This is a simple voltmeter that takes A0 as an input, the Pin 13 LED blinks everytime a value is sent

/***************************************
 * Simple Arduino Voltmeter
 * Measures Voltage on A0 analog input
 * CC-BY-NC-SA sonline.neocities.org
 * 2014-11-29 Ver.: 0.1
 ****************************************/

#define ANAIN A0

int vout;

void setup() {
  Serial.begin(9600);
  pinMode(ANAIN, INPUT);
  pinMode(13, OUTPUT);
}

void loop() {
  while (Serial.available() > 0) {
   vout = (analogRead(ANAIN) * 5.0) / 1024.0; 
   if(Serial.println(vout)){
     digitalWrite(13, HIGH);
     digitalWrite(13, LOW);
     }
  }
}