Traffic light simulation in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple traffic light sequence

What will be the output of this Raspberry Pi traffic light simulation code snippet?

Raspberry Pi

import time

lights = ['Red', 'Green', 'Yellow']
for light in lights:
    print(f"Light is {light}")
    time.sleep(1)

A

Light is Red
Light is Green
Light is Yellow

B

Light is Red
Light is Yellow
Light is Green

C

Light is Green
Light is Yellow
Light is Red

D

Light is Yellow
Light is Green
Light is Red

💡 Hint

Look at the order of the lights in the list and how the loop prints them.

The code loops through the list in order: Red, Green, then Yellow, printing each light.

🧠 Conceptual

intermediate

Understanding GPIO pin setup for traffic lights

Which statement correctly describes the purpose of setting GPIO pins as outputs in a Raspberry Pi traffic light program?

A To read the current state of the traffic light buttons

B To send signals to turn the LEDs on or off

C To power the Raspberry Pi board

D To connect the Raspberry Pi to the internet

💡 Hint

Think about what output pins do in controlling LEDs.

Output pins send electrical signals to devices like LEDs to control their state.

🔧 Debug

advanced

Identify the error in this traffic light timing code

What error will this Raspberry Pi traffic light code produce when run?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.OUT)

try:
    while True:
        GPIO.output(17, True)
        time.sleep(2)
        GPIO.output(17, False)
        time.sleep('2')
finally:
    GPIO.cleanup()

A No error, the code runs correctly

B SyntaxError due to missing colon after while True

C RuntimeError because GPIO pin 17 is not set as output

D TypeError because time.sleep() received a string instead of a number

💡 Hint

Check the argument types passed to time.sleep.

time.sleep() requires a number (int or float), but '2' is a string causing a TypeError.

📝 Syntax

advanced

Correct syntax for turning on multiple LEDs

Which option correctly turns on GPIO pins 17, 27, and 22 to simulate a traffic light?

A GPIO.output([17, 27, 22], True)

B GPIO.output({17, 27, 22}, True)

C GPIO.output(17, True); GPIO.output(27, True); GPIO.output(22, True)

D GPIO.output(17 and 27 and 22, True)

💡 Hint

Check how GPIO.output accepts pin numbers.

RPi.GPIO.output accepts a single pin or a list/tuple of pins to set multiple at once. A uses a list, correct. B uses set, unsupported (TypeError). C works but multiple calls. D invalid.

🚀 Application

expert

Calculate total cycle time of traffic light simulation

Given this traffic light timing code, what is the total time in seconds for one full cycle?

Raspberry Pi

import time

sequence = [
    ('Red', 5),
    ('Green', 3),
    ('Yellow', 2)
]

def run_cycle():
    for color, duration in sequence:
        print(f"Light: {color}")
        time.sleep(duration)

run_cycle()

A 15 seconds

B 5 seconds

C 10 seconds

D 20 seconds

💡 Hint

Add the durations for all colors in the sequence.

The total cycle time is 5 + 3 + 2 = 10 seconds.