void setup() {

pinMode(9, OUTPUT);

}

void loop() {

for (int i = 0; i <= 180; i += 1) {

digitalWrite(9, HIGH);

delayMicroseconds(500 + i);

digitalWrite(9, LOW);

delayMicroseconds(20000 - i * 100);

}

delay(1000);

for (int i = 180; i >= 0; i -= 1) {

digitalWrite(9, HIGH);

delayMicroseconds(500 + i);

digitalWrite(9, LOW);

delayMicroseconds(20000 - i * 100);

}

delay(1000);

}