Build a Traffic Light Simulator with Arduino on Velxio

Introduction

Ever wanted to build a traffic light but didn’t have the components handy? With Velxio, you can simulate an entire Arduino-based traffic light right in your browser. This project uses three LEDs (red, yellow, green) connected to an Arduino Uno to mimic the classic traffic light sequence: red → yellow → green → yellow → red. It’s a perfect beginner project to learn digital outputs, timing, and sequential logic.

Circuit Walkthrough

Let’s look at the circuit. The diagram below shows the complete wiring:

Circuit diagram of traffic-light on the Velxio simulator canvas

Components

Arduino Uno – the brain of the operation.
Red LED – connected to digital pin 13.
Yellow LED – connected to digital pin 12.
Green LED – connected to digital pin 11.
Resistors – each LED needs a current-limiting resistor (typically 220Ω) in series with its anode. In Velxio, the built-in LEDs already have internal resistors, so you don’t need to add them manually.

Wiring Details

Each LED has two legs: anode (longer leg, marked as pin “A”) and cathode (shorter leg, marked as pin “C”). The wiring is straightforward:

Red LED: Anode to Arduino pin 13 via a red wire (#ff0000). Cathode to Arduino GND via a black wire.
Yellow LED: Anode to Arduino pin 12 via a yellow wire (#ffaa00). Cathode to GND.
Green LED: Anode to Arduino pin 11 via a green wire (#00ff00). Cathode to GND.

All three LEDs share the same GND connection on the Arduino. This is a common ground configuration.

Code Walkthrough

Now let’s examine the code that makes the lights change. Open the code editor in Velxio:

Source code for traffic-light in the Velxio Monaco editor

// Traffic Light Simulator
// Red -> Yellow -> Green -> Yellow -> Red

const int RED_PIN = 13;
const int YELLOW_PIN = 12;
const int GREEN_PIN = 11;

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(YELLOW_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
}

void loop() {
  // Red light
  digitalWrite(RED_PIN, HIGH);
  delay(3000);
  digitalWrite(RED_PIN, LOW);

  // Yellow light
  digitalWrite(YELLOW_PIN, HIGH);
  delay(1000);
  digitalWrite(YELLOW_PIN, LOW);

  // Green light
  digitalWrite(GREEN_PIN, HIGH);
  delay(3000);
  digitalWrite(GREEN_PIN, LOW);

  // Yellow light again
  digitalWrite(YELLOW_PIN, HIGH);
  delay(1000);
  digitalWrite(YELLOW_PIN, LOW);
}

How It Works

Pin Definitions: We define constants for the three pins: RED_PIN = 13, YELLOW_PIN = 12, GREEN_PIN = 11. These match the wiring.
Setup: In setup(), we set all three pins as OUTPUT using pinMode(). This tells the Arduino to send voltage out through these pins.
Loop: The loop() function runs forever. It follows this sequence:
- Turn red LED on (HIGH), wait 3 seconds, turn it off (LOW).
- Turn yellow LED on, wait 1 second, turn it off.
- Turn green LED on, wait 3 seconds, turn it off.
- Turn yellow LED on again, wait 1 second, turn it off.
- Then the loop repeats from the red light.

This creates the familiar traffic light pattern.

Key Concepts for Students

Digital Outputs: digitalWrite(pin, HIGH) sets a pin to 5V (on), LOW sets it to 0V (off).
Timing with delay(): delay(milliseconds) pauses the program. Be careful: while delay() runs, the Arduino can’t do anything else.
Sequential Logic: The order of operations matters. The code runs line by line, so the lights change in a predictable sequence.
Common Ground: All LEDs share the same ground connection. This is typical in digital circuits.

Common Pitfalls and Debugging Tips

Wrong Pin Numbers: Double-check that the pin numbers in the code match the wiring. If the red LED is on pin 13 but you wrote 12, it won’t light.
LED Orientation: LEDs are polarized. The anode (long leg) must connect to the Arduino pin, and the cathode (short leg) to GND. In Velxio, the LED component is correctly oriented by default.
Missing Resistors: In real life, always use a current-limiting resistor (220Ω–1kΩ) in series with each LED to prevent damage. Velxio’s built-in LEDs include resistors, but if you use custom components, add them.
delay() Blocks: If you want to add buttons or sensors later, delay() will freeze the program. Consider using millis() for non-blocking timing.

Suggested Extensions

Once you have the basic traffic light working, try these enhancements:

Add a Pedestrian Button: Use a push button to request a walk signal. When pressed, the traffic light changes to red, then a white LED (pedestrian walk) lights up.
Use millis() Instead of delay(): Rewrite the code to use millis() for timing. This allows the Arduino to check for button presses while waiting.
Add a Buzzer: Connect a piezo buzzer to a pin and make it beep when the green light is about to turn yellow.
Simulate a Crosswalk: Add two more LEDs (red and green) for a pedestrian crossing, and coordinate them with the traffic lights.

Try It Yourself

Ready to build your own traffic light? Open the example in Velxio and start experimenting:

Open Traffic Light Example on Velxio

You can edit the code, change the timing, or add more LEDs. The simulator runs entirely in your browser – no downloads, no hardware. Use the toolbar to compile and run your code:

Velxio editor toolbar with compile and run controls