MQ7-esp/MQ7-ESP.ino

/* sensor MQ7.  alternate heater 5V/1.4V  60sec/90sec
  // read sensor @ 88sec(of 90-period)
  // Kn.Ny '16
  // This program to be used as CO-alarm in tents wintertime during night. (gasoline or firewood burners)
  // http://www.savvysolutions.info/savvymicrocontrollersolutions/arduino.php?topic=arduino-mq7-CO-gas-sensor
  // http://www.savvysolutions.info/savvymicrocontrollersolutions/index.php?sensor=mq-7-gas-sensors
  // ****************************************************************************************************/

#define serial
#define warnlevel 50    //CO ppm warnlevel
#define continousMeasurement
#define HeatOnPin D0
#define Buzzer D5
#define ADCOffset 3
#define lotime 90000UL  // 90 sek low heating
#define hitime 60000UL  // 60 sek high heating
#define read_time 148000UL // read 2 sek ahead of end low period
#define thresholdMeasurement 10 //ToDo


/*
   struct for rtc memory
*/
struct {
  bool burnIn = false;    //set to false in productive
  unsigned int minADC = 65535;  //save minimal adc value
  byte ringPointer = 0;
  unsigned int adcRingBuf[thresholdMeasurement] = { 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 }; //ring buffer for ten values
} rtcData;


// Adjust vRef to be the true supply voltage in mV.
const float vRef = 4470.0; //mVolt
const float RL = 10.0;    //kOhm
const float CleanAirRatio = 26.09;
float Rs = 0.0;
float R0 = 0.0;
float ambTemp = 20.0;
float ambHum = 65.0;
unsigned int measurement = 0;


uint32_t calculateCRC32( const uint8_t *data, size_t length ) {
  uint32_t crc = 0xffffffff;
  while ( length-- ) {
    uint8_t c = *data++;
    for ( uint32_t i = 0x80; i > 0; i >>= 1 ) {
      bool bit = crc & 0x80000000;
      if ( c & i ) {
        bit = !bit;
      }

      crc <<= 1;
      if ( bit ) {
        crc ^= 0x04c11db7;
      }
    }
  }

  return crc;
}

void updateRTC() {
  //ToDo save rtcData
  //crc32 stuff
}

void getRTC() {
  //crc32 stuff
  //ToDo get rtcData
}

void saveMeasurement (unsigned int measurement) {
  rtcData.adcRingBuf[rtcData.ringPointer] = measurement;
  rtcData.ringPointer = (rtcData.ringPointer + 1) % thresholdMeasurement;
}

float CalcRsFromVRL(float VRL) {
  //  VRL = RL voltage in mV.
  //  vRef = supply voltage, 5000 mV
  //  RL = load resistor in k ohms
  //  The equation Rs = (Vc - Vo)*(RL/Vo)
  //  is derived from the voltage divider
  //  principle:  Vo = RL * Vc (Rs + RL)
  //
  //  Note.  Alternatively you could calc
  //         Rs from ADC value using
  //         Rs = RL * (1024 - ADC) / ADC
  if ( VRL == 0.0 )
  {
    return vRef * VRL;
  }
  return ((vRef - VRL) * (RL / VRL));
}

float CalcRsFromADC(float ADC) {
  if ( ADC == 0 )
  {
    return 10240;
  }
  return (RL * (1024 - ADC) / ADC);
}

unsigned int GetCOPpmForRatioRsRo(float RsRo_ratio) {
  /*
     x=Rs/R0; y=ppm
     1,6122158133673148; 50,488950942143966
     0,999928961393748; 100
     0,39070475612941435; 396,4363929567069
     0,22807477512659288; 1000
     0,09483831585572762; 3964,363929567065
     y = 100.607 * (Rs/R0)^-1.54901
     this regression needs to be shifted by the CleanAirRatio.
     R0 = Rs/CleanAirRatio => Rs/R0 => Rs/(Rs_old/CleanAirRatio)
  */
  float ppm = 0;
  ppm = 100.607 * pow( (RsRo_ratio * CleanAirRatio), -1.54901);
  return (unsigned int) ppm;
}

