The Synchronous Clock: Telling Time by the Power Station's Cycles

/*
  grid_clock.ino -- a clock that counts the mains cycles, the way Warren's clock does.

  SAFETY: never connect anything here to the mains. Use a plug-in adapter whose OUTPUT is a
  low-voltage AC (for example 9 V AC) -- check the label says AC OUTPUT, not DC.

  Wiring: adapter AC output -> 1 kohm resistor -> opto-isolator LED anode; LED cathode -> the
  other adapter wire. Put a diode across the LED, reversed (anode to cathode), so the LED never
  sees the reverse half-cycle. Opto transistor: collector -> pin 2, emitter -> GND. The pin uses
  its internal pull-up, so it goes LOW once per mains cycle.

  Set MAINS_HZ to 50 or 60 for your country.
*/

const int PIN_CYCLE = 2;
const unsigned int MAINS_HZ = 50;

volatile unsigned long cycles = 0;
void onCycle() { cycles++; }

unsigned long lastPrinted = 0;

void setup() {
  pinMode(PIN_CYCLE, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(PIN_CYCLE), onCycle, FALLING);
  Serial.begin(9600);
  Serial.println(F("grid_time  (compare with a network-synced clock)"));
}

void loop() {
  noInterrupts();
  unsigned long c = cycles;
  interrupts();
  unsigned long seconds = c / MAINS_HZ;
  if (seconds != lastPrinted && seconds % 10 == 0) {
    lastPrinted = seconds;
    unsigned long h = seconds / 3600, m = (seconds / 60) % 60, s = seconds % 60;
    if (h < 10) Serial.print('0');
    Serial.print(h); Serial.print(':');
    if (m < 10) Serial.print('0');
    Serial.print(m); Serial.print(':');
    if (s < 10) Serial.print('0');
    Serial.println(s);
  }
}