Traffic light simulation in Raspberry Pi - Step-by-Step Execution

Concept Flow - Traffic light simulation

Start Simulation

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Turn RED ON

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Wait RED duration

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Turn RED OFF, Turn GREEN ON

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Wait GREEN duration

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Turn GREEN OFF, Turn YELLOW ON

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Wait YELLOW duration

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Turn YELLOW OFF

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Repeat cycle

The traffic light cycles through red, green, and yellow lights with waits in between, repeating continuously.

Execution Sample

Raspberry Pi

import time
from gpiozero import LED

red = LED(17)
green = LED(27)
yellow = LED(22)

while True:
    red.on()
    time.sleep(5)
    red.off()
    green.on()
    time.sleep(5)
    green.off()
    yellow.on()
    time.sleep(2)
    yellow.off()

This code turns on each traffic light color for a set time, then switches to the next color in a loop.

Execution Table

Step	Action	LED State (Red, Green, Yellow)	Wait (seconds)	Next Step
1	Turn RED ON	Red=ON, Green=OFF, Yellow=OFF	5	2
2	Turn RED OFF, GREEN ON	Red=OFF, Green=ON, Yellow=OFF	5	3
3	Turn GREEN OFF, YELLOW ON	Red=OFF, Green=OFF, Yellow=ON	2	4
4	Turn YELLOW OFF	Red=OFF, Green=OFF, Yellow=OFF	0	1 (repeat)

💡 Simulation runs infinitely; cycle repeats after step 4.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
red	OFF	ON	OFF	OFF	OFF
green	OFF	OFF	ON	OFF	OFF
yellow	OFF	OFF	OFF	ON	OFF

Key Moments - 3 Insights

Why does the red light turn off before the green light turns on?

Because the code explicitly calls red.off() before green.on() in step 2, ensuring only one light is on at a time as shown in execution_table row 2.

What happens if we remove the time.sleep() calls?

Without time.sleep(), the lights would change instantly without delay, so the lights would appear to flicker or all turn off quickly, losing the traffic light effect (see Wait column in execution_table).

Why does the simulation never stop?

Because the while True loop repeats the steps endlessly, as noted in the exit_note and the Next Step column looping back to step 1.

Visual Quiz - 3 Questions

Look at the execution_table, what is the LED state after step 3?

A Red=OFF, Green=OFF, Yellow=ON

B Red=ON, Green=OFF, Yellow=OFF

C Red=OFF, Green=ON, Yellow=OFF

D All OFF

💡 Hint

Check the LED State column in execution_table row 3.

At which step does the green light turn on?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the Action column in execution_table where green.on() is called.

If the time.sleep(5) after turning red on is changed to time.sleep(3), what changes in variable_tracker?

A Green variable stays ON longer

B No change in variable values, only timing changes

C Red variable changes to OFF earlier

D Yellow variable turns ON earlier

💡 Hint

Variable_tracker shows LED states, not timing; timing changes do not affect ON/OFF states directly.

Concept Snapshot

Traffic light simulation cycles through red, green, yellow LEDs.
Use GPIO pins to control LEDs on Raspberry Pi.
Turn one LED ON, wait, then turn OFF and switch to next.
Use time.sleep() to pause between changes.
Repeat cycle endlessly with while True loop.

Full Transcript

This traffic light simulation code runs on a Raspberry Pi using GPIO pins to control three LEDs representing red, green, and yellow lights. The program turns the red LED on, waits 5 seconds, then turns it off and turns the green LED on for 5 seconds. Next, it turns green off and yellow on for 2 seconds, then turns yellow off. This cycle repeats forever. The execution table shows each step with LED states and wait times. The variable tracker shows how each LED's state changes after each step. Key moments clarify why lights turn off before the next turns on, the importance of delays, and the infinite loop. The quiz tests understanding of LED states at steps, when green turns on, and effects of changing wait times.