Bidirectional visitor counter on an Arduino Uno

// Bidirectional visitor counter: two IR obstacle sensors across a doorway
// tell people coming in from people going out, an I2C LCD shows the count,
// and a relay keeps the room light on while anyone is inside.
//
//   sensor A   the outer FC-51 (the corridor side): OUT on pin 2, VCC 5V, GND
//   sensor B   the inner FC-51 (the room side):     OUT on pin 3, VCC 5V, GND
//   LCD        16x2 with an I2C backpack at 0x27: SDA A4, SCL A5, VCC 5V, GND
//   relay      IN on pin 7 (high trigger), VCC 5V, GND
//              COM to the 12 V adapter's +, NO to the lamp, the lamp's other
//              lead to the adapter's - (which also joins the Uno's GND)
//
// HOW IT WORKS
//   An FC-51 pulls OUT LOW while something is in front of it. Someone coming
//   in breaks A first and then B; someone going out breaks B first and then
//   A. The sketch remembers which beam broke first, counts the person when
//   the other one breaks, and then waits for both beams to be clear before
//   it looks for the next person, so one person is counted once.
//
//   A beam that breaks and nothing follows within PASS_MS (someone who
//   looked in and turned back) is forgotten. Going out of an empty room is
//   not counted, so the count inside never goes below zero.
//
//   The light is on while the room count is above zero and goes off when the
//   last person leaves.
//
// WHAT TO CHANGE
//   PASS_MS   how long a person may take from one beam to the other
//   Mount the two sensors a hand's width apart, at waist height, so one
//   person cannot break both at the same instant.

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

LiquidCrystal_I2C lcd(0x27, 16, 2);

const int SENSOR_A = 2;   // outside
const int SENSOR_B = 3;   // inside
const int RELAY_PIN = 7;

const unsigned long PASS_MS = 1500;

int entered = 0;
int left = 0;
int inside = 0;

char first = 0;           // 'A' or 'B': the beam that broke first, or 0
unsigned long firstAt = 0;
bool waitClear = false;

bool blocked(int pin) {
  return digitalRead(pin) == LOW;
}

void show() {
  lcd.setCursor(0, 0);
  lcd.print("IN ");
  lcd.print(entered);
  lcd.print("  OUT ");
  lcd.print(left);
  lcd.print("     ");
  lcd.setCursor(0, 1);
  lcd.print("Inside: ");
  lcd.print(inside);
  lcd.print(inside > 0 ? "  light" : "       ");
  digitalWrite(RELAY_PIN, inside > 0 ? HIGH : LOW);
}

void report(const char *what) {
  Serial.print(what);
  Serial.print("  in ");
  Serial.print(entered);
  Serial.print("  out ");
  Serial.print(left);
  Serial.print("  inside ");
  Serial.println(inside);
}

void setup() {
  Serial.begin(115200);
  pinMode(SENSOR_A, INPUT);
  pinMode(SENSOR_B, INPUT);
  digitalWrite(RELAY_PIN, LOW);
  pinMode(RELAY_PIN, OUTPUT);
  lcd.init();
  lcd.backlight();
  show();
  Serial.println("visitor counter ready");
}

void loop() {
  bool a = blocked(SENSOR_A);
  bool b = blocked(SENSOR_B);

  if (waitClear) {
    if (!a && !b) waitClear = false;
  } else if (first == 0) {
    if (a && !b) first = 'A';
    else if (b && !a) first = 'B';
    if (first) firstAt = millis();
  } else if (first == 'A' && b) {
    entered++;
    inside++;
    report("in");
    show();
    first = 0;
    waitClear = true;
  } else if (first == 'B' && a) {
    if (inside > 0) {
      left++;
      inside--;
      report("out");
      show();
    } else {
      Serial.println("out ignored: nobody was inside");
    }
    first = 0;
    waitClear = true;
  } else if (millis() - firstAt > PASS_MS) {
    first = 0;  // turned back: forget it
    waitClear = true;
  }
  delay(5);
}