PWMLED for brightness in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output brightness level?

Consider this Python code using gpiozero's PWMLED to set brightness. What brightness value will be printed?

Raspberry Pi

from gpiozero import PWMLED
led = PWMLED(17)
led.value = 0.5
print(led.value)

A 0.0

B None

C 1.0

D 0.5

💡 Hint

The value property sets brightness from 0 (off) to 1 (full brightness).

The PWMLED.value property holds the brightness level as a float between 0 and 1. Setting it to 0.5 means half brightness, so printing led.value outputs 0.5.

❓ Predict Output

intermediate

What happens if you set PWMLED value to 1.5?

What will happen when running this code snippet?

Raspberry Pi

from gpiozero import PWMLED
led = PWMLED(17)
led.value = 1.5
print(led.value)

A ValueError is raised

B led.value prints 0.0

C led.value prints 1.5

D led.value prints 1.0

💡 Hint

PWMLED.value must be between 0 and 1 inclusive.

Setting led.value to 1.5 is invalid because brightness must be between 0 and 1. This raises a ValueError.

🔧 Debug

advanced

Why does this PWMLED code not change brightness?

This code tries to set brightness but the LED stays off. What is the problem?

Raspberry Pi

from gpiozero import PWMLED
led = PWMLED(17)
led.value = -0.1
print(led.value)

A PWMLED does not support setting value directly

B LED pin 17 is wrong, so no brightness change

C Negative brightness is invalid and ignored, so LED stays off (value 0)

D The print statement causes the LED to turn off

💡 Hint

Brightness values must be between 0 and 1 inclusive.

Setting led.value to a negative number is invalid. The PWMLED class clamps the value to 0, so the LED stays off.

🧠 Conceptual

advanced

How does PWMLED control brightness?

Which best describes how PWMLED changes LED brightness?

A It turns the LED on and off very fast to simulate brightness levels

B It changes the voltage supplied to the LED continuously

C It changes the LED color to appear dimmer

D It uses a resistor to reduce current for brightness control

💡 Hint

PWM means Pulse Width Modulation.

PWMLED uses Pulse Width Modulation: it switches the LED on and off very quickly. The ratio of on-time to off-time controls perceived brightness.

❓ Predict Output

expert

What is the final brightness value after this sequence?

What value will be printed after running this code?

Raspberry Pi

from gpiozero import PWMLED
led = PWMLED(17)
led.value = 0.3
led.value += 0.4
led.value -= 0.1
print(round(led.value, 2))

A 0.0

B 0.6

C 0.7

D 0.5

💡 Hint

Remember PWMLED.value clamps between 0 and 1.

Starting at 0.3, adding 0.4 gives 0.7, subtracting 0.1 gives 0.6. The final value is 0.6.

Practice

1. What does the PWMLED class control on a Raspberry Pi?

easy

A. The speed of the Raspberry Pi fan

B. The color of an LED by mixing red, green, and blue lights

C. The temperature of the Raspberry Pi CPU

D. The brightness of an LED by changing how long it is on and off quickly

Solution

1. Step 1: Understand PWMLED purpose

   PWMLED is used to control LED brightness by adjusting the time it stays on and off rapidly.

2. Step 2: Compare options

   Only The brightness of an LED by changing how long it is on and off quickly describes brightness control using on/off timing, which matches PWMLED's function.

3. Final Answer:

   The brightness of an LED by changing how long it is on and off quickly -> Option D

4. Quick Check:

   PWMLED controls brightness = A [OK]

Hint: PWM means brightness by fast on/off timing [OK]

Common Mistakes:

• Confusing brightness with color control
• Thinking PWMLED controls temperature or fan speed

2. Which of the following is the correct way to set the brightness of a PWMLED named led to half brightness?

easy

A. led.set_brightness = 0.5

B. led.brightness(0.5)

C. led.brightness = 0.5

D. led.brightness = 50

Solution

1. Step 1: Recall PWMLED brightness syntax

   Brightness is set by assigning a float between 0 and 1 to the brightness attribute.

2. Step 2: Check each option

   led.brightness = 0.5 uses correct attribute assignment with 0.5 (half brightness). Others use wrong syntax or wrong value type.

