/***************************************************

 DFRobot Gravity: Analog TDS Sensor / Meter For Arduino

 <https://www.dfrobot.com/wiki/index.php/Gravity:_Analog_TDS_Sensor_/_Meter_For_Arduino_SKU:_SEN0244>

 Created 2017-8-22

 By Jason <jason.ling@dfrobot.com@dfrobot.com>

 GNU Lesser General Public License.

 See <http://www.gnu.org/licenses/> for details.

 All above must be included in any redistribution

 /***********Notice and Trouble shooting***************

 1. This code is tested on Arduino Uno and Leonardo with Arduino IDE 1.0.5 r2 and 1.8.2.

 2. More details, please click this link: <https://www.dfrobot.com/wiki/index.php/Gravity:_Analog_TDS_Sensor_/_Meter_For_Arduino_SKU:_SEN0244>

 ****************************************************/

#define TdsSensorPin A1

#define VREF 5.0      // analog reference voltage(Volt) of the ADC

#define SCOUNT  30           // sum of sample point

int analogBuffer[SCOUNT];    // store the analog value in the array, read from ADC

int analogBufferTemp[SCOUNT];

int analogBufferIndex = 0,copyIndex = 0;

float averageVoltage = 0,tdsValue = 0,temperature = 25;

void setup()

{

    Serial.begin(9600);

    pinMode(TdsSensorPin,INPUT);

}

void loop()

{

   static unsigned long analogSampleTimepoint = millis();

   if(millis()-analogSampleTimepoint > 40U)     //every 40 milliseconds,read the analog value from the ADC

   {

     analogSampleTimepoint = millis();

     analogBuffer[analogBufferIndex] = analogRead(TdsSensorPin);    //read the analog value and store into the buffer

     analogBufferIndex++;

     if(analogBufferIndex == SCOUNT) 

         analogBufferIndex = 0;

   }   

   static unsigned long printTimepoint = millis();

   if(millis()-printTimepoint > 800U)

   {

      printTimepoint = millis();

      for(copyIndex=0;copyIndex<SCOUNT;copyIndex++)

        analogBufferTemp[copyIndex]= analogBuffer[copyIndex];

      averageVoltage = getMedianNum(analogBufferTemp,SCOUNT) * (float)VREF / 1024.0; // read the analog value more stable by the median filtering algorithm, and convert to voltage value

      float compensationCoefficient=1.0+0.02*(temperature-25.0);    //temperature compensation formula: fFinalResult(25^C) = fFinalResult(current)/(1.0+0.02*(fTP-25.0));

      float compensationVolatge=averageVoltage/compensationCoefficient;  //temperature compensation

      tdsValue=(133.42*compensationVolatge*compensationVolatge*compensationVolatge - 255.86*compensationVolatge*compensationVolatge + 857.39*compensationVolatge)*0.5; //convert voltage value to tds value

      //Serial.print("voltage:");

      //Serial.print(averageVoltage,2);

      //Serial.print("V   ");

      Serial.print("TDS Value:");

      Serial.print(tdsValue,0);

      Serial.println("ppm");

   }

}

int getMedianNum(int bArray[], int iFilterLen) 

{

      int bTab[iFilterLen];

      for (byte i = 0; i<iFilterLen; i++)

      bTab[i] = bArray[i];

      int i, j, bTemp;

      for (j = 0; j < iFilterLen - 1; j++) 

      {

      for (i = 0; i < iFilterLen - j - 1; i++) 

          {

        if (bTab[i] > bTab[i + 1]) 

            {

        bTemp = bTab[i];

            bTab[i] = bTab[i + 1];

        bTab[i + 1] = bTemp;

         }

      }

      }

      if ((iFilterLen & 1) > 0)

    bTemp = bTab[(iFilterLen - 1) / 2];

      else

    bTemp = (bTab[iFilterLen / 2] + bTab[iFilterLen / 2 - 1]) / 2;

      return bTemp;

}