int tripin= 7;
int echopin= 8;
int led1 = 9;
int duration, distance;
void setup()
{
  pinMode(tripin, OUTPUT);
  pinMode(led1, OUTPUT);
  pinMode(echopin, INPUT);
  Serial.begin(9600);
}
void loop()
{
  digitalWrite(tripin, LOW);
  delay(2);
  digitalWrite(tripin, HIGH);
  delay(10);
  digitalWrite(tripin, LOW);
  duration=pulseIn(echopin,HIGH);
  distance = duration*0.034/2;
  Serial.print("distance: ");
  Serial.print(distance);
  Serial.println("CM ");
  if(distance<20)
  {
    digitalWrite(led1, HIGH);
    Serial.println(" ");
  }
  else
  {
    digitalWrite(led1, LOW);
    Serial.println(" ");
  }
  delay(100);
}