float vLow = 0.2;
float vHigh = 20;
float x;
int delayTime;
void setup() {
  pinMode(27, INPUT); //啟動27號 gpio腳位
  pinMode(18, OUTPUT); //啟動18號 gpio腳位
  Serial.begin(9600); //啟動序列埠
}

void loop() {
  x=analogRead(27);
  x = (x/4096)*(vHigh - vLow)+vLow;
  delayTime=500/x;
  //delay的時間=一半的週期=1000/(2*頻率)   因為單位是毫秒，需乘1000
  Serial.print("LED閃爍頻率 = ");
  Serial.print(x);
  Serial.println(" Hz");
  digitalWrite(18, HIGH);
  delay(delayTime);
  digitalWrite(18, LOW);
  delay(delayTime);  
}