void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  double value = analogRead(A0);
  char x;
  if (Serial.available()) {
    x = Serial.read();

    if (x == ' ') {
      value *= 3.0 * 5.0 / 1024;
      Serial.println(value);
    }
  }
}