/***************************************************
 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 A4
#define VREF 3.3      // 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(115200);
    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 compensationVoltage=averageVoltage/compensationCoefficient;  //temperature compensation
      tdsValue=(133.42*compensationVoltage*compensationVoltage*compensationVoltage - 255.86*compensationVoltage*compensationVoltage + 857.39*compensationVoltage)*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;
}