FREE SIMULATOR EXAMPLE · INTERMEDIATE

[Pi 5] Traffic Light

This example runs a red/yellow/green traffic-light state machine on the Raspberry Pi 5 — three LEDs on GPIO17/27/22 with a 3-3-1 second cycle. Upload the Python script, run it, and the emulated intersection cycles forever.

Board	Raspberry Pi 5 (Cortex-A76 + RP1 southbridge, QEMU-emulated Linux)
Difficulty	intermediate
Category	basics
Libraries	None — just the Arduino core
Runs in	Sensbench browser simulator — free, no account

The exact circuit you get when you open this example — already wired, ready to run.

The Python script

#!/usr/bin/env python3
# Raspberry Pi 5 - Traffic light state machine
# Wiring: red->GPIO17, yellow->GPIO27, green->GPIO22 (each LED- -> GND).
from gpiozero import LED
from time import sleep

red = LED(17)
yellow = LED(27)
green = LED(22)
print("Traffic light running - Ctrl-C to stop")
try:
    while True:
        red.on(); yellow.off(); green.off()
        print("RED")
        sleep(3)
        red.off(); green.on()
        print("GREEN")
        sleep(3)
        green.off(); yellow.on()
        print("YELLOW")
        sleep(1)
        yellow.off()
except KeyboardInterrupt:
    red.off(); yellow.off(); green.off()
    print("Stopped")

How it works

Three gpiozero LED objects — red on GPIO17, yellow on GPIO27, green on GPIO22 — are driven by an explicit state machine. Each loop iteration sets exactly one LED on and the others off, prints the active color, and sleeps: red 3 s, green 3 s, yellow 1 s. The single-LED-on discipline is the whole logic.

The state machine is written as three explicit transitions rather than a table, which keeps the example beginner-approachable. The KeyboardInterrupt handler turns all three LEDs off on exit — a small courtesy that prevents a real Pi from being left with a glowing red after Ctrl-C.

Before you build it for real

Get the parts

Related examples

FAQ

Why this red, green, yellow sequence?

It is a simplified traffic cycle: red for 3 seconds, green for 3 seconds, yellow for 1 second, then repeat. Real intersections interleave left-turn arrows and all-red clearance phases; the 3-3-1 timing here is the teaching version, long enough to read each state clearly.

Why use three separate LED objects instead of a list?

Named objects (red, yellow, green) make the state machine readable: red.on(); yellow.off(); green.off() reads like English. A list would be more compact but less obvious. Either works; readability wins for a demo whose point is the sequence itself.