Electronic voting machine on an Arduino Nano

// Electronic voting machine: four buttons to vote, one to see the result,
// and a 16x2 I2C LCD, on an Arduino Nano.
//
//   vote buttons   red -> D2, blue -> D3, green -> D4, yellow -> D5, each to GND
//   result button  D6 to GND
//   LCD            SDA -> A4, SCL -> A5, VCC -> 5 V, GND -> GND (address 0x27)
//   piezo          D12 and GND
//
// The class votes for a team color. Press a color to vote for it: the
// machine beeps, says thank you, and ignores every button for two seconds,
// so one press is one vote and the next voter steps up.
//
// Press the result button to see every count and the winner for five
// seconds. Hold it for three seconds to clear all the votes and start a new
// election.
//
// # The votes survive a power cut
//
// Every vote is written to the EEPROM, the Nano's own memory that keeps its
// contents with the power off. When the sketch starts it reads the counts
// back, so pulling the plug halfway through the vote loses nothing. A blank
// EEPROM reads 255 in every byte, so a marker byte says whether the counts
// there were written by this sketch at all.
//
// # One press, one vote
//
// A button is read as "pressed" only on the change from up to down, and 30 ms
// of quiet after a change is needed before the next one counts. Holding a
// button down, or the contacts bouncing, can never add a second vote.

#include <EEPROM.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const uint8_t VOTE[4] = {2, 3, 4, 5};
const char *const TEAM[4] = {"Red", "Blue", "Green", "Yellow"};
const uint8_t RESULT = 6;
const uint8_t PIEZO = 12;

const int MARKER_ADDRESS = 8;  // the four counts take bytes 0 to 7
const uint8_t MARKER = 0x5A;
const unsigned long LOCK_MS = 2000;
const unsigned long SHOW_MS = 5000;
const unsigned long CLEAR_HOLD_MS = 3000;

LiquidCrystal_I2C lcd(0x27, 16, 2);

unsigned int votes[4];
bool lastState[5];             // the four vote buttons, then the result button
unsigned long changedAt[5];
unsigned long busyUntil = 0;   // the screen shows a message until then
unsigned long resultDownAt = 0;
bool resultHeld = false;

unsigned int total() {
  return votes[0] + votes[1] + votes[2] + votes[3];
}

void saveVotes() {
  for (uint8_t i = 0; i < 4; i++) EEPROM.put(i * 2, votes[i]);
  EEPROM.write(MARKER_ADDRESS, MARKER);
}

void loadVotes() {
  if (EEPROM.read(MARKER_ADDRESS) != MARKER) {
    for (uint8_t i = 0; i < 4; i++) votes[i] = 0;
    saveVotes();
    return;
  }
  for (uint8_t i = 0; i < 4; i++) EEPROM.get(i * 2, votes[i]);
}

void showWaiting() {
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Press to vote");
  lcd.setCursor(0, 1);
  lcd.print("Votes so far: ");
  lcd.print(total());
}

void castVote(uint8_t team) {
  votes[team]++;
  saveVotes();
  tone(PIEZO, 1047, 150);
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Vote: ");
  lcd.print(TEAM[team]);
  lcd.setCursor(0, 1);
  lcd.print("Thank you!");
  Serial.print("vote for ");
  Serial.print(TEAM[team]);
  Serial.print(", ");
  Serial.print(total());
  Serial.println(total() == 1 ? " vote in all" : " votes in all");
  busyUntil = millis() + LOCK_MS;
}

void showResults() {
  // Who has the most? A tie is said as a tie.
  unsigned int best = 0;
  for (uint8_t i = 0; i < 4; i++) best = max(best, votes[i]);
  lcd.clear();
  for (uint8_t i = 0; i < 4; i++) {
    lcd.setCursor((i % 2) * 8, i / 2);
    lcd.print(TEAM[i][0]);  // R, B, G and Y
    lcd.print(':');
    lcd.print(votes[i]);
  }
  Serial.print("results: ");
  for (uint8_t i = 0; i < 4; i++) {
    Serial.print(TEAM[i]);
    Serial.print(' ');
    Serial.print(votes[i]);
    Serial.print(i < 3 ? ", " : "\n");
  }
  uint8_t leaders = 0;
  for (uint8_t i = 0; i < 4; i++) {
    if (votes[i] == best) leaders++;
  }
  if (best == 0) {
    Serial.println("no votes yet");
  } else if (leaders == 1) {
    for (uint8_t i = 0; i < 4; i++) {
      if (votes[i] != best) continue;
      Serial.print("winner: ");
      Serial.println(TEAM[i]);
      lcd.setCursor(12, 0);
      lcd.print("WIN");
      lcd.setCursor(12, 1);
      lcd.print(TEAM[i][0]);
    }
  } else {
    Serial.print("a tie between ");
    Serial.print(leaders);
    Serial.println(" teams");
    lcd.setCursor(12, 0);
    lcd.print("TIE");
  }
  busyUntil = millis() + SHOW_MS;
}

void clearVotes() {
  for (uint8_t i = 0; i < 4; i++) votes[i] = 0;
  saveVotes();
  tone(PIEZO, 440, 400);
  lcd.clear();
  lcd.print("Votes cleared");
  Serial.println("votes cleared: a new election");
  busyUntil = millis() + LOCK_MS;
}

// Has button `i` on `pin` just gone down?
bool pressed(uint8_t i, uint8_t pin, unsigned long now) {
  bool state = digitalRead(pin);
  bool down = false;
  if (state != lastState[i] && now - changedAt[i] > 30) {
    down = state == LOW;
    lastState[i] = state;
    changedAt[i] = now;
  }
  return down;
}

void setup() {
  Serial.begin(115200);
  for (uint8_t i = 0; i < 4; i++) pinMode(VOTE[i], INPUT_PULLUP);
  pinMode(RESULT, INPUT_PULLUP);
  pinMode(PIEZO, OUTPUT);
  for (uint8_t i = 0; i < 5; i++) lastState[i] = HIGH;
  lcd.init();
  lcd.backlight();
  loadVotes();
  Serial.print("Voting machine ready: ");
  Serial.print(total());
  Serial.println(" votes stored");
  showWaiting();
}

void loop() {
  unsigned long now = millis();
  bool busy = (long)(busyUntil - now) > 0;

  for (uint8_t i = 0; i < 4; i++) {
    // Read every button every time, so a press during the lock is not saved
    // up and counted the moment it ends.
    if (pressed(i, VOTE[i], now) && !busy) castVote(i);
  }

  if (pressed(4, RESULT, now)) {
    resultDownAt = now;
    resultHeld = true;
    showResults();
  }
  if (resultHeld && lastState[4] == HIGH) resultHeld = false;
  if (resultHeld && now - resultDownAt >= CLEAR_HOLD_MS) {
    resultHeld = false;
    clearVotes();
  }

  // Back to "press to vote" when a message has been up long enough.
  static bool wasBusy = false;
  busy = (long)(busyUntil - now) > 0;
  if (wasBusy && !busy) showWaiting();
  wasBusy = busy;
}