const int LED = 12;
int alt = HIGH;

void setup() {
  Serial.begin(9600);
  pinMode(LED, OUTPUT);
}

void loop() {
  digitalWrite(LED, alt);
  alt = alt ? LOW : HIGH;

  int value = analogRead(A0);
  float voltage = value * (5.0 / 1023);
  Serial.println(voltage);
  
  delay(1000);
}