3. Final Answer:

   led.brightness = 0.5 -> Option C

4. Quick Check:

   Set brightness by attribute = C [OK]

Hint: Use led.brightness = value between 0 and 1 [OK]

Common Mistakes:

• Calling brightness like a function
• Using integer 50 instead of 0.5
• Trying to assign to a method

3. What will be the output behavior of this code snippet?

from gpiozero import PWMLED
import time
led = PWMLED(17)
led.brightness = 0.2
time.sleep(2)
led.brightness = 0.8
time.sleep(2)
led.off()

medium

A. LED glows dimly for 2 seconds, then brightly for 2 seconds, then turns off

B. LED glows brightly for 2 seconds, then dimly for 2 seconds, then turns off

C. LED stays off the whole time

D. Code causes an error because brightness is set incorrectly

Solution

1. Step 1: Analyze brightness settings

   First brightness is 0.2 (dim), then after 2 seconds it changes to 0.8 (bright).

2. Step 2: Understand timing and off command

   Each brightness level lasts 2 seconds, then LED is turned off.

3. Final Answer:

   LED glows dimly for 2 seconds, then brightly for 2 seconds, then turns off -> Option A

4. Quick Check:

   Brightness changes 0.2 to 0.8 then off = A [OK]

Hint: Brightness values control glow level over time [OK]

Common Mistakes:

• Mixing order of brightness values
• Thinking brightness values cause errors
• Assuming LED stays on after off()

4. Identify the error in this code that tries to set LED brightness:

from gpiozero import PWMLED
led = PWMLED(18)
led.brightness(0.7)

medium

A. Using parentheses after brightness instead of assignment

B. Wrong pin number for PWMLED

C. Missing import for time module

D. PWMLED does not support brightness control

Solution

1. Step 1: Check how brightness is set

   Brightness is an attribute, so it should be assigned a value, not called like a function.

2. Step 2: Verify other options

   Pin 18 is valid, time module is not needed here, and PWMLED supports brightness.

3. Final Answer:

   Using parentheses after brightness instead of assignment -> Option A

4. Quick Check:

   brightness is attribute, not method = D [OK]

Hint: Set brightness with =, not parentheses [OK]

Common Mistakes:

• Calling brightness like a function
• Assuming pin number causes error
• Thinking time import is required here

5. You want to smoothly increase an LED's brightness from off to full over 5 seconds using PWMLED. Which code snippet correctly achieves this?

hard

A. from gpiozero import PWMLED
import time
led = PWMLED(21)
for i in range(5):
led.brightness = i
time.sleep(1)
led.off()

B. from gpiozero import PWMLED
import time
led = PWMLED(21)
for i in range(6):
led.brightness = i / 5
time.sleep(1)
led.off()

C. from gpiozero import PWMLED
import time
led = PWMLED(21)
for i in range(0, 1, 0.2):
led.brightness = i
time.sleep(1)
led.off()

D. from gpiozero import PWMLED
import time
led = PWMLED(21)
for i in range(5):
led.brightness = 1 - i / 5
time.sleep(1)
led.off()

Solution

1. Step 1: Understand brightness range and timing

   Brightness must go from 0 to 1 in steps over 5 seconds, so dividing i by 5 in 6 steps (0 to 5) works.

2. Step 2: Check each code snippet

   from gpiozero import PWMLED
   import time
   led = PWMLED(21)
   for i in range(6):
       led.brightness = i / 5
       time.sleep(1)
   led.off()

   correctly uses i/5 for brightness from 0 to 1 in 1-second steps.

   from gpiozero import PWMLED
   import time
   led = PWMLED(21)
   for i in range(5):
       led.brightness = i
       time.sleep(1)
   led.off()

   sets brightness to integers >1 (invalid).

   from gpiozero import PWMLED
   import time
   led = PWMLED(21)
   for i in range(0, 1, 0.2):
       led.brightness = i
       time.sleep(1)
   led.off()

   uses range with float step (invalid syntax).

   from gpiozero import PWMLED
   import time
   led = PWMLED(21)
   for i in range(5):
       led.brightness = 1 - i / 5
       time.sleep(1)
   led.off()

   decreases brightness instead of increasing.

3. Final Answer:

   Code that increases brightness smoothly from 0 to 1 over 5 seconds using i/5 steps -> Option B

4. Quick Check:

   Brightness from 0 to 1 in steps = B [OK]

Hint: Divide loop index by max steps for smooth brightness [OK]

Common Mistakes:

• Using integer brightness values >1
• Trying to use float step in range (not allowed)
• Decreasing brightness instead of increasing