Test_pH.ino

#define SensorPin A3            //pH meter Analog output to Arduino Analog Input 0
#define Offset 0.04            //deviation compensate
#define first_ph_reference_voltage 1.73
#define first_standard_solution 6.7
#define second_ph_reference_voltage 1.29
#define second_standard_solution 4.1
#define LED 13
#define samplingInterval 1000
#define printInterval 1000
#define ArrayLenth  40    //times of collection
int pHArray[ArrayLenth];   //Store the average value of the sensor feedback
int pHArrayIndex=0;    
void setup(void)
{
  pinMode(LED,OUTPUT);  
  Serial.begin(115200);  
  Serial.println("pH meter experiment!");    //Test the serial monitor
}

void loop(void)
{
  static bool up = false;
  static bool down = false;
  static unsigned long samplingTime = millis();
  static unsigned long printTime = millis();
  static float pHValue,voltage;
  if(millis()-samplingTime > samplingInterval)
  {
      pHArray[pHArrayIndex++]=analogRead(SensorPin);
      if(pHArrayIndex==ArrayLenth)pHArrayIndex=0;
      voltage = avergearray(pHArray, ArrayLenth)*5.0/1024;
      float slope = (first_standard_solution-second_standard_solution)/((first_ph_reference_voltage-1.5)/3-(second_ph_reference_voltage-1.5)/3);
      float intercept = first_standard_solution - slope * (first_ph_reference_voltage-1.50) / 3;
      pHValue = slope * (voltage - 1.5)/3 + intercept + Offset;
      samplingTime=millis();
  }
  if(millis() - printTime > printInterval)   //Every 800 milliseconds, print a numerical, convert the state of the LED indicator
  {
  Serial.print("Voltage:");
        Serial.print(voltage,2);
        Serial.print("    pH value: ");
  Serial.println(pHValue,2);
        digitalWrite(LED,digitalRead(LED)^1);
        printTime=millis();
  }
}
double avergearray(int* arr, int number){
  int i;
  int max,min;
  double avg;
  long amount=0;
  if(number<=0){
    Serial.println("Error number for the array to avraging!/n");
    return 0;
  }
  if(number<5){   //less than 5, calculated directly statistics
    for(i=0;i<number;i++){
      amount+=arr[i];
    }
    avg = amount/number;
    return avg;
  }else{
    if(arr[0]<arr[1]){
      min = arr[0];max=arr[1];
    }
    else{
      min=arr[1];max=arr[0];
    }
    for(i=2;i<number;i++){
      if(arr[i]<min){
        amount+=min;        //arr<min
        min=arr[i];
      }else {
        if(arr[i]>max){
          amount+=max;    //arr>max
          max=arr[i];
        }else{
          amount+=arr[i]; //min<=arr<=max
        }
      }//if
    }//for
    avg = (double)amount/(number-2);
  }//if
  return avg;
}