volatile int flow_frequency; // Measures flow sensor pulses
unsigned char flowsensor = 2; // Sensor Input
unsigned long currentTime;
unsigned long cloopTime;
int pinOut = 10;
void flow () // Interrupt function
{
   flow_frequency++;
}
void setup()
{
   pinMode(flowsensor, INPUT);
   pinMode(pinOut, OUTPUT);
   digitalWrite(flowsensor, HIGH); // Optional Internal Pull-Up
   attachInterrupt(0, flow, RISING); // Setup Interrupt
   sei(); // Enable interrupts
   currentTime = millis();
   cloopTime = currentTime;
}
void loop ()
{
   currentTime = millis();
   // Every second, calculate and print litres/hour
   if(currentTime >= (cloopTime + 1000))
   {
      cloopTime = currentTime; // Updates cloopTime
      // Pulse frequency (Hz) = 7.5Q, Q is flow rate in L/min.
      if(flow_frequency > 0)
      {
        
        digitalWrite(pinOut, LOW);
      }
      else
      {
        
        digitalWrite(pinOut, HIGH);
      }
      flow_frequency = 0; // Reset Counter
      
   }
}