Why PWM is needed for analog-like control in Raspberry Pi - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use PWM for analog-like control on Raspberry Pi?

On a Raspberry Pi, why is Pulse Width Modulation (PWM) used to simulate analog output instead of directly outputting a varying voltage?

A Because Raspberry Pi GPIO pins can only output digital signals (on/off), PWM creates an average voltage effect by switching rapidly.

B Because Raspberry Pi GPIO pins can output true analog voltages, PWM is just a simpler alternative.

C Because PWM reduces the power consumption of the Raspberry Pi by lowering voltage permanently.

D Because PWM signals are easier to read by sensors than analog voltages.

💡 Hint

Think about what kind of signals Raspberry Pi pins can produce physically.

Raspberry Pi GPIO pins are digital, meaning they can only be fully on (3.3V) or off (0V). PWM rapidly switches the pin on and off to create an average voltage effect, which devices perceive as analog-like control.

❓ Predict Output

intermediate

PWM Duty Cycle Effect on LED Brightness

What will be the brightness effect on an LED connected to a Raspberry Pi GPIO pin when the PWM duty cycle is set to 75%?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)  # 1kHz frequency
pwm.start(0)

pwm.ChangeDutyCycle(75)
time.sleep(2)
pwm.stop()
GPIO.cleanup()

A The LED will be off because 75% duty cycle is too high.

B The LED will be bright, approximately 75% of full brightness.

C The LED will blink on and off slowly at 75% intervals.

D The LED will be at full brightness regardless of duty cycle.

💡 Hint

Duty cycle controls how long the LED is on during each cycle.

Setting the duty cycle to 75% means the LED is on 75% of the time and off 25%, making it appear about 75% as bright as fully on.

🔧 Debug

advanced

Why does this PWM code not dim the LED?

Given this Raspberry Pi PWM code, why does the LED stay fully on instead of dimming?

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(100)

for dc in range(0, 101, 20):
    pwm.ChangeDutyCycle(dc)
    time.sleep(0.5)
pwm.stop()
GPIO.cleanup()

A Because the GPIO pin was not set up as output, the PWM does not work.

B Because the PWM was started at 100% duty cycle, the LED stays fully on and the ChangeDutyCycle calls have no effect.

C Because the PWM was started at 100% duty cycle, the LED starts fully on and the loop changes duty cycle correctly, so the LED dims as expected.

D Because the frequency is too high, the LED cannot dim.

💡 Hint

Check how the duty cycle changes inside the loop.

The PWM starts at 100% duty cycle (fully on). The loop changes the duty cycle from 0 to 100 in steps, so the LED dims and brightens as expected.

📝 Syntax

advanced

Identify the syntax error in PWM setup code

Which option contains a syntax error in setting up PWM on Raspberry Pi?

Raspberry Pi

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

A

GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)
pwm.start(50)

B

PIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)
pwm.start(50)

C

)05(trats.mwp
)0001 ,81(MWP.OIPG = mwp
)TUO.OIPG ,81(putes.OIPG

D

GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18 1000)
pwm.start(50)

💡 Hint

Look for missing commas or parentheses.

Option D is missing a comma between 18 and 1000 in the PWM constructor, causing a syntax error.

🚀 Application

expert

Calculate average voltage from PWM duty cycle

A Raspberry Pi GPIO pin outputs 3.3V when on. If PWM is set to 40% duty cycle, what is the average voltage the connected device experiences?

A 1.32 volts

B 0.40 volts

C 1.98 volts

D 3.30 volts

💡 Hint

Multiply the duty cycle percentage by the voltage when on.

Average voltage = 3.3V * 0.40 = 1.32V