LED brightness control in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the PWM class from the gpiozero library.

Raspberry Pi

from gpiozero import [1]

Drag options to blanks, or click blank then click option'

APWMLED

BLED

CButton

DOutputDevice

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'LED' instead of 'PWMLED' which only supports on/off control.

The PWMLED class allows control of LED brightness using PWM signals.

2fill in blank

medium

Complete the code to create a PWMLED object on GPIO pin 18.

Raspberry Pi

led = PWMLED([1])

Drag options to blanks, or click blank then click option'

A17

B18

C22

D23

Attempts:

3 left

💡 Hint

Common Mistakes

Using a pin that does not support PWM or is not connected to the LED.

GPIO pin 18 is commonly used for PWM output on Raspberry Pi.

3fill in blank

hard

Fix the error in setting the LED brightness to 50%.

Raspberry Pi

led.value = [1]

Drag options to blanks, or click blank then click option'

A50

B1

C0.5

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer 50 instead of decimal 0.5 causes the LED to stay fully on or off.

The value property expects a float between 0 (off) and 1 (full brightness). 50% brightness is 0.5.

4fill in blank

hard

Fill both blanks to create a dictionary that maps brightness levels to LED values for 0%, 50%, and 100%.

Raspberry Pi

brightness_levels = {0: [1], 50: [2], 100: 1.0}

Drag options to blanks, or click blank then click option'

A0.0

B0.5

C1

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer percentages instead of decimal values for LED brightness.

Brightness 0% is 0.0, 50% is 0.5, and 100% is 1.0 for the LED value.

5fill in blank

hard

Fill all three blanks to set the LED brightness to 75% using the dictionary and print the value.

Raspberry Pi

level = 75
led.value = brightness_levels.get([1], [2])
print(f"LED brightness set to [3]")

Drag options to blanks, or click blank then click option'

Alevel

B0.75

D75%

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong key or default value causes the LED to not set correctly.

We use the variable level to get the brightness from the dictionary. If not found, default to 0.75. Then print '75%'.

Practice

1. What does PWM stand for in the context of controlling LED brightness on a Raspberry Pi?

easy

A. Pulse Wire Mode

B. Power Wave Management

C. Pin Wiring Method

D. Pulse Width Modulation

Solution

1. Step 1: Understand PWM meaning

   PWM is a technique used to control power by switching it on and off quickly.

2. Step 2: Match PWM to full form

   Pulse Width Modulation means changing the width of the pulse to control power, which fits LED brightness control.

3. Final Answer:

   Pulse Width Modulation -> Option D

4. Quick Check:

   PWM = Pulse Width Modulation [OK]

Hint: PWM controls brightness by changing pulse width [OK]

Common Mistakes:

• Confusing PWM with power management
• Thinking PWM is a wiring method
• Mixing up acronyms

2. Which of the following is the correct way to start PWM on GPIO pin 18 with a frequency of 1000 Hz using RPi.GPIO?

easy

A. pwm = GPIO.PWM(18, 1000); pwm.start(0)

B. pwm = GPIO.PWM(18); pwm.start(1000)

C. pwm = GPIO.PWM(1000, 18); pwm.start(0)

D. pwm = GPIO.PWM(18, 0); pwm.start(1000)

Solution

1. Step 1: Check PWM constructor parameters

   RPi.GPIO PWM takes pin number first, then frequency in Hz.

2. Step 2: Verify start method usage

   start() takes initial duty cycle (0-100), so start(0) is correct for off initially.

3. Final Answer:

   pwm = GPIO.PWM(18, 1000); pwm.start(0) -> Option A

4. Quick Check:

   GPIO.PWM(pin, freq) then start(duty) [OK]

Hint: PWM(pin, frequency) then start(duty cycle) [OK]

Common Mistakes:

• Swapping frequency and pin
• Starting PWM with frequency instead of duty
• Using zero frequency

3. What will be the output brightness level of the LED after running this code snippet?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 500)
pwm.start(0)
pwm.ChangeDutyCycle(75)

medium

A. LED will be at 25% brightness

B. LED will be off

C. LED will be at 75% brightness

D. Code will cause an error

Solution

1. Step 1: Understand duty cycle effect

   Duty cycle 75 means LED receives power 75% of the time, so brightness is 75%.

2. Step 2: Check code correctness

   Code sets up PWM on pin 18 at 500 Hz, starts at 0, then changes duty cycle to 75 correctly.

3. Final Answer:

   LED will be at 75% brightness -> Option C

4. Quick Check:

   ChangeDutyCycle(75) = 75% brightness [OK]

Hint: Duty cycle % equals brightness % [OK]

Common Mistakes:

• Confusing duty cycle with frequency
• Thinking 0 means full brightness
• Forgetting to start PWM before changing duty

4. Identify the error in this code that controls LED brightness and fix it:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)
pwm.ChangeDutyCycle(50)
pwm.start(50)
GPIO.cleanup()

medium

A. Frequency 1000 is too high, use 100

B. ChangeDutyCycle called before start; swap their order

C. GPIO.cleanup() should be before pwm.start()

D. Missing GPIO.setwarnings(False)

Solution

1. Step 1: Check PWM start and ChangeDutyCycle order

   ChangeDutyCycle must be called after pwm.start(), not before.

2. Step 2: Correct the order

   Swap lines so pwm.start(50) comes before pwm.ChangeDutyCycle(50).

3. Final Answer:

   ChangeDutyCycle called before start; swap their order -> Option B

4. Quick Check:

   start() must come before ChangeDutyCycle() [OK]

Hint: Always start PWM before changing duty cycle [OK]

Common Mistakes:

• Calling ChangeDutyCycle before start
• Cleaning up GPIO too early
• Incorrect frequency assumptions

5. You want to smoothly increase LED brightness from 0% to 100% over 5 seconds using PWM on pin 18. Which code snippet correctly implements this?

hard

A. import time
for dc in range(0, 101):
pwm.ChangeDutyCycle(dc)
time.sleep(0.05)

B. import time
for dc in range(0, 101, 10):
pwm.ChangeDutyCycle(dc)
time.sleep(0.5)

C. import time
for dc in range(0, 101, 20):
pwm.ChangeDutyCycle(dc)
time.sleep(1)

D. import time
for dc in range(0, 51):
pwm.ChangeDutyCycle(dc*2)
time.sleep(0.1)

Solution

1. Step 1: Calculate total steps and delay

   To increase from 0 to 100 in 5 seconds smoothly, small steps with short delay are needed.

2. Step 2: Analyze each option timing

   import time for dc in range(0, 101): pwm.ChangeDutyCycle(dc) time.sleep(0.05) uses 101 steps with 0.05s delay = 5.05s total, smooth increase.

3. Final Answer:

   import time for dc in range(0, 101): pwm.ChangeDutyCycle(dc) time.sleep(0.05) -> Option A

4. Quick Check:

   101 steps * 0.05s = ~5 seconds smooth ramp [OK]

Hint: Use small steps with short delay for smooth brightness [OK]

Common Mistakes:

• Using too few steps causing jumps
• Incorrect total time calculation
• Multiplying duty cycle incorrectly