24_hour_reminder.ino

// Define our physical setup
const int EARLIER_PIN = 13;
const int LED_PIN = 8;
const int HAVE_FED_PIN = 2;

// Define some configuration
// The "units" of our program
#define SECONDS 1000
#define HOURS 3600000
// Seconds for testing
// Hours for production
const unsigned long UNIT = HOURS;

const unsigned long TICKS = 24;
const unsigned long INTERVAL_MS = UNIT * TICKS;

// Declare some state-tracking variables
int haveFedButtonPressed = 0;
int alertSoonerButtonPressed = 0;
boolean alertSoonerButtonActive = false;
long previousMillis = 0;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(HAVE_FED_PIN, INPUT);
  pinMode(EARLIER_PIN, INPUT);
}

boolean itHasBeen(unsigned long intervalMillis) {
  unsigned long currentMillis = millis();
  if ((unsigned long)(currentMillis - previousMillis) >= intervalMillis) {
    previousMillis = currentMillis;
    return true;
  } else {
    return false;
  }
}

void alertSooner(long unit) {
  previousMillis -= UNIT;
  // Guard against underflow (yes, it would take a while)
  if (previousMillis == UNIT) {
    previousMillis = 0;
  }
}

void loop() {
  // Maybe alert the human to feed the fish
  if (itHasBeen(INTERVAL_MS)) {
    // Turn the LED ON
    digitalWrite(LED_PIN, HIGH);
  }

  // Check if the human told us they fed the fish
  haveFedButtonPressed = digitalRead(HAVE_FED_PIN);
  if (haveFedButtonPressed) {
    // Turn the LED OFF
    digitalWrite(LED_PIN, LOW);
  }

  // Check if the human told us to alert 1 UNIT sooner
  alertSoonerButtonPressed = digitalRead(EARLIER_PIN);
  if (alertSoonerButtonPressed && !alertSoonerButtonActive) {
    alertSooner(UNIT);
    alertSoonerButtonActive = true;
  }
  // Try to de-bounce the button
  if (!alertSoonerButtonPressed) {
    alertSoonerButtonActive = false;
  }

  // Wait some time
  // TODO: it would be better for battery life to enter some
  //  low-power state, which we would exit on a button press.
  delay(50);
}