int relay = 13; // Plug the relay into Digital Pin 13

void setup() {
    //Initialize serial monitor at 9600 baudrate
  Serial.begin(9600);

  pinMode(relay, OUTPUT);

}

void loop() 
{

  //Reading analog input from A0 pin - it reads as a digital value 0-1023
  int digitalValue = analogRead(0);

  //Converting digital value to voltage - so we can print it as Voltage
  double voltage = (digitalValue * 5 ) / 1023.0;

//Turn the LED on only if the voltage is between 2.9 and 4.2.
//Under 2.9 or above 4.2 - turn off the LED
if ( voltage > 2.9  && voltage < 4.2) {

 digitalWrite(relay, HIGH); // Turn on the relay
 DisplayData(digitalValue, voltage);
 if (voltage == 2.9){
  Serial.print("Exactly 2.9V!");
 }
}

if ( voltage < 2.9 or voltage > 4.2) {
 digitalWrite(relay, LOW); // Turnoff  the relay 
 DisplayData(digitalValue, voltage);
 for (int i = 0; i < 6; i++) {
    Serial.print(analogRead(i));
    Serial.print("--");
    delay(500);
}
}

/*
  digitalWrite(relay, HIGH); // Turn the relay on
  delay(1000); // Wait 1 second
  digitalWrite(relay, LOW); // Turn the relay Off
  delay(1000); // Wait 1 second
*/
}

void DisplayData(int DigValue, double Volt){
  //Printing digital value in serial monitor
  Serial.print("Digital Value: ");
  Serial.print(DigValue);

  //Printing measured input voltage in serial monitor
  Serial.print(", input voltage: ");
  Serial.println(Volt, 3);
  //delay(500);

}