connect arduino to a reed switch to read the water flow from a water meter

reed_switch.c

// this is for an arduino uno.
// this was inspired by http://www.instructables.com/id/Arduino-Reed-Switch/?ALLSTEPS but
// changed to use the arduino internal pull up resistor:
//
//    +-----------------+
//    |    arduino  GND -----reed_switch----\
//    |                 |                   |
//    |              12 --------------------/
//    +-----------------+
//
// tested with a window reed switch alike http://www.aliexpress.com/item/New-Arrival-High-Quality-10W-Recessed-Magnetic-Window-Door-Contacts-Alarm-Security-Reed-Switch-Home-Safely/32550780759.html

const int pinReedSwitch = 12;
const int pinLed = 13;

int lastValue;
unsigned long lastValueTime;
unsigned int counter = 0;

void setup() {
  // initialize pins.
  pinMode(pinReedSwitch, INPUT_PULLUP);
  pinMode(pinLed, OUTPUT);

  // initialize serial and wait for the port to open.
  Serial.begin(19200);
  while (!Serial) {
    // wait for serial port to connect.
    // needed for native USB port only.
  }
  Serial.println("hello");

  lastValue = !digitalRead(pinReedSwitch);
  lastValueTime = millis();
}

void loop() {
  int value = !digitalRead(pinReedSwitch);
  if (value != lastValue) {
    lastValue = value;
    digitalWrite(pinLed, value == HIGH ? HIGH : LOW);
    if (value) {
      ++counter;
      unsigned long valueTime = millis();
      unsigned long timeDifference = valueTime >= lastValueTime
        ? valueTime - lastValueTime
        : lastValueTime - valueTime;
      lastValueTime = valueTime;
      Serial.print(counter);
      Serial.print(" dt=");
      Serial.print(timeDifference);
      Serial.print('\n');
    }
  }
}