int measurementAnalog() {
  //measure x values and get the mean value
  static unsigned int x = 10;
  unsigned int valueADC = 0;
  for (unsigned int i = 0; i < x; i++) {
    valueADC += analogRead(A0);
    delay(10);
  }
  Serial.println("DEBUG: measurement ADC " + String(valueADC / x));
  return (valueADC / x);
}


void burnIn(bool trigger) {
  /*
     if not flag in eeprom set
     then burn in sensor
  */
  unsigned int measureRow[thresholdMeasurement] = { 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 };
  int counter = 0;
  //set timer with the current milli counter
  unsigned long starttime = millis();
  unsigned long currenttime = millis();
  unsigned long static burnInTime = 172800000; //48 hours * 60 minutes * 60 seconds * 1000 milli
  Serial.println("DEBUG: burnIn");
  if ( !rtcData.burnIn ) {
    digitalWrite(HeatOnPin, HIGH); // => 5V
    while ( currenttime < burnInTime ) {
      delay(1000);
      currenttime = abs(millis() - starttime);
      Serial.print("\r" + String((int)((currenttime / 1000) / 360)) + ":" + String((int)((currenttime / 1000) / 60)) + ":" + String((int)((currenttime / 1000) % 60)) + "..");
      if (trigger) {
        unsigned int tmp = CalcRsFromADC(measurementAnalog());
        measureRow[counter] = tmp;
        counter = ( counter + 1 ) % thresholdMeasurement;
        Serial.println("BurnIn: Current RS from ADC " + String(tmp));
        //ToDo a value which means that the sensor is clean enough.
        //However the resistance drop in first sigth and stabilize then at a specific value, mine approx. 37k Ohm.
        /*if (tmp >= 36.0) {
          Serial.println("BurnIn: Trigger");
          break;
          }*/
        int sum = 0;
        for (int i = 0; i < thresholdMeasurement - 1; i++) {
          sum = sum + abs(measureRow[i] - measureRow[i + 1]);
        }
        Serial.println("DEBUG: Currentsum value " + String(sum));
        if (sum <= 0) {
          Serial.println("BurnIn: Stabilization");
          break;
        }
      }
    }
    //set initial lowest value
    rtcData.minADC = measurementAnalog();
    rtcData.burnIn = true;
    Serial.println("DEBUG BurnIn:\t" + String(currenttime / 1000) + " seconds| rtc.minADC" + String(rtcData.minADC) + " || RS " + String(CalcRsFromADC(rtcData.minADC)));
  }
}


/*
   a method to react to changing environmental parameters.
   ToDo: The values are NOT usuable!

  void compensateDriftOld() {
  if (Rs <= 20.0 && Rs > R0) {
    R0 = Rs;
    Serial.println("DEBUG: Drift compensate done. New R0:\t" + String(R0));
    return;
  }
  if ( Rs > 20.0) {
    Serial.println("DEBUG: Sensor needs cleaning!");
    burnIn(false);
    return;
  }
  }
*/

/*
   a method to react to changing environmental parameters.
   take the ADC values and calculate the summed up difference.
   renew the R0 if stabilizied ADc values in a row and the old R0 value needs to be smaller
*/
void compensateDrift() {
  int sum = 0;

  for (int i = 0; i < thresholdMeasurement - 1 ; i++) {
    sum = sum + (abs(rtcData.adcRingBuf[i] - rtcData.adcRingBuf[i + 1]));
  }

  if (R0 > Rs && sum <= 1) {
    R0 = Rs;
    Serial.println("DEBUG: Drift compensate done. New R0:\t" + String(R0));
    return;
  }
  //check for lowest resistence after heating
  if ( rtcData.adcRingBuf[rtcData.ringPointer] > rtcData.minADC) {
    Serial.println("DEBUG: Sensor needs cleaning!");
    burnIn(false);
    return;
  }
}

int measurementCycle() {
  //set timer with the current milli counter
  unsigned long starttime = millis();
  unsigned long currenttime = 0;
  static unsigned long next_time = hitime + lotime;  //set complete runtime for one measurement
  unsigned int currentMeasurement, lastMeasurement = 0;
  boolean done_reading = true; //only a simple toggle switch

