  co2Serial.setTimeout(500);

  if (co2Serial.readBytes(response, 9) == 9 &&
      response[0] == 0xFF && response[1] == 0x86) {

    // Verify the checksum before trusting the data
    byte checksum = 0;
    for (int i = 1; i < 8; i++) checksum += response[i];
    checksum = 0xFF - checksum + 1;

    if (checksum == response[8]) {
      int ppm = response[2] * 256 + response[3];
      Serial.print("CO2: ");
      Serial.print(ppm);
      Serial.println(" ppm");
    } else {
      Serial.println("Checksum error - reading discarded");
    }
  } else {
    Serial.println("No response - check wiring and 5V supply");
  }

  delay(5000);  // The sensor updates slowly; 5 s polling is plenty
}

// MH-Z19C CO2 Sensor - ESP32 Example (raw UART protocol)
// Sensor Tx -> GPIO 16 (RX2), Sensor Rx -> GPIO 17 (TX2), Vin -> VIN (5V)

HardwareSerial co2Serial(2);  // UART2

const uint8_t readCmd[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};

void setup() {
  Serial.begin(115200);
  co2Serial.begin(9600, SERIAL_8N1, 16, 17);  // baud, config, RX, TX
  Serial.println("Warming up (about 60 s after power-on)...");
}

void loop() {
  uint8_t response[9];

  co2Serial.flush();
  while (co2Serial.available()) co2Serial.read();  // clear stale bytes

  co2Serial.write(readCmd, 9);
  co2Serial.setTimeout(500);

  if (co2Serial.readBytes(response, 9) == 9 &&
      response[0] == 0xFF && response[1] == 0x86) {

    uint8_t checksum = 0;
    for (int i = 1; i < 8; i++) checksum += response[i];
    checksum = 0xFF - checksum + 1;

    if (checksum == response[8]) {
      int ppm = response[2] * 256 + response[3];
      Serial.printf("CO2: %d ppm\n", ppm);
    }
  } else {
    Serial.println("No response - check wiring and 5V supply");
  }

  delay(5000);
}

#!/usr/bin/env python3
# MH-Z19C CO2 Sensor - Raspberry Pi Example (raw UART protocol)
# Sensor Tx -> GPIO 15 (pin 10), Sensor Rx -> GPIO 14 (pin 8), Vin -> 5V
# Setup: sudo raspi-config (disable serial console, enable serial port)
#        pip3 install pyserial

import time
import serial

READ_CMD = bytes([0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79])

ser = serial.Serial('/dev/serial0', baudrate=9600, timeout=1)

print('Warming up (about 60 s after power-on)...')

try:
    while True:
        ser.reset_input_buffer()
        ser.write(READ_CMD)
        response = ser.read(9)

        if len(response) == 9 and response[0] == 0xFF and response[1] == 0x86:
            checksum = (0xFF - (sum(response[1:8]) & 0xFF) + 1) & 0xFF
            if checksum == response[8]:
                ppm = response[2] * 256 + response[3]
                print('CO2: {} ppm'.format(ppm))
            else:
                print('Checksum error - reading discarded')
        else:
            print('No response - check wiring and 5V supply')

        time.sleep(5)

except KeyboardInterrupt:
    print('Stopped by user')
finally:
    ser.close()

# MH-Z19C CO2 Sensor - Pico MicroPython Example (raw UART protocol)
# Sensor Tx -> GP1 (UART0 RX), Sensor Rx -> GP0 (UART0 TX), Vin -> VBUS

from machine import UART, Pin
import time

uart = UART(0, baudrate=9600, tx=Pin(0), rx=Pin(1), timeout=500)

READ_CMD = bytes([0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79])

print("Warming up (about 60 s after power-on)...")

while True:
    # Clear any stale bytes, then request a reading
    while uart.any():
        uart.read()
    uart.write(READ_CMD)
    time.sleep_ms(200)

    response = uart.read(9)

    if response and len(response) == 9 and \
       response[0] == 0xFF and response[1] == 0x86:
        checksum = (0xFF - (sum(response[1:8]) & 0xFF) + 1) & 0xFF
        if checksum == response[8]:
            ppm = response[2] * 256 + response[3]
            print("CO2:", ppm, "ppm")
        else:
            print("Checksum error - reading discarded")
    else:
        print("No response - check wiring and 5V supply")

    time.sleep(5)