PWMLED for brightness in Raspberry Pi - Step-by-Step Execution

Concept Flow - PWMLED for brightness

Start

↓

Set PWMLED pin

↓

Set brightness value (0.0 to 1.0)

↓

PWM signal adjusts LED brightness

↓

LED glows with set brightness

↓

End or change brightness

This flow shows how setting a brightness value changes the LED brightness using PWM signals.

Execution Sample

Raspberry Pi

from gpiozero import PWMLED
led = PWMLED(17)
led.value = 0.5  # half brightness

This code sets up a PWM LED on pin 17 and sets its brightness to half.

Execution Table

Step	Action	PWMLED.value	LED Brightness Level	Notes
1	Create PWMLED on pin 17	0.0	Off	LED starts off by default
2	Set led.value = 0.5	0.5	Medium brightness	LED glows at half brightness
3	Set led.value = 1.0	1.0	Full brightness	LED glows fully bright
4	Set led.value = 0.0	0.0	Off	LED turns off
5	Set led.value = 0.75	0.75	Bright	LED glows bright but not full
6	End	0.75	Bright	Program ends, LED stays at last brightness

💡 Program ends after setting brightness; LED stays at last set brightness.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
led.value	0.0	0.5	1.0	0.0	0.75	0.75
LED Brightness	Off	Medium	Full	Off	Bright	Bright

Key Moments - 3 Insights

Why does led.value accept numbers between 0.0 and 1.0?

Because led.value controls the brightness as a fraction of full power, where 0.0 means off and 1.0 means fully on, as shown in steps 2 to 5 in the execution_table.

What happens if you set led.value to a number greater than 1 or less than 0?

The PWMLED class expects values only between 0.0 and 1.0; values outside this range may cause errors or unexpected behavior, so always keep values within this range as shown in the execution_table.

Does the LED turn off immediately when led.value is set to 0?

Yes, setting led.value to 0 turns the LED off immediately, as seen in step 4 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the LED brightness level after step 3?

A Off

B Medium brightness

C Full brightness

D Bright but not full

💡 Hint

Check the 'LED Brightness Level' column at step 3 in the execution_table.

At which step does the LED turn off?

A Step 4

B Step 2

C Step 5

D Step 3

💡 Hint

Look for 'Off' in the 'LED Brightness Level' column in the execution_table.

If led.value is set to 0.25 instead of 0.5 at step 2, how would the LED brightness change?

A LED brightness would be full

B LED brightness would be dimmer than medium

C LED brightness would be off

D LED brightness would be brighter than full

💡 Hint

Compare the brightness levels for different led.value settings in the variable_tracker.

Concept Snapshot

PWMLED controls LED brightness by setting led.value between 0.0 (off) and 1.0 (full brightness).
Changing led.value adjusts the pulse width modulation signal to vary brightness smoothly.
Use led.value = x where x is a float from 0.0 to 1.0.
Setting led.value to 0 turns LED off immediately.
Values outside 0.0-1.0 are invalid and should be avoided.

Full Transcript

This lesson shows how to control LED brightness on a Raspberry Pi using PWMLED from gpiozero. We start by creating a PWMLED object on a pin, then set its value between 0.0 and 1.0 to adjust brightness. The execution table traces steps where brightness changes from off to medium, full, off again, and bright. The variable tracker shows how led.value and LED brightness change step by step. Key moments clarify why values must be between 0.0 and 1.0 and what happens when setting led.value to 0. The quiz tests understanding of brightness levels at different steps and effects of changing led.value. The snapshot summarizes the main points for quick reference.

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