Automatic pet feeder on an Arduino Uno

// Automatic pet feeder: a DS3231 keeps the time, and at each meal time a
// servo opens the food gate for one portion and closes it again.
//
//   DS3231   SDA and SCL to the Uno's SDA and SCL, VCC 5V, GND
//   LCD      16x2 with an I2C backpack at 0x27: SDA A4, SCL A5, VCC 5V, GND
//            (A4 and A5 are the same I2C bus as SDA and SCL)
//   servo    MG996R signal on pin 9, VCC from its own 5 to 6 V supply,
//            GND joined to the Uno's
//   button   FEED NOW on pin 2, to GND (the pin's pull-up holds it HIGH)
//   buzzer   active buzzer on pin 8, other leg to GND
//
// HOW IT WORKS
//   The screen shows the time and the next meal. When the clock reaches a
//   meal time the buzzer chirps three times (so the pet learns the sound),
//   the gate opens to OPEN_ANGLE for PORTION_MS and closes. Each meal is
//   logged on the serial monitor with the time it was served.
//
//   A meal is served once: the sketch remembers the day and minute it last
//   fed, so the minute the clock sits on 07:30 gives one portion, not sixty.
//   FEED NOW gives an extra portion at any time, and is logged as one.
//
// WHAT TO CHANGE
//   MEALS        the meal times, as hour and minute, up to eight
//   PORTION_MS   how long the gate stays open: a longer time, a bigger meal
//   OPEN_ANGLE   how far the gate opens; CLOSED_ANGLE where it rests
//
// SETTING THE CLOCK. A DS3231 keeps time on its coin cell. To set a real
// one, run rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); once from setup()
// and upload again without it. In Mokxi the clock starts at the time in its
// "start" property.

#include <Wire.h>
#include <RTClib.h>
#include <LiquidCrystal_I2C.h>
#include <Servo.h>

RTC_DS3231 rtc;
LiquidCrystal_I2C lcd(0x27, 16, 2);
Servo gate;

const int SERVO_PIN = 9;
const int BUTTON_PIN = 2;
const int BUZZER_PIN = 8;

struct Meal {
  uint8_t hour;
  uint8_t minute;
};
const Meal MEALS[] = {{7, 30}, {12, 0}, {18, 0}};
const int MEAL_COUNT = sizeof(MEALS) / sizeof(MEALS[0]);

const unsigned long PORTION_MS = 1500;
const int OPEN_ANGLE = 90;
const int CLOSED_ANGLE = 0;

long lastFed = -1;   // the day and minute of the last scheduled meal
int lastSecond = -1;

void two(Print &out, int n) {
  if (n < 10) out.print('0');
  out.print(n);
}

void showTime(Print &out, const DateTime &t, bool seconds) {
  two(out, t.hour());
  out.print(':');
  two(out, t.minute());
  if (seconds) {
    out.print(':');
    two(out, t.second());
  }
}

// The next meal after this minute, wrapping round to tomorrow's first.
int nextMeal(const DateTime &t) {
  int now = t.hour() * 60 + t.minute();
  for (int i = 0; i < MEAL_COUNT; i++) {
    if (MEALS[i].hour * 60 + MEALS[i].minute > now) return i;
  }
  return 0;
}

void chirp() {
  for (int i = 0; i < 3; i++) {
    digitalWrite(BUZZER_PIN, HIGH);
    delay(100);
    digitalWrite(BUZZER_PIN, LOW);
    delay(100);
  }
}

void feed(const char *why) {
  DateTime t = rtc.now();
  lcd.setCursor(0, 1);
  lcd.print("Feeding...      ");
  chirp();
  gate.write(OPEN_ANGLE);
  delay(PORTION_MS);
  gate.write(CLOSED_ANGLE);
  showTime(Serial, t, true);
  Serial.print("  ");
  Serial.println(why);
  lastSecond = -1;  // redraw the screen at once
}

void setup() {
  Serial.begin(115200);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(BUZZER_PIN, OUTPUT);
  gate.attach(SERVO_PIN);
  gate.write(CLOSED_ANGLE);
  lcd.init();
  lcd.backlight();
  if (!rtc.begin()) {
    Serial.println("no DS3231 at 0x68: check SDA, SCL and VCC");
    lcd.print("No clock found");
    while (true) delay(10);
  }
  if (rtc.lostPower()) Serial.println("the clock lost power: set the time");
  Serial.print("pet feeder ready, meals at");
  for (int i = 0; i < MEAL_COUNT; i++) {
    Serial.print(' ');
    two(Serial, MEALS[i].hour);
    Serial.print(':');
    two(Serial, MEALS[i].minute);
  }
  Serial.println();
}

void loop() {
  DateTime t = rtc.now();

  // A meal time: once per day and minute, however long the minute lasts.
  long stamp = (long)(t.unixtime() / 60);
  for (int i = 0; i < MEAL_COUNT; i++) {
    if (t.hour() == MEALS[i].hour && t.minute() == MEALS[i].minute && stamp != lastFed) {
      lastFed = stamp;
      char why[] = "meal 0 served";
      why[5] = '1' + i;
      feed(why);
    }
  }

  if (digitalRead(BUTTON_PIN) == LOW) {
    feed("extra portion (FEED NOW)");
    while (digitalRead(BUTTON_PIN) == LOW) delay(10);
  }

  if (t.second() != lastSecond) {
    lastSecond = t.second();
    lcd.setCursor(0, 0);
    lcd.print("Time ");
    showTime(lcd, t, true);
    lcd.print("   ");
    const Meal &next = MEALS[nextMeal(t)];
    lcd.setCursor(0, 1);
    lcd.print("Next meal ");
    two(lcd, next.hour);
    lcd.print(':');
    two(lcd, next.minute);
    lcd.print(' ');
  }
  delay(50);
}