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.

Practice

1. What is the main purpose of using GPIO pins in a Raspberry Pi traffic light simulation?

easy

A. To control the LEDs turning on and off

B. To increase the Raspberry Pi processing speed

C. To connect the Raspberry Pi to the internet

D. To store the traffic light timing data

Solution

1. Step 1: Understand GPIO pin function

   GPIO pins on Raspberry Pi are used to send signals to external devices like LEDs.

2. Step 2: Connect GPIO pins to LEDs

   In traffic light simulation, GPIO pins turn LEDs on or off to show red, yellow, or green lights.

3. Final Answer:

   To control the LEDs turning on and off -> Option A

4. Quick Check:

   GPIO pins control LEDs = A [OK]

Hint: GPIO pins control hardware like LEDs in simulations [OK]

Common Mistakes:

• Thinking GPIO speeds up the CPU
• Confusing GPIO with internet connection
• Assuming GPIO stores data

2. Which of the following Python code snippets correctly sets up GPIO pin 17 as an output for the traffic light LED?

easy

A. GPIO.output(17, GPIO.IN)

B. GPIO.setup(17, GPIO.OUT)

C. GPIO.setup(17, GPIO.IN)

D. GPIO.output(17, GPIO.OUT)

Solution

1. Step 1: Identify correct GPIO setup function

   GPIO.setup(pin_number, mode) is used to set pin mode; mode can be GPIO.OUT or GPIO.IN.

2. Step 2: Choose correct mode for LED output

   LEDs need output mode, so GPIO.OUT is correct for pin 17.

3. Final Answer:

   GPIO.setup(17, GPIO.OUT) -> Option B

4. Quick Check:

   Setup pin as output = B [OK]

Hint: Use GPIO.setup(pin, GPIO.OUT) to set output pins [OK]

Common Mistakes:

• Using GPIO.output instead of GPIO.setup to set mode
• Setting pin as GPIO.IN instead of GPIO.OUT
• Confusing setup and output functions

3. What will be the output behavior of this Python code snippet controlling a traffic light LED on GPIO 18?

GPIO.output(18, GPIO.HIGH)
time.sleep(3)
GPIO.output(18, GPIO.LOW)

medium

A. The LED on pin 18 stays off permanently

B. The LED on pin 18 blinks rapidly for 3 seconds

C. The program will cause a syntax error

D. The LED on pin 18 turns on for 3 seconds, then turns off

Solution

1. Step 1: Analyze GPIO.output commands

   GPIO.HIGH turns the LED on; GPIO.LOW turns it off.

2. Step 2: Understand time.sleep effect

   time.sleep(3) pauses the program for 3 seconds while LED stays on.

3. Final Answer:

   The LED on pin 18 turns on for 3 seconds, then turns off -> Option D

4. Quick Check:

   HIGH then sleep then LOW = C [OK]

Hint: HIGH turns LED on, LOW turns it off; sleep pauses timing [OK]

Common Mistakes:

• Thinking LED blinks rapidly without loop
• Assuming syntax error due to time.sleep
• Confusing HIGH with LOW states

4. Identify the error in this traffic light simulation code snippet:

GPIO.setup(22, GPIO.OUT)
GPIO.output(22, HIGH)
time.sleep(2)
GPIO.output(22, LOW)

medium

A. HIGH and LOW should be prefixed with GPIO.

B. GPIO.setup should use GPIO.IN instead of GPIO.OUT

C. time.sleep cannot be used with GPIO.output

D. Pin 22 cannot be used for output

Solution

1. Step 1: Check usage of HIGH and LOW constants

   HIGH and LOW must be written as GPIO.HIGH and GPIO.LOW to be recognized.

2. Step 2: Verify other code parts

   GPIO.setup with GPIO.OUT is correct; time.sleep works fine; pin 22 is valid.

3. Final Answer:

   HIGH and LOW should be prefixed with GPIO. -> Option A

4. Quick Check:

   HIGH/LOW require GPIO. prefix = D [OK]

Hint: Always prefix HIGH/LOW with GPIO. to avoid NameError [OK]

Common Mistakes:

• Omitting GPIO. prefix for HIGH/LOW
• Changing pin mode incorrectly
• Thinking time.sleep causes error

5. You want to simulate a traffic light cycle with red, yellow, and green LEDs connected to GPIO pins 5, 6, and 13 respectively. Which Python code snippet correctly cycles through the lights with 2 seconds red, 1 second yellow, and 3 seconds green, repeating forever?

hard

A. GPIO.output(5, GPIO.HIGH)
GPIO.output(6, GPIO.HIGH)
GPIO.output(13, GPIO.HIGH)
time.sleep(6)
GPIO.output(5, GPIO.LOW)
GPIO.output(6, GPIO.LOW)
GPIO.output(13, GPIO.LOW)

B. for light in [5,6,13]:
GPIO.output(light, GPIO.HIGH)
time.sleep(2)
GPIO.output(light, GPIO.LOW)

C. while True:
GPIO.output(5, GPIO.HIGH)
time.sleep(2)
GPIO.output(5, GPIO.LOW)
GPIO.output(6, GPIO.HIGH)
time.sleep(1)
GPIO.output(6, GPIO.LOW)
GPIO.output(13, GPIO.HIGH)
time.sleep(3)
GPIO.output(13, GPIO.LOW)

D. while True:
GPIO.output(13, GPIO.HIGH)
time.sleep(3)
GPIO.output(13, GPIO.LOW)
GPIO.output(6, GPIO.HIGH)
time.sleep(1)
GPIO.output(6, GPIO.LOW)
GPIO.output(5, GPIO.HIGH)
time.sleep(2)
GPIO.output(5, GPIO.LOW)

Solution

1. Step 1: Check order and timing of lights

   Traffic lights cycle red (2s), yellow (1s), green (3s) in that order repeatedly.

2. Step 2: Verify code matches timing and order

   while True: GPIO.output(5, GPIO.HIGH) time.sleep(2) GPIO.output(5, GPIO.LOW) GPIO.output(6, GPIO.HIGH) time.sleep(1) GPIO.output(6, GPIO.LOW) GPIO.output(13, GPIO.HIGH) time.sleep(3) GPIO.output(13, GPIO.LOW) uses while True loop, turns on red (pin 5) for 2s, then yellow (pin 6) for 1s, then green (pin 13) for 3s, turning off each after delay.

3. Final Answer:

   while True loop with red (2s), yellow (1s), green (3s) -> Option C

4. Quick Check:

   Correct order and timing in loop = A [OK]

Hint: Use while True and sequence GPIO.output with time.sleep [OK]

Common Mistakes:

• Wrong order of lights
• Using for loop without timing per light
• Turning all LEDs on at once