Example project · Microcontrollers

Traffic light + pedestrian crossing

A real intersection controller on the Uno: red, yellow and green run the cars while a WALK lamp waits its turn. CLICK AND HOLD THE BUTTON to request a crossing; green yields after its minimum time, yellow bridges, then WALK lights while the cars sit at red and flashes out before green returns. A polled state machine with an input pull-up button, narrated phase by phase on the serial monitor. The Code tab holds the same logic as an editable Arduino sketch.

Open this project in the simulator →All example projects

What's on the bench

4× LED
4× Resistor
Arduino Uno
Battery
Button

The code

This Arduino C++ sketch lives in the Code tab; sign in and press Compile & upload to build real firmware for the emulated board.

// The same crossing controller as an Arduino sketch. Edit it, then
// Compile & upload (sign-in needed): the server builds real AVR firmware.
const int RED_PIN = 12, YELLOW_PIN = 11, GREEN_PIN = 10;
const int WALK_PIN = 8, BUTTON_PIN = 2;

void lights(bool r, bool y, bool g, bool walk) {
  digitalWrite(RED_PIN, r);
  digitalWrite(YELLOW_PIN, y);
  digitalWrite(GREEN_PIN, g);
  digitalWrite(WALK_PIN, walk);
}

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(YELLOW_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
  pinMode(WALK_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);  // the button closes to ground
  Serial.begin(9600);
}

void loop() {
  lights(false, false, true, false);
  Serial.println("GREEN  cars flow");
  unsigned long start = millis();
  bool requested = false;
  // Green holds at least 1.5 s, then until someone asks to cross.
  while (millis() - start < 1500 || !requested) {
    if (digitalRead(BUTTON_PIN) == LOW) requested = true;
    delay(10);
  }
  lights(false, true, false, false);
  Serial.println("YELLOW  crossing requested");
  delay(1000);
  lights(true, false, false, true);
  Serial.println("RED  WALK on");
  delay(3000);
  Serial.println("WALK flashing  clear the road");
  for (int i = 0; i < 4; i++) {
    digitalWrite(WALK_PIN, LOW);
    delay(250);
    digitalWrite(WALK_PIN, HIGH);
    delay(250);
  }
  digitalWrite(WALK_PIN, LOW);
}