参考了一个卖传感器的商家的代码，当年二叔买这个传感器花了40+还有运费，一直扔着没用呢
http://detail.etao.com/18934469888.htm

1. int pin = 8;
   
2. unsigned long duration;
   
3. unsigned long starttime;
   
4. unsigned long sampletime_ms = 5000;
   
5. unsigned long lowpulseoccupancy = 0;
   
6. float ratio = 0;
   
7. 
   
8. void setup() {
   
9.   Serial.begin(9600);
   
10.   pinMode(pin,INPUT);
    
11.   starttime = millis();
    
12. }
    
13. 
    
14. void loop() {
    
15.   duration = pulseIn(pin, LOW);
    
16.   lowpulseoccupancy++;
    
17.   if ((millis()-starttime) > sampletime_ms)
    
18.   {
    
19.     ratio = lowpulseoccupancy*1000/sampletime_ms;
    
20.     Serial.print(ratio);
    
21.     Serial.println("Hz");
    
22.     lowpulseoccupancy = 0;
    
23.     starttime = millis();
    
24.   }
    
25. }

复制代码