MHZ19.h

#include <SoftwareSerial.h>

class MHZ19
{
    SoftwareSerial* co2Serial;
  public:
    MHZ19(int rx, int tx) {
      co2Serial = new SoftwareSerial(rx, tx);
    }
    ~MHZ19() {
      stop();
      delete co2Serial;
    }
    void start() {
      co2Serial->begin(9600); //Init sensor MH-Z19(14)
    }
    void stop() {
      co2Serial->end();
    }
    int read()
    {

      byte cmd[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};
      // command to ask for data
      byte response[9]; // for answer

      co2Serial->write(cmd, 9); //request PPM CO2

      // The serial stream can get out of sync. The response starts with 0xff, try to resync.
      while (co2Serial->available() > 0 && (unsigned char)co2Serial->peek() != 0xFF)
      {
        co2Serial->read();
      }

      memset(response, 0, 9);
      co2Serial->readBytes(response, 9);

      if (response[1] != 0x86)
      {
        Serial.println("Invalid response from co2 sensor!");
        return -1;
      }

      byte crc = 0;
      for (int i = 1; i < 8; i++)
      {
        crc += response[i];
      }
      crc = 255 - crc + 1;

      if (response[8] == crc)
      {
        int responseHigh = (int)response[2];
        int responseLow = (int)response[3];
        int ppm = (256 * responseHigh) + responseLow;
        return ppm;
      }
      else
      {
        Serial.println("CRC error!");
        return -1;
      }
    }
};