LED brightness control in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is PWM and why is it used for LED brightness control?

PWM stands for Pulse Width Modulation. It controls LED brightness by switching the LED on and off very fast. The longer the LED is on compared to off, the brighter it looks.

beginner

How do you set up a GPIO pin on Raspberry Pi for PWM to control an LED?

You first import the GPIO library, set the pin mode to BCM or BOARD, then set the pin as output. After that, you create a PWM instance on that pin with a frequency, and start it with a duty cycle.

beginner

What does the duty cycle in PWM represent?

The duty cycle is the percentage of time the signal is ON during one cycle. For example, 50% means the LED is on half the time and off half the time, making it appear half as bright.

intermediate

Why can't we just use a resistor to change LED brightness instead of PWM?

Using a resistor changes current but wastes power as heat and is less efficient. PWM controls brightness by switching on/off quickly, saving power and allowing smooth brightness control.

beginner

What Python library is commonly used on Raspberry Pi to control GPIO pins for LED brightness?

The RPi.GPIO library is commonly used. It allows you to set up pins, create PWM signals, and control LED brightness easily.

What does PWM stand for in LED brightness control?

A Pulse Width Modulation

B Power Wave Management

C Pulse Wave Measurement

D Power Width Modulation

PWM means Pulse Width Modulation, which controls brightness by changing how long the LED is on in each cycle.

In PWM, what happens if the duty cycle is 100%?

A LED is fully on

B LED is off

C LED blinks slowly

D LED is half bright

A 100% duty cycle means the LED is on all the time, so it is fully bright.

Which Raspberry Pi pin mode can be used to refer to pins by their physical location?

A BCM

B BOARD

C PWM

D GPIO

BOARD mode refers to pins by their physical position on the Raspberry Pi header.

Why is PWM preferred over using a resistor to control LED brightness?

A PWM wastes more power

B Resistor is more efficient

C PWM allows smooth brightness control and saves power

D Resistor changes color

PWM saves power and allows smooth brightness changes by switching the LED on and off quickly.

Which Python command starts PWM on a GPIO pin with a duty cycle of 75%?

A GPIO.output(75)

B GPIO.setup(75)

C pwm.start(75)

D pwm.ChangeDutyCycle(75)

The pwm.start(75) command starts PWM with a 75% duty cycle, making the LED 75% bright.

Explain how PWM controls LED brightness on a Raspberry Pi.

Think about how turning the LED on and off quickly changes how bright it looks.

Describe the steps to set up PWM on a Raspberry Pi GPIO pin to control an LED.

Remember the order of commands to prepare the pin and start PWM.

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