  //start cleaning sensitive part of sensor
  Serial.println("-------------------------------------------------------------");
  digitalWrite(HeatOnPin, HIGH); // => 5V
  Serial.print("5V:\t on");

  while (currenttime < next_time) {
    //refresh current time
    currenttime = abs(millis() - starttime);
#ifndef continousMeasurement
    lastMeasurement = currentMeasurement;
    currentMeasurement = measurementAnalog();
    if (thresholdMeasurement > abs(lastMeasurement - currentMeasurement) ) {
      delay(10);
    }

    if (done_reading && currenttime >= hitime) //sensor reach heating time limit
    {
      digitalWrite(HeatOnPin, LOW); //  => 1.4V
      Serial.print("\n\r1.4V:\t on\n");
      done_reading = false;
    }
    if (currenttime >= next_time ) {
      break;
    }
#else
    if (done_reading && currenttime >= hitime) //sensor reach heating time limit
    {
      digitalWrite(HeatOnPin, LOW); //  => 1.4V
      Serial.print("\n\r1.4V:\t on\n");
      done_reading = false;
    }
    if (currenttime > read_time) // then do a reading now
    {
      currentMeasurement = measurementAnalog();
      done_reading = true;
      break;
      //add nynquist thereom and sums up multiple measurements
    }
    delay(50);
#endif
  }
  return currentMeasurement;
}

void calibrateCleanAir() {
  Serial.println("DEBUG: calibrate Clean AIR");
  //wait until sensor reach low end resistance
  unsigned int i = 0;
  while ( true ) {
    i++;
    Rs = CalcRsFromADC(measurementCycle());
    Serial.println("Calibrate Clean Air:\t#" + String(i) + "\tRs: \t" + String(Rs));
    /*
       At least three measurements to settle down the sensor
       ToDo : the datasheet mentions that the Rs is between 2k and 20k for 100ppm in air. Therefore the values multiplies with the clean_air_ratio of 26.09
       That leads to a range: 5,2MOhm up to 52,2MOhm which is not in any reach of the real world (17.06.18: current value is ADC 2 up to 3, which is 3403 or 5110 Ohm.
       the resistance get lower with higher CO measurements.
       I guess the maximum value of 20k for 100ppm in air (a cold sensor has Rs = 330k)
    */
    //if (i >= 3 && Rs <= 20.0) {
    if (i >= 3 && Rs >= 3000.0) {
      //Serial.println ("Rs <= 20.0");
      Serial.println ("Rs >= 3000.0");
      break;
    }
  }
  R0 = Rs;
  Serial.println("R0: \t" + String(R0));
}

void ambientMeasurement() {

}

float correlateAmbient() {
  return 1.0;
}


// the setup routine runs once when you press reset:
void setup() {
  // initialize serial communication at 115200 bits per second:
  Serial.begin(115200);
  pinMode(HeatOnPin, OUTPUT);
  burnIn(true);
  calibrateCleanAir();
}


void loop() {
  float ppm = 0;
  //save adc data
  saveMeasurement(measurementCycle());
  Rs = CalcRsFromADC(rtcData.adcRingBuf[rtcData.ringPointer]);
  Serial.println("Rs: \t" + String(Rs) + " | R0:\t" + String(R0) + " || Ratio:\t" + String(Rs / R0) + "| Ratio:\t" + String(Rs / R0 * CleanAirRatio) );
  ppm = GetCOPpmForRatioRsRo(Rs / R0);
  Serial.println("CO ppm: \t" + String(ppm));
  compensateDrift();
}