// pin's config
const int Trig = 2;
const int Echo = 3;
 
// current  led state
int led_state=0;
 
long led_period_on = 10; //periods = 300*10=3s - how long wait after object disappear?
long led_left_on; //if this is 0 - turn off
 
void setup()
{
  pinMode(Trig, OUTPUT);
  pinMode(Echo, INPUT);
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
  //default turn on
  turn_led(1);
  led_left_on = led_period_on;
}
 
void loop()
{
  if ( led_left_on-- > 0 )
  {
    if (led_state == 0)
      turn_led(1);
  }
  else
  {
    if (led_state == 1)
       turn_led(0);
  }
  digitalWrite(Trig, LOW); // clear
  delayMicroseconds(2); // wait 2 mks
  digitalWrite(Trig, HIGH); // run
  delayMicroseconds(10); // wait 10 mks
  digitalWrite(Trig, LOW); // clear
  long time_us=pulseIn(Echo, HIGH); // How long was signal?
 
  if ( time_us != 0 && time_us/58 ) //  to santimeters
  {
    led_left_on = led_period_on;
    if (led_state == 0)
      turn_led(1);
  }
 
  delay(300); // run again
}
 
// turn led on/off
void turn_led( int led_state_)
{
   led_state = led_state_;
   digitalWrite(11, led_state == 0?LOW:HIGH);
   digitalWrite(10, led_state == 0?HIGH:LOW);