Pot-of-gold treasure box on an Arduino Uno

// Pot-of-gold treasure box: a box that only opens for the lucky code. Type it
// on the keypad and the lid unlocks, the gold inside lights up and a little
// tune plays. Get it wrong and it grumbles.
//
//   keypad    rows -> pins 9, 8, 7, 6, columns -> pins 5, 4, 3, 2
//   latch     servo PWM -> pin 10, VCC -> 5 V, GND -> GND
//   gold      12 WS2812B LEDs in a ring, DIN <- 330 ohms <- pin 11
//   piezo     pin 12 -> piezo -> GND
//
// The code is 317, for March 17th, then # to try it. * clears what you have
// typed. The sketch keeps up to eight keys, so a long guess cannot run off
// the end of the buffer.
//
// The Keypad library scans the 16 keys by driving one row at a time and
// reading the four columns, so 16 keys need only 8 pins.
//
// Make it a hunt: hide clues to the code around the house, and fill the box
// with chocolate coins. A servo makes a good latch: its horn turns across the
// lid's edge to lock it and swings clear to open.
//
// Things to try. Type 3 1 7 #. Try a wrong code. Change CODE to your own.

#include <Keypad.h>
#include <Servo.h>
#include <Adafruit_NeoPixel.h>

const char CODE[] = "317";
const int LATCH_PIN = 10, GOLD_PIN = 11, PIEZO_PIN = 12;
const int LOCKED = 0, UNLOCKED = 90;
const int LEDS = 12;

const byte ROWS = 4, COLS = 4;
char keys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'},
};
byte rowPins[ROWS] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};
Keypad pad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
Servo latch;
Adafruit_NeoPixel gold(LEDS, GOLD_PIN, NEO_GRB + NEO_KHZ800);

char typed[9];
int count = 0;

void shimmer(unsigned long ms) {
  unsigned long until = millis() + ms;
  while (millis() < until) {
    for (int i = 0; i < LEDS; i++) {
      int v = random(0, 8) == 0 ? 255 : random(120, 200);
      gold.setPixelColor(i, v, v * 3 / 5, 0);
    }
    gold.show();
    delay(40);
  }
  gold.clear();
  gold.show();
}

void open() {
  Serial.println("Lucky! The pot of gold is open");
  latch.write(UNLOCKED);
  const int tune[] = {659, 784, 880, 1047, 880, 1047};
  for (int i = 0; i < 6; i++) {
    tone(PIEZO_PIN, tune[i], 120);
    delay(150);
  }
  shimmer(5000);
  latch.write(LOCKED);
  Serial.println("Locked again");
}

void wrong() {
  Serial.println("Not this time, try again");
  tone(PIEZO_PIN, 160, 400);
  for (int i = 0; i < 3; i++) {
    gold.fill(gold.Color(40, 0, 0));
    gold.show();
    delay(120);
    gold.clear();
    gold.show();
    delay(120);
  }
}

void setup() {
  Serial.begin(9600);
  latch.attach(LATCH_PIN);
  latch.write(LOCKED);
  gold.begin();
  gold.setBrightness(180);
  gold.show();
  Serial.println("Treasure box locked: type the lucky code, then #");
}

void loop() {
  char key = pad.getKey();
  if (!key) return;
  tone(PIEZO_PIN, 1200, 30);
  if (key == '*') {
    count = 0;
    Serial.println("Cleared");
  } else if (key == '#') {
    typed[count] = 0;
    if (strcmp(typed, CODE) == 0) open();
    else wrong();
    count = 0;
  } else if (count < 8) {
    typed[count++] = key;
    Serial.print("Key ");
    Serial.println(key);
  }
}