co2mointor.ino

void setup() {
  Serial.begin(9600);
  analogReference(DEFAULT);
}

unsigned int co2mon(unsigned long delta) {
  static const unsigned int SensorIn = A0;
  static const unsigned int BaseVoltage = 480;
  static const unsigned int PollInterval = 10;

  static unsigned int pollTime = 0;
  static unsigned int concentrationSamples0[8] = {0};
  static unsigned int concentrationSamples1[8] = {0};
  static unsigned int pos0 = 0;
  static unsigned int pos1 = 0;
  static unsigned int concentrationCache = 0;

  if ((pollTime += (unsigned int)delta) < PollInterval) return concentrationCache;
  pollTime = 0;

  unsigned int sensorVal = analogRead(SensorIn);
  unsigned int voltage = sensorVal * (long)5000 / 1024;

  unsigned int concentrationNow = 0;
  if (voltage > BaseVoltage) {  // initialized
    voltage -= BaseVoltage;
    concentrationNow = (unsigned long)voltage * 50 / 16;
  }
  // Max 5000 ppm
  if (concentrationNow > 5000) concentrationNow = 5000;

  // Set sample0
  concentrationSamples0[++pos0] = concentrationNow;
  if (pos0 != 8) return concentrationCache;

  // Set sample1 by average of samples0
  unsigned int concentrationSum0 = 0;
  do {
    concentrationSum0 += concentrationSamples0[--pos0];
  } while (pos0);
  concentrationSamples1[pos1++ % 8] = concentrationSum0 / 8;
  
  // Moving window average of samples1
  unsigned int pos1New = pos1 - 8;
  unsigned int concentrationSum1 = 0;
  do {
    concentrationSum1 += concentrationSamples1[--pos1 % 8];
  } while (pos1 != pos1New);

  concentrationCache = concentrationSum1 / 8;
  return concentrationCache;
}

void loop() {
  static unsigned long timeOld = 0;
  unsigned long timeNow = millis();
  unsigned long delta = timeNow - timeOld;
  timeOld = timeNow;

  unsigned int concentration = co2mon(delta);

  if (Serial.available()) {
    char c = Serial.read();
    if (c == 'c') {
      Serial.println(concentration);
    }
  }
}