#define TRIG 8 
#define ECHO 9
#define led 3
#define buzzer 4
float distance,duration;
#define limitation 50

void setup() {
Serial.begin(9600);
pinMode(TRIG, OUTPUT);
pinMode(ECHO,INPUT);
pinMode(led, OUTPUT);
pinMode(buzzer, OUTPUT);

}

void loop() {
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
 duration=pulseIn(ECHO,HIGH);
 distance=0.017*duration;
if(distance<limitation){
  digitalWrite(led, HIGH);
  digitalWrite(buzzer, HIGH);
  tone(buzzer,1000,1000);}
else
  digitalWrite(led, LOW);
  digitalWrite(buzzer,LOW);

  Serial.print("Distance:");
  Serial.print(distance);
  Serial.println("cm");
  delay(500);

 }