LED brightness control in Raspberry Pi - Step-by-Step Execution

Concept Flow - LED brightness control

Start

↓

Setup GPIO pin for PWM

↓

Set initial brightness (duty cycle)

↓

Enter loop

↓

Increase brightness step

↓

Check if brightness > max

↓

Reset to min

↓

Apply brightness to LED

↓

Wait short delay

↓

Repeat loop

The program sets up a PWM pin on the Raspberry Pi to control LED brightness by changing the duty cycle in a loop, increasing brightness step by step and resetting after reaching max.

Execution Sample

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)
pwm.start(0)

try:
    while True:
        for dc in range(0, 101, 10):
            pwm.ChangeDutyCycle(dc)
            time.sleep(0.2)
finally:
    pwm.stop()
    GPIO.cleanup()

This code smoothly increases LED brightness from off (0%) to full (100%) in steps of 10%, pausing briefly at each step.

Execution Table

Step	Duty Cycle (dc)	Action	LED Brightness	Delay (seconds)
1	0	Set PWM duty cycle to 0%	LED off	0.2
2	10	Increase duty cycle to 10%	Dim LED	0.2
3	20	Increase duty cycle to 20%	Slightly brighter	0.2
4	30	Increase duty cycle to 30%	Brighter	0.2
5	40	Increase duty cycle to 40%	More bright	0.2
6	50	Increase duty cycle to 50%	Half brightness	0.2
7	60	Increase duty cycle to 60%	More than half bright	0.2
8	70	Increase duty cycle to 70%	Bright LED	0.2
9	80	Increase duty cycle to 80%	Very bright	0.2
10	90	Increase duty cycle to 90%	Almost full brightness	0.2
11	100	Increase duty cycle to 100%	Full brightness	0.2
12	Loop repeats	Restart from 0%	LED off	0.2

💡 Loop runs indefinitely until program is stopped manually.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9	After Step 10	After Step 11	Final
dc (duty cycle)	0	0	10	20	30	40	50	60	70	80	90	100	0 (loop restart)

Key Moments - 3 Insights

Why does the LED brightness change smoothly instead of instantly?

Because the duty cycle changes in small steps (10% increments) with a short delay (0.2 seconds) between each step, as shown in execution_table rows 1-11.

What happens when the duty cycle reaches 100%?

The program resets the duty cycle back to 0% to start the brightness cycle again, as seen in execution_table row 12.

Why do we use PWM to control LED brightness?

PWM changes how long the LED is on versus off very quickly, which our eyes see as brightness changes. This is shown by changing the duty cycle variable 'dc' in the code and execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at Step 5. What is the LED brightness when duty cycle is 40%?

A More bright

B LED off

C Dim LED

D Full brightness

💡 Hint

Check the 'LED Brightness' column at Step 5 in the execution_table.

At which step does the duty cycle reach full brightness (100%)?

A Step 10

B Step 11

C Step 12

D Step 9

💡 Hint

Look at the 'Duty Cycle (dc)' column in the execution_table to find 100%.

If the delay time is increased from 0.2 to 1 second, how would the execution_table change?

A The LED brightness would skip some levels.

B The duty cycle values would change to larger steps.

C The 'Delay (seconds)' column would show 1.0 instead of 0.2 for each step.

D The loop would stop after one cycle.

💡 Hint

Focus on the 'Delay (seconds)' column in the execution_table.

Concept Snapshot

LED brightness control uses PWM on a Raspberry Pi GPIO pin.
Set up PWM with frequency, then change duty cycle (0-100%) to adjust brightness.
Increase duty cycle in steps with delays for smooth brightness change.
Loop to repeat brightness cycle.
Use GPIO.cleanup() to reset pins after program ends.

Full Transcript

This example shows how to control LED brightness on a Raspberry Pi using PWM. First, the GPIO pin is set up for PWM output. The program starts PWM with 0% duty cycle, meaning LED is off. Then, inside a loop, the duty cycle increases from 0% to 100% in steps of 10%. Each step changes the LED brightness slightly brighter. After reaching 100%, the duty cycle resets to 0%, and the cycle repeats. A short delay after each step makes the brightness change smooth and visible. The variable 'dc' tracks the duty cycle value. The program runs indefinitely until stopped. Finally, PWM is stopped and GPIO pins are cleaned up to avoid issues. This method uses PWM to simulate different brightness levels by changing how long the LED is on in each cycle.