LED toggle with button in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when the button is pressed once?

Consider this Python code running on a Raspberry Pi. It toggles an LED on or off each time a button is pressed.

import RPi.GPIO as GPIO
import time

LED_PIN = 18
BUTTON_PIN = 17

GPIO.setmode(GPIO.BCM)
GPIO.setup(LED_PIN, GPIO.OUT)
GPIO.setup(BUTTON_PIN, GPIO.IN, pull_up_down=GPIO.PUD_UP)

led_state = False

while True:
    button_state = GPIO.input(BUTTON_PIN)
    if button_state == GPIO.LOW:
        led_state = not led_state
        GPIO.output(LED_PIN, led_state)
        print(f"LED is {'ON' if led_state else 'OFF'}")
        time.sleep(0.3)  # debounce delay
    time.sleep(0.1)

What will be printed after pressing the button once?

A No output

B LED is OFF

C LED is ON

D

LED is ON
LED is OFF

💡 Hint

Think about the initial state of the LED and what toggling means.

The LED starts off (False). Pressing the button toggles it to True, so the output is "LED is ON".

🧠 Conceptual

intermediate

Why is there a delay after detecting the button press?

In the LED toggle code, there is a time.sleep(0.3) after toggling the LED when the button is pressed. Why is this delay important?

A To wait for the LED to physically turn on

B To keep the LED on for 0.3 seconds

C To reduce CPU usage by slowing the loop

D To debounce the button and avoid multiple toggles from one press

💡 Hint

Think about what happens when a mechanical button is pressed and released quickly.

Mechanical buttons can cause multiple rapid signals (bounces) when pressed. The delay helps ignore these extra signals so the LED toggles only once per press.

🔧 Debug

advanced

What error will this code produce?

Look at this code snippet for toggling an LED with a button on Raspberry Pi:

import RPi.GPIO as GPIO
import time

LED_PIN = 18
BUTTON_PIN = 17

GPIO.setmode(GPIO.BCM)
GPIO.setup(LED_PIN, GPIO.OUT)
GPIO.setup(BUTTON_PIN, GPIO.IN)

led_state = False

while True:
    if GPIO.input(BUTTON_PIN) == GPIO.LOW:
        led_state = not led_state
        GPIO.output(LED_PIN, led_state)
        print(f"LED is {'ON' if led_state else 'OFF'}")
        time.sleep(0.3)
    time.sleep(0.1)

What error or problem will this code cause when running?

A RuntimeError: No pull-up or pull-down resistor configured for BUTTON_PIN

B SyntaxError due to missing colon

C TypeError when toggling led_state

D No error, code runs fine

💡 Hint

Think about the button input setup and electrical noise.

The button pin is set as input but no pull-up or pull-down resistor is configured. This can cause a RuntimeError or unstable input readings on Raspberry Pi.

📝 Syntax

advanced

Which option fixes the syntax error in this code?

Find the option that fixes the syntax error in this LED toggle code snippet:

import RPi.GPIO as GPIO
import time

LED_PIN = 18
BUTTON_PIN = 17

GPIO.setmode(GPIO.BCM)
GPIO.setup(LED_PIN, GPIO.OUT)
GPIO.setup(BUTTON_PIN, GPIO.IN, pull_up_down=GPIO.PUD_UP)

led_state = False

while True
    if GPIO.input(BUTTON_PIN) == GPIO.LOW:
        led_state = not led_state
        GPIO.output(LED_PIN, led_state)
        print(f"LED is {'ON' if led_state else 'OFF'}")
        time.sleep(0.3)
    time.sleep(0.1)

A Add colon after while True: (while True:)

B Add parentheses to if statement (if (GPIO.input(BUTTON_PIN) == GPIO.LOW):)

C Remove the print statement

D Change GPIO.setup to GPIO.setup(LED_PIN, GPIO.OUT, pull_up_down=GPIO.PUD_UP)

💡 Hint

Check the line that starts the while loop.

The while loop is missing a colon at the end of the line. Adding it fixes the syntax error.

🚀 Application

expert

How many times will the LED toggle if the button is held down for 2 seconds?

Given this code snippet:

import RPi.GPIO as GPIO
import time

LED_PIN = 18
BUTTON_PIN = 17

GPIO.setmode(GPIO.BCM)
GPIO.setup(LED_PIN, GPIO.OUT)
GPIO.setup(BUTTON_PIN, GPIO.IN, pull_up_down=GPIO.PUD_UP)

led_state = False

start_time = time.time()
while time.time() - start_time < 2:
    if GPIO.input(BUTTON_PIN) == GPIO.LOW:
        led_state = not led_state
        GPIO.output(LED_PIN, led_state)
        print(f"LED is {'ON' if led_state else 'OFF'}")
        time.sleep(0.3)  # debounce delay
    time.sleep(0.1)

If the button is pressed and held down continuously for 2 seconds, how many times will the LED toggle?

A 1 time

B 7 times

C 5 times

D 10 times

💡 Hint

Calculate how many toggle cycles fit in 2 seconds considering the delays.

The loop runs for 2 seconds. Each toggle cycle takes about 0.3 + 0.1 = 0.4 seconds. 2 / 0.4 = 5 cycles approximately, but the first toggle happens immediately, so total toggles are about 7.