void setup() {
  // put your setup code here, to run once:
pinMode(4, OUTPUT); //trigger
pinMode(3, INPUT); //Echo
pinMode(11, OUTPUT);
pinMode(5, OUTPUT);
Serial.begin(9600);
}


void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(4, LOW);
delayMicroseconds(2);
digitalWrite(4, HIGH);
delayMicroseconds(10);
digitalWrite(4, LOW);
float duration = pulseIn(3,HIGH);
float distance = ((0.034*duration)/2);
Serial.print(distance);
Serial.print("cm");
Serial.println();

if(distance<=20){

digitalWrite(11,HIGH);
tone(5,500);
delay(200);
digitalWrite(11,LOW);
noTone(5);
delay(200);
}
else{
digitalWrite(11,LOW);
noTone(5);
}
}