Door Alarm using Arduino

#include <NewPing.h>
#define TRIGGER_PIN  12  // Arduino pin tied to trigger pin on ping sensor.
#define ECHO_PIN     11  // Arduino pin tied to echo pin on ping sensor.
#define MAX_DISTANCE 500 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE); // NewPing setup of pins and maximum distance.

unsigned int pingSpeed = 50; // How frequently are we going to send out a ping (in milliseconds). 50ms would be 20 times a second.
unsigned long pingTimer;
int flag = 0; // Holds the next ping time.

void setup() {
  Serial.begin(115200); // Open serial monitor at 115200 baud to see ping results.
  pingTimer = millis();
  pinMode(10, OUTPUT); // Start now.
  // Start now.
}

void loop() {
  // Notice how there's no delays in this sketch to allow you to do other processing in-line while doing distance pings.
  if (millis() >= pingTimer) {   // pingSpeed milliseconds since last ping, do another ping.
    pingTimer += pingSpeed;      // Set the next ping time.
    sonar.ping_timer(echoCheck); // Send out the ping, calls "echoCheck" function every 24uS where you can check the ping status.
  }
  if (flag == 1)
  {
    digitalWrite(10, HIGH);

    delay(500);
    digitalWrite(10, LOW);

    delay(500);
    digitalWrite(10, HIGH);

    delay(500);
    digitalWrite(10, LOW);

    delay(500);
  }
  else
  {
    digitalWrite(10, LOW);
  }
}

void echoCheck() { // Timer2 interrupt calls this function every 24uS where you can check the ping status.
  if (sonar.check_timer()) { // This is how you check to see if the ping was received.
    // Here's where you can add code.
    Serial.print("Ping: ");
    Serial.print(sonar.ping_result / US_ROUNDTRIP_CM); // Ping returned, uS result in ping_result, convert to cm with US_ROUNDTRIP_CM.
    Serial.println("cm");
    if ((sonar.ping_result / US_ROUNDTRIP_CM) < 50)
      flag = 1;
    else if ((sonar.ping_result / US_ROUNDTRIP_CM) > 50)
      flag = 0;
  }
}