Automatic railway level-crossing gate model (Arduino Uno + IR sensors + servo)

// Automatic railway level-crossing gate model — Arduino Uno + 2 IR sensors + SG90 + buzzer + WS2812B.
// Sensor A and sensor B sit on the track on either side of the road. Whichever one sees the train
// first closes the gate (road lights flash red, buzzer beeps). The gate opens only after the train
// has passed the sensor on the OTHER side and that sensor is clear again — so it works for trains
// from either direction.
// IR sensor A OUT → D2, sensor B OUT → D3 (LOW = train), SG90 signal → D9,
// buzzer module I/O → D8, WS2812B DIN → D6 through 330 Ω.
#include <Servo.h>
#include <Adafruit_NeoPixel.h>

constexpr uint8_t SENSOR_A = 2, SENSOR_B = 3, LED_PIN = 6, BUZZER_PIN = 8, SERVO_PIN = 9;
constexpr uint8_t NUM_LEDS = 2;              // the two red road lamps
constexpr uint8_t GATE_OPEN = 90, GATE_CLOSED = 0;   // servo angles — adjust to your boom
constexpr unsigned long CLEAR_MS = 2000;     // exit sensor must stay clear this long
constexpr unsigned long STUCK_MS = 60000;    // no train at the exit after this: warn, stay closed

Servo gate;
Adafruit_NeoPixel lamps(NUM_LEDS, LED_PIN, NEO_GRB + NEO_KHZ800);

enum State : uint8_t { OPEN, CLOSING_WAIT_EXIT, EXIT_SEEN };
State state = OPEN;
uint8_t exitPin = 0;
unsigned long closedAt = 0, exitClearSince = 0;
int angle = GATE_OPEN, target = GATE_OPEN;
unsigned long lastStep = 0, lastBlink = 0;
bool blinkPhase = false, warned = false;

bool train(uint8_t pin) { return digitalRead(pin) == LOW; }

void lampsIdle() {
  lamps.setPixelColor(0, 0x002000);  // dim green: road open
  lamps.setPixelColor(1, 0x002000);
  lamps.show();
}

void setup() {
  pinMode(SENSOR_A, INPUT_PULLUP);  // FC-51 has its own pull-up; this one just keeps an unplugged input HIGH
  pinMode(SENSOR_B, INPUT_PULLUP);
  pinMode(BUZZER_PIN, OUTPUT);
  Serial.begin(115200);
  Serial.println(F("Level crossing ready — gate open. Block sensor A (D2) or B (D3) to send a train."));
  lamps.begin();
  lamps.setBrightness(120);
  lampsIdle();
  gate.attach(SERVO_PIN);
  gate.write(GATE_OPEN);
}

void loop() {
  unsigned long now = millis();

  switch (state) {
    case OPEN:
      if (train(SENSOR_A) || train(SENSOR_B)) {
        exitPin = train(SENSOR_A) ? SENSOR_B : SENSOR_A;
        state = CLOSING_WAIT_EXIT;
        closedAt = now;
        warned = false;
        target = GATE_CLOSED;
        Serial.print(F("Train at sensor "));
        Serial.print(exitPin == SENSOR_B ? 'A' : 'B');
        Serial.println(F(" — closing gate"));
      }
      break;
    case CLOSING_WAIT_EXIT:
      if (train(exitPin)) {
        state = EXIT_SEEN;
        exitClearSince = 0;
        Serial.println(F("Train reached the far sensor"));
      } else if (!warned && now - closedAt > STUCK_MS) {
        // Fail-safe: never open just because time passed. A stopped train is still on the road.
        warned = true;
        Serial.println(F("WARNING: train not seen at the far sensor for 60 s — gate stays closed"));
      }
      break;
    case EXIT_SEEN:
      if (train(exitPin)) exitClearSince = 0;
      else if (!exitClearSince) exitClearSince = now;
      else if (now - exitClearSince >= CLEAR_MS) {
        state = OPEN;
        target = GATE_OPEN;
        Serial.println(F("Last coach gone — opening gate"));
      }
      break;
  }

  // Move the boom 1° every 20 ms (about 2 s end to end) instead of snapping it.
  if (angle != target && now - lastStep >= 20) {
    lastStep = now;
    angle += angle < target ? 1 : -1;
    gate.write(angle);
  }

  // Warning: lamps flash alternately and the buzzer beeps while the gate is closed or moving.
  bool warning = state != OPEN || angle != GATE_OPEN;
  if (warning && now - lastBlink >= 400) {
    lastBlink = now;
    blinkPhase = !blinkPhase;
    lamps.setPixelColor(0, blinkPhase ? 0xFF0000 : 0);
    lamps.setPixelColor(1, blinkPhase ? 0 : 0xFF0000);
    lamps.show();
    // Beep only while the boom is moving and for the first seconds after closing, like a real crossing bell.
    digitalWrite(BUZZER_PIN, blinkPhase && (angle != target || now - closedAt < 5000));
  } else if (!warning && lastBlink) {
    lastBlink = 0;
    digitalWrite(BUZZER_PIN, LOW);
    lampsIdle();
  }
}

