mh_z19_test.ino

//
// test program for esp-wroom-02 & mh-z19 
//
//   ESP-WROOM-02 development board
//       https://www.switch-science.com/catalog/2500/
//
//   MH-Z19
//       http://www.winsen-sensor.com/products/ndir-co2-sensor/mh-z19.html
//
// License:
//   Copyright (c) 2016 yoggy <[email protected]>
//   Released under the MIT license
//   http://opensource.org/licenses/mit-license.php;
//

#include <SoftwareSerial.h>  // https://github.com/plerup/espsoftwareserial

SoftwareSerial sws = SoftwareSerial(4, 5); // RX, TX
byte buf[9];

void setup() {
  Serial.begin(9600);
  sws.begin(9600);
}

void loop() {
  int co2 = read_data_from_mh_z19();

  if (co2 < 0) {
    Serial.println("read failed...");
  }
  else {
    Serial.print("co2 = ");
    Serial.print(co2);
    Serial.print(" ppm");
  }
  
  delay(1000); 
}

int read_data_from_mh_z19() {
  for (int i = 0; i < 10; ++i) {
    // send command
    sws.write((uint8_t)0xff);
    sws.write((uint8_t)0x01);
    sws.write((uint8_t)0x86);
    sws.write((uint8_t)0x00);
    sws.write((uint8_t)0x00);
    sws.write((uint8_t)0x00);
    sws.write((uint8_t)0x00);
    sws.write((uint8_t)0x00);
    sws.write((uint8_t)0x79);
    sws.flush();

    // read
    memset(buf, 0, 9);
    sws.readBytes(buf, 9);

    // parse
    if (buf[0] == 0xff && buf[1] == 0x86) {
      int co2 = buf[2] * 256 + buf[3];
      Serial.println(co2);
      return co2;
    }
    delay(100);
  }
  return -1;
}