void setup() {
  pinMode(A0, INPUT);
  Serial.begin(9600);
  for (int pin = 4 ; pin <= 13 ; pin++) pinMode(pin, OUTPUT);
}

void loop() {
  int data = analogRead(A0);
  float resolution = 5.0 / 1023 ;
  float volt = data * resolution ;
  Serial.print("Data = " + String(data));
  Serial.println("\t Volt= " + String(volt));
  for (int pin = 13 ; pin >= 4 ; pin--) digitalWrite(pin, LOW);
  float d = 5.0 / 10 ;// delta = 0.5v
  int n = volt / d ;  // LED number
  digitalWrite(4 + n, HIGH);
  delay(100);
}