Technical Architecture

#define TdsSensorPin A1
#define VREF 5.0      // analog reference voltage(Volt) of the ADC
#define SCOUNT  30    // sum of sample point
#include <SoftwareSerial.h>

SoftwareSerial mySerial(3, 2); //SIM800L Tx & Rx is connected to Arduino #3 & #2


int analogBuffer[SCOUNT];    // store the analog value in the array, read from ADC
int analogBufferTemp[SCOUNT];
int analogBufferIndex = 0,copyIndex = 0;
float averageVoltage = 0,temperature = 25;
double tdsValue = 0;
int counter=0;


void setup()
{
    
    Serial.begin(9600);
    pinMode(TdsSensorPin,INPUT);

    
}

void loop()
{
   static unsigned long analogSampleTimepoint = millis();
   if(millis()-analogSampleTimepoint > 60U)     //every 60 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 > 60U)
   {
      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
    
      if (tdsValue>0)
      {
        Serial.println(tdsValue,0);
      }
      
      
   }   
}
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;

     
}

void updateSerial()
{
  delay(500);
  while (Serial.available()) 
  {
    mySerial.write(Serial.read());//Forward what Serial received to Software Serial Port
  }
  while(mySerial.available()) 
  {
    Serial.write(mySerial.read());//Forward what Software Serial received to Serial Port
  }
}

// to be inserted and modified according to developers preference
   if (tdsValue > 50 and counter==0)
   {
     mySerial.begin(9600);

    Serial.println("Initializing..."); 
    delay(1000);
  
    mySerial.println("AT");  
    updateSerial();
  
    mySerial.println("AT+CMGF=1"); // Configuring TEXT mode
    updateSerial();
    mySerial.println("AT+CMGS=\"+xxxxxxxxxxx\""); //change country code or number
    updateSerial();
    mySerial.print("TDS value: ");
    mySerial.print(tdsValue,0); //text content
    mySerial.print("\n");
    mySerial.print("The value is above RO limits");
    updateSerial();
    mySerial.write(26);

    counter=1;

    }