🛠 Variateur de lumière : PWM sur une LED

Objectives

Understand Pulse Width Modulation (PWM): vary the brightness of an LED by changing the duty cycle. The J8 connector on the Pi 2 exposes a hardware PWM output on GPIO18; you will first use the software PWM from RPi.GPIO (simple and flexible), then enable hardware PWM via config.txt.

Hardware

• Raspberry Pi 2 Model B running 32-bit Raspberry Pi OS
• LED + 330 Ω resistor on GPIO18 (pin 12 of J8)
• Package: sudo apt install python3-rpi.gpio

Complete Code

#!/usr/bin/env python3
"""pwm_led.py — rampe d'intensite PWM (logiciel) sur GPIO18."""
import RPi.GPIO as GPIO
from time import sleep

PIN = 18  # aussi la sortie PWM materielle du J8
GPIO.setmode(GPIO.BCