Object counter with an E18-D80NK on an Arduino Uno

// Object counter: an E18-D80NK infrared proximity sensor counts what passes
// it, on a 16x2 I2C LCD, with a beep from an active buzzer for each one.
//
//   2       the E18-D80NK's black wire, OUT (brown on 5 V, blue on GND)
//   8       the active buzzer's plus leg, its other leg on GND
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// The E18-D80NK sends a beam of modulated infrared out of its lens and looks
// for it coming back. Anything inside its range, set by the screw on its
// back (3 to 80 cm, 40 here), pulls OUT low. OUT is an NPN transistor's
// collector and nothing else: the sensor has no pull-up of its own, so the
// pin's INPUT_PULLUP holds it high while nothing is there. A loose input pin
// would count noise.
//
// One object is one high-to-low change on the pin, not every reading that is
// low, so the count goes up once however long a box stands in the beam. A
// box that rocks on the edge of the beam could still count twice, so a new
// count needs the pin to have been high for 50 ms first.
//
// Press Run, then drag the target closer than 40 cm on the sensor and back.

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

const int SENSOR_PIN = 2;
const int BUZZER_PIN = 8;
const unsigned long CLEAR_MS = 50;  // the beam must be clear this long between objects

LiquidCrystal_I2C lcd(0x27, 16, 2);
unsigned long count = 0;
bool present = false;
unsigned long lastBlocked = 0;
unsigned long beepUntil = 0;

void show() {
  lcd.setCursor(0, 1);
  lcd.print("Count: ");
  lcd.print(count);
  lcd.print("     ");
}

void setup() {
  pinMode(SENSOR_PIN, INPUT_PULLUP);
  pinMode(BUZZER_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Object counter");
  lcd.init();
  lcd.backlight();
  lcd.print("Object counter");
  show();
}

void loop() {
  unsigned long now = millis();
  bool blocked = digitalRead(SENSOR_PIN) == LOW;

  if (blocked) {
    // A new object: the beam was clear a moment ago, and for long enough.
    if (!present && now - lastBlocked >= CLEAR_MS) {
      count++;
      // The beep first: writing the LCD over I2C takes tens of milliseconds.
      digitalWrite(BUZZER_PIN, HIGH);
      beepUntil = now + 100;
      show();
      Serial.print("Object ");
      Serial.println(count);
    }
    present = true;
    lastBlocked = now;
  } else {
    present = false;
  }

  if (beepUntil != 0 && now >= beepUntil) {
    digitalWrite(BUZZER_PIN, LOW);
    beepUntil = 0;
  }
}