int trigger = 12;

int echo = 13;

int led = 8;

long duration = 0;

int cm = 0;

int inch = 0;

void setup() {

  // put your setup code here, to run once:

  Serial.begin(9600);

  pinMode(trigger, OUTPUT);

  pinMode(echo, INPUT);

  pinMode(led, OUTPUT);

}

void loop() {

  // put your main code here, to run repeatedly:

  digitalWrite(trigger, LOW);

  digitalWrite(trigger, HIGH);

  digitalWrite(trigger, LOW);

  duration = pulseIn(echo, HIGH);

  cm = duration*0.034/2;

  inch = duration*0.0133/2;

  if (inch < 50){

    digitalWrite(led, HIGH);

  } else {

    digitalWrite(led, LOW);

  }

  if (inch < 100 ) {

    Serial.print("Inches: ");

    Serial.println(inch);

    Serial.print("Cm: ");

    Serial.println(cm);

  }

  delay(500);

}

Ultrasonic sensors are distance sensors that use sound waves to detect how far away an object is. They send out high frequency bursts of sound and listen for its echo. They then determine how far away the object is based on how long it takes for the sound to return to the sensor. This variety requires an Arduino library to operate.

NewPing Library

#include <NewPing.h>

NewPing mysensor(5, 6, 200);

void setup() {
  Serial.begin(9600);
}

void loop() {
  int pingTime = mysensor.ping();

  int distance = mysensor.ping_in();

  int distance_cm = mysensor.ping_cm();

  Serial.println(distance);
}