Traffic Light

A traffic light is one of the most classic projects in physical computing. It uses three LEDs — red, yellow and green — cycling in a well-defined order. It is a great next step after learning how to blink and alternate LEDs, because the logic is a small state machine instead of a simple loop.

In this project we build a functioning traffic light using a Raspberry Pi and Pi4J.

Source code: projects/traffic-light/TrafficLight.java

What You Will Learn

How to control three independent LEDs from Java
How to model a small state machine with GPIO output
How different timings shape the "feel" of a traffic light
How to organize repeatable phases in a Pi4J program

Components

Raspberry Pi
1 red LED
1 yellow LED
1 green LED
3 × 220Ω resistors
Breadboard
Jumper wires

Wiring

Connect each LED's anode (longer leg) to a distinct GPIO pin through a 220Ω resistor.
Connect all cathodes (shorter legs) to any GND pin on the Pi.
Double-check the polarity before powering the Pi.

How It Works

The program configures three GPIO pins as digital outputs. It then cycles through the classic phases:

Green — traffic can pass. Long duration.
Yellow — get ready to stop. Short duration.
Red — traffic stops. Long duration.
Back to green.

At any given moment only one LED is on. When a phase ends, its LED is turned off before the next one is turned on, exactly like a real traffic light controller.

Possible Enhancements

Add a pedestrian button that requests a red phase
Add a second traffic light for a crossing (they should always be complementary)
Blink the green a few times before turning yellow
Add a countdown displayed on an LCD or a set of extra LEDs
Make timings configurable from a properties file

Why It Matters

Traffic lights teach a fundamental idea in embedded systems: at any moment, the hardware is in exactly one state, and each state has a defined output and a defined transition. Once you internalize that, you can drive far more complex devices — vending machines, elevators, robots — with the same mental model.