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--- arduino:mh-z19b [2020/03/02 21:35] – created aistis
+++ arduino:mh-z19b [2020/03/02 21:36] (current) – aistis
@@ Line 4: / Line 4: @@
 carbon dioxide
+https://forum.arduino.cc/index.php?topic=525459.msg3587557#msg3587557
 {{:arduino:pasted:20200302-213502.png}}
+{{:arduino:pasted:20200302-213536.png}}
+<code cpp>
+#include <SoftwareSerial.h>
+SoftwareSerial co2Serial(7, 6); // define MH-Z19 RX TX
+unsigned long startTime = millis();
+void setup() {
+  Serial.begin(115200);
+  co2Serial.begin(9600);
+  pinMode(9, INPUT);
+}
+void loop() {
+  int ppm_uart = readCO2UART();
+  //int ppm_pwm = readCO2PWM();
+  delay(1000);
+}
+int readCO2UART(){
+  byte cmd[9] = {0xFF,0x01,0x86,0x00,0x00,0x00,0x00,0x00,0x79};
+  byte response[9]; // for answer
+  co2Serial.write(cmd, 9); //request PPM CO2
+  // clear the buffer
+  memset(response, 0, 9);
+  int i = 0;
+  while (co2Serial.available() == 0) {
+//    Serial.print("Waiting for response ");
+//    Serial.print(i);
+//    Serial.println(" s");
+    delay(100);
+    i++;
+  }
+  if (co2Serial.available() > 0) {
+      co2Serial.readBytes(response, 9);
+  }
+  // print out the response in hexa
+  for (int i = 0; i < 9; i++) {
+    //Serial.print(String(response[i], HEX));
+    //Serial.print("   ");
+  }
+  //Serial.println("");
+  // checksum
+  byte check = getCheckSum(response);
+  if (response[8] != check) {
+    Serial.println("Checksum not OK!");
+    Serial.print("Received: ");
+    Serial.println(response[8]);
+    Serial.print("Should be: ");
+    Serial.println(check);
+  }
+  // ppm
+  int ppm_uart = 256 * (int)response[2] + response[3];
+  Serial.print(ppm_uart);
+  // temp
+  byte temp = response[4] - 40;
+  Serial.print(",");
+  Serial.println(temp);
+  // status
+  byte status = response[5];
+  //Serial.println(status);
+  if (status == 0x40) {
+    Serial.println("Status OK");
+  }
+  return ppm_uart;
+}
+byte getCheckSum(char *packet) {
+  byte i;
+  unsigned char checksum = 0;
+  for (i = 1; i < 8; i++) {
+    checksum += packet[i];
+  }
+  checksum = 0xff - checksum;
+  checksum += 1;
+  return checksum;
+}
+int readCO2PWM() {
+  unsigned long th, tl, ppm_pwm = 0;
+  do {
+    th = pulseIn(9, HIGH, 1004000) / 1000;
+    tl = 1004 - th;
+    ppm_pwm = 5000 * (th-2)/(th+tl-4);
+  } while (th == 0);
+  Serial.print("PPM PWM: ");
+  Serial.println(ppm_pwm);
+  return ppm_pwm;
+}
+</code>