#define PIN_TRIG 3
#define PIN_ECHO 2
#define BUZZER_PIN  7
int channel = 0;
int resolution =8;
void setup() {
  Serial.begin(115200);
  ledcSetup(channel, freq, resolution);
  pinMode(PIN_TRIG, OUTPUT);
  pinMode(PIN_ECHO, INPUT);
  tone(7, 262, 250);
}

void loop() {

  digitalWrite(PIN_TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(PIN_TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(PIN_TRIG, LOW);
  long duration = pulseIn(PIN_ECHO, HIGH);
  float distance = duration/10;
  Serial.print("distance:");
  Serial.print(distance);

  if (distance <= 100) {
    tone(BUZZER_PIN, 1000);
    delay(2000);
    noTone(BUZZER_PIN);
    delay(2000);
  }
}