#define pwmPin A0
#define LedPin 13
 
int prevVal = LOW;
long th, tl, h, l, ppm;
 
void setup() {
  Serial.begin(9600);
  pinMode(pwmPin, INPUT);
  pinMode(LedPin, OUTPUT);
}
 
void loop() {
  long tt = millis();
  int myVal = digitalRead(pwmPin);
 
  if (myVal == HIGH) {
    digitalWrite(LedPin, HIGH);
    if (myVal != prevVal) {
      h = tt;
      tl = h - l;
      prevVal = myVal;
    }
  }  else {
    digitalWrite(LedPin, LOW);
    if (myVal != prevVal) {
      l = tt;
      th = l - h;
      prevVal = myVal;
      ppm = 5000 * (th - 2) / (th + tl - 4);
      Serial.println("PPM = " + String(ppm));
    }
  }
}

/*
  MH-Z19
  Intelligent Infrared CO2 Module
 
  MsTimer2 is a small and very easy to use library to interface Timer2 with
  humans. It's called MsTimer2 because it "hardcodes" a resolution of 1
  millisecond on timer2
  For Details see: http://www.arduino.cc/playground/Main/MsTimer2
 
  2017/08/20
  NextStep/Kaz Ueno
*/
 
#include <MsTimer2.h>
 
/* Switch on LED on and off each half second */
 
#define     LedPin 13                               /* LED to pin 13 */
#define     pwmPin A0                               /* CO2 data input to pin A0 */
 
boolean     prevVal = LOW;
 
uint32_t    CO2_data = 0L;
uint8_t     data_count = 0L;
 
uint32_t    th, tl, h, l, ppm, ppm_now;
 
uint32_t    ppm_befor = 0L;
uint32_t    tt = 0L;
 
 
 
void flash()
{
  uint8_t myVal = digitalRead(pwmPin);
 
  tt++ ;
  if (myVal == HIGH) {
    digitalWrite(LedPin, HIGH);
    if (myVal != prevVal) {
      tl = tt ; 
      prevVal = myVal;
      tt =0L;
    }
  }  else {
    digitalWrite(LedPin, LOW);
    if (myVal != prevVal) {
      th = tt; 
      prevVal = myVal;
      ppm = 5000 * (th - 2) / (th + tl - 4);
      ppm_now = ppm_befor * 0.8 + ppm * 0.2 ;
      ppm_befor = ppm_now;
      data_count++ ;
      CO2_data += ppm_now;
      tt = 0L;
    }
  }
}
 
void setup()
{
  Serial.begin(9600);
  pinMode(pwmPin, INPUT);
  pinMode(LedPin, OUTPUT);
 
  MsTimer2::set(1, flash);                            // 1ms毎に割り込みを発生させる
  MsTimer2::start();
  delay(5000);
}
 
void loop()
{
  uint32_t    CO2_data_ave;
  CO2_data_ave = CO2_data / data_count;
  Serial.println("PPM = " + String(CO2_data_ave) +"," + String(ppm_now) +"," + String(ppm));
  CO2_data = 0L;
  data_count = 0L;
 
  delay(5000);
}