问Arduino超声波传感器pulseIn只有3或4个

EN

#define SPEAKER 11
#define TRIGGER 2
#define ECHO 3
#define SPLIT 40

unsigned long duration;

int alarm1()
{
  Serial.print("alarmtest");
  analogWrite(SPEAKER, 100);
  delay(100);
  analogWrite(SPEAKER, 0);
  delay(100);
  analogWrite(SPEAKER, 100);
  delay(100);
  analogWrite(SPEAKER, 0);
  delay(500);
}

void setup() 
{
  Serial.begin(9600);
  pinMode(SPEAKER, OUTPUT);
  pinMode(TRIGGER, OUTPUT);
  pinMode(ECHO, INPUT);
}

void loop() 
{       
  digitalWrite(TRIGGER, LOW);
  delayMicroseconds(50);
  digitalWrite(TRIGGER, HIGH);
  delayMicroseconds(50);
  digitalWrite(TRIGGER, LOW);
  duration = pulseIn(ECHO, HIGH);
  delay(200);

  Serial.print(duration);
  if (duration < SPLIT)
  {
   // alarm1();
  }
}