#include <NewPing.h>
#define relayPin 3
#define TRIGGER_PIN  5  // Arduino pin tied to trigger pin on the ultrasonic sensor.
#define ECHO_PIN     4  // Arduino pin tied to echo pin on the ultrasonic sensor.
#define MAX_DISTANCE 200 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.

int distance;

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE);
void setup() {
    Serial.begin(115200);
    pinMode(relayPin, OUTPUT);
  // put your setup code here, to run once:

}

void loop() {
   readDist();
  Serial.println(distance);// put your main code here, to run repeatedly:
if (distance < 50){
  digitalWrite(relayPin, HIGH);
}else{
  digitalWrite(relayPin, LOW);
}
}

void readDist(){
  delay(50);
  distance = sonar.ping_cm();
  if(distance == 0){
    distance = 200;
  }
}