Knock lock on the book’s pins, on an Arduino Uno

// Knock lock: three knocks open it, the button locks it again.
//
//   knock sensor  a force sensor from 5V to A0, and 10k from A0 to GND
//   button        pin 2, with 10k to GND (pressed reads HIGH)
//   yellow LED    pin 3, through 220 ohm: flashes for every knock heard
//   green LED     pin 4, through 220 ohm: unlocked
//   red LED       pin 5, through 220 ohm: locked
//   servo         signal on pin 9: the bolt, 0 degrees open, 90 locked
//
// On the bench the knock sensor is a piezo disc on A0 with a 1 M resistor
// across it: a knock bends the disc and it makes a small voltage. Mokxi's
// piezo only plays sound, so a force sensor stands in for it here, on the
// same pin and with the same code: press it once for each knock.
//
// The sketch starts unlocked. Press the button to lock: the bolt turns and
// the red LED lights. Then knock three times and the lock opens again.

#include <Servo.h>

const int SENSOR = A0;
const int BUTTON = 2;
const int YELLOW = 3;
const int GREEN = 4;
const int RED = 5;
const int KNOCK_LEVEL = 100;  // a reading over this counts as a knock
const int KNOCKS_TO_OPEN = 3;

Servo bolt;
bool locked = false;
int knocks = 0;
bool wasPressed = false;

void lock() {
  locked = true;
  knocks = 0;
  bolt.write(90);
  digitalWrite(GREEN, LOW);
  digitalWrite(RED, HIGH);
  Serial.println("locked");
}

void unlock() {
  locked = false;
  bolt.write(0);
  digitalWrite(GREEN, HIGH);
  digitalWrite(RED, LOW);
  Serial.println("unlocked");
}

void setup() {
  pinMode(BUTTON, INPUT);
  pinMode(YELLOW, OUTPUT);
  pinMode(GREEN, OUTPUT);
  pinMode(RED, OUTPUT);
  bolt.attach(9);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: knock lock. Press the button to lock, knock three times to open.");
  unlock();
}

void loop() {
  if (!locked) {
    if (digitalRead(BUTTON) == HIGH) lock();
    return;
  }
  bool pressed = analogRead(SENSOR) > KNOCK_LEVEL;
  if (pressed && !wasPressed) {
    knocks++;
    digitalWrite(YELLOW, HIGH);
    Serial.print("knock ");
    Serial.println(knocks);
    delay(50);
    digitalWrite(YELLOW, LOW);
    if (knocks >= KNOCKS_TO_OPEN) unlock();
  }
  wasPressed = pressed;
  delay(10);
}