#define sensor 2
unsigned long startMillis;  //some global variables available anywhere in     the program
unsigned long currentMillis;
const unsigned long period = 1000;  //the value is a number of milliseconds
long freq = 0;
int state = 0;
int lstate = 0;
void setup() {
Serial.begin(115200);
pinMode(sensor, INPUT);
  startMillis = millis();  //initial start time
}

void loop() {
  currentMillis = millis();
  state = digitalRead(sensor);

 if (state != lstate) {
  freq = freq +1;
  lstate = state;
  }
   if (currentMillis - startMillis >= period)  //test whether the period has elapsed
  {
  Serial.println(freq);
  startMillis = currentMillis;
  freq = 0;
  }

  }