const int PIN_Echo = 4;
const int PIN_Trigger = 3;
const int PIN_Led = 2;

void setup(){
   Serial.begin(115200);
   pinMode(PIN_Trigger, OUTPUT);
   pinMode(PIN_Echo, INPUT);
   pinMode(PIN_Led, OUTPUT);
}

void loop(){
   int CM = ping(PIN_Trigger, PIN_Echo);

   //Serial.print("Distance: ");
   if (4 <= CM <= 400)
      Serial.println(CM);
   if (CM > 400)
      Serial.println("Invalid measures. Farther than 4m.");
   if (CM < 4)
      Serial.println("Invalid measures. Closer than 4cm.");
   
   digitalWrite(PIN_Led, HIGH);
   delay(100);
   digitalWrite(PIN_Led, LOW);
   delay(100);
}

int ping(int PIN_Trigger, int PIN_Echo){
   long duration;
   long distanceCM;
   
   digitalWrite(PIN_Trigger, LOW);  //para generar un pulso limpio ponemos a LOW 4us
   delayMicroseconds(4);
   digitalWrite(PIN_Trigger, HIGH);  //generamos Trigger (disparo) de 10us
   delayMicroseconds(10);
   digitalWrite(PIN_Trigger, LOW);
   
   duration = pulseIn(PIN_Echo, HIGH);  //medimos el tiempo entre pulsos, en microsegundos
   
   distanceCM = duration * 10 / 292/ 2;   //convertimos a distancia, en mm
   return distanceCM;
}