Keypad lock on an Arduino Uno

// Keypad lock: type the code, the relay clicks, the door opens.
//
//   pins 9, 8, 7, 6   keypad rows R1 to R4
//   pins 5, 4, 3, 2   keypad columns C1 to C4
//   pin 10, 11, 12    the RGB LED's red, green and blue legs, 220 ohm each
//   pin 13            the relay module's IN (VCC to 5 V, GND to GND)
//
// The code is 1234 and # sends it. Get it right and the LED goes green and
// the relay pulls in for three seconds, which on a real door is the strike
// plate letting go. Get it wrong and the LED flashes red. * clears what you
// have typed, and every key you press blinks blue so you know it landed.
//
// # Scanning a keypad
//
// A keypad has no chip in it. Under each key is a contact that shorts one of
// four row wires to one of four column wires, so the sketch has to ask: drive
// one row low, read the four columns with their pull-ups on, and a column that
// reads low is a key held in that row. mokxi_keypad.h does that scan and
// hands back one character per press.
//
// # The relay
//
// The board pin cannot switch a door strike, a lamp or a motor on its own, and
// it should not try. The relay module takes a logic level on IN and closes a
// contact that has nothing to do with the board's 5 V. COM is wired to the
// supply and NO to the door lamp, so the lamp lights only while the relay is
// pulled in.

#include "mokxi_keypad.h"

const uint8_t ROWS[4] = {9, 8, 7, 6};
const uint8_t COLS[4] = {5, 4, 3, 2};

const int RED = 10;
const int GREEN = 11;
const int BLUE = 12;
const int RELAY = 13;

const char CODE[] = "1234";
const int CODE_LEN = 4;
const int OPEN_MS = 3000;

Keypad pad(ROWS, COLS);

char typed[8];
int count = 0;

static void setColor(int r, int g, int b) {
  digitalWrite(RED, r);
  digitalWrite(GREEN, g);
  digitalWrite(BLUE, b);
}

static void clearTyped() {
  count = 0;
  typed[0] = 0;
}

static bool matches() {
  if (count != CODE_LEN) return false;
  for (int i = 0; i < CODE_LEN; i++) {
    if (typed[i] != CODE[i]) return false;
  }
  return true;
}

static void open() {
  Serial.println("code accepted: door open");
  setColor(LOW, HIGH, LOW);
  digitalWrite(RELAY, HIGH);
  delay(OPEN_MS);
  digitalWrite(RELAY, LOW);
  setColor(HIGH, LOW, LOW);
  Serial.println("locked again");
}

static void refuse() {
  Serial.println("wrong code");
  for (int i = 0; i < 3; i++) {
    setColor(LOW, LOW, LOW);
    delay(120);
    setColor(HIGH, LOW, LOW);
    delay(120);
  }
}

void setup() {
  pinMode(RELAY, OUTPUT);
  pinMode(RED, OUTPUT);
  pinMode(GREEN, OUTPUT);
  pinMode(BLUE, OUTPUT);
  digitalWrite(RELAY, LOW);
  setColor(HIGH, LOW, LOW);
  pad.begin();
  Serial.begin(115200);
  Serial.println("Mokxi Uno: keypad lock. Type 1234 then #.");
  clearTyped();
}

void loop() {
  char c = pad.pressed();
  if (c == 0) {
    delay(5);
    return;
  }

  // A blue wink on every key, so a press that lands is a press you can see.
  setColor(LOW, LOW, HIGH);
  delay(60);
  setColor(HIGH, LOW, LOW);

  if (c == '#') {
    if (matches()) {
      open();
    } else {
      refuse();
    }
    clearTyped();
    return;
  }
  if (c == '*') {
    Serial.println("cleared");
    clearTyped();
    return;
  }
  if (count < 7) {
    typed[count++] = c;
    typed[count] = 0;
  }
  Serial.print("typed ");
  Serial.println(typed);
}