#define trigger 4
#define echo A3
#define led 6
void setup() {
pinMode(led, OUTPUT); 
pinMode(echo, INPUT);
pinMode(trigger, OUTPUT);
}

void loop() {
long duracion=pulseIn(echo,HIGH);
long distancia=(duracion/2)/29;
delay(500);

digitalWrite(trigger, HIGH);
delay(10);
digitalWrite(trigger, LOW);
if(distancia<100|| distancia>20){
digitalWrite(led, HIGH);}
else{digitalWrite(led, LOW);

}

}