// Automatic railway level-crossing gate model — Arduino Uno + 2 IR sensors + SG90 + buzzer + WS2812B.
// Sensor A and sensor B sit on the track on either side of the road. Whichever one sees the train
// first closes the gate (road lights flash red, buzzer beeps). The gate opens only after the train
// has passed the sensor on the OTHER side and that sensor is clear again — so it works for trains
// from either direction.
// IR sensor A OUT → D2, sensor B OUT → D3 (LOW = train), SG90 signal → D9,
// buzzer module I/O → D8, WS2812B DIN → D6 through 330 Ω.
#include <Servo.h>
#include <Adafruit_NeoPixel.h>

constexpr uint8_t SENSOR_A = 2, SENSOR_B = 3, LED_PIN = 6, BUZZER_PIN = 8, SERVO_PIN = 9;
constexpr uint8_t NUM_LEDS = 2;              // the two red road lamps
constexpr uint8_t GATE_OPEN = 90, GATE_CLOSED = 0;   // servo angles — adjust to your boom
constexpr unsigned long CLEAR_MS = 2000;     // exit sensor must stay clear this long
constexpr unsigned long STUCK_MS = 60000;    // no train at the exit after this: warn, stay closed

Servo gate;
Adafruit_NeoPixel lamps(NUM_LEDS, LED_PIN, NEO_GRB + NEO_KHZ800);

enum State : uint8_t { OPEN, CLOSING_WAIT_EXIT, EXIT_SEEN };
State state = OPEN;
uint8_t exitPin = 0;
unsigned long closedAt = 0, exitClearSince = 0;
int angle = GATE_OPEN, target = GATE_OPEN;
unsigned long lastStep = 0, lastBlink = 0;
bool blinkPhase = false, warned = false;

bool train(uint8_t pin) { return digitalRead(pin) == LOW; }

void lampsIdle() {
  lamps.setPixelColor(0, 0x002000);  // dim green: road open
  lamps.setPixelColor(1, 0x002000);
  lamps.show();
}

void setup() {
  pinMode(SENSOR_A, INPUT_PULLUP);  // FC-51 has its own pull-up; this one just keeps an unplugged input HIGH
  pinMode(SENSOR_B, INPUT_PULLUP);
  pinMode(BUZZER_PIN, OUTPUT);
  Serial.begin(115200);
  Serial.println(F("Level crossing ready — gate open. Block sensor A (D2) or B (D3) to send a train."));
  lamps.begin();
  lamps.setBrightness(120);
  lampsIdle();
  gate.attach(SERVO_PIN);
  gate.write(GATE_OPEN);
}

void loop() {
  unsigned long now = millis();

  switch (state) {
    case OPEN:
      if (train(SENSOR_A) || train(SENSOR_B)) {
        exitPin = train(SENSOR_A) ? SENSOR_B : SENSOR_A;
        state = CLOSING_WAIT_EXIT;
        closedAt = now;
        warned = false;
        target = GATE_CLOSED;
        Serial.print(F("Train at sensor "));
        Serial.print(exitPin == SENSOR_B ? 'A' : 'B');
        Serial.println(F(" — closing gate"));
      }
      break;
    case CLOSING_WAIT_EXIT:
      if (train(exitPin)) {
        state = EXIT_SEEN;
        exitClearSince = 0;
        Serial.println(F("Train reached the far sensor"));
      } else if (!warned && now - closedAt > STUCK_MS) {
        // Fail-safe: never open just because time passed. A stopped train is still on the road.
        warned = true;
        Serial.println(F("WARNING: train not seen at the far sensor for 60 s — gate stays closed"));
      }
      break;
    case EXIT_SEEN:
      if (train(exitPin)) exitClearSince = 0;
      else if (!exitClearSince) exitClearSince = now;
      else if (now - exitClearSince >= CLEAR_MS) {
        state = OPEN;
        target = GATE_OPEN;
        Serial.println(F("Last coach gone — opening gate"));
      }
      break;
  }

  // Move the boom 1° every 20 ms (about 2 s end to end) instead of snapping it.
  if (angle != target && now - lastStep >= 20) {
    lastStep = now;
    angle += angle < target ? 1 : -1;
    gate.write(angle);
  }

  // Warning: lamps flash alternately and the buzzer beeps while the gate is closed or moving.
  bool warning = state != OPEN || angle != GATE_OPEN;
  if (warning && now - lastBlink >= 400) {
    lastBlink = now;
    blinkPhase = !blinkPhase;
    lamps.setPixelColor(0, blinkPhase ? 0xFF0000 : 0);
    lamps.setPixelColor(1, blinkPhase ? 0 : 0xFF0000);
    lamps.show();
    // Beep only while the boom is moving and for the first seconds after closing, like a real crossing bell.
    digitalWrite(BUZZER_PIN, blinkPhase && (angle != target || now - closedAt < 5000));
  } else if (!warning && lastBlink) {
    lastBlink = 0;
    digitalWrite(BUZZER_PIN, LOW);
    lampsIdle();
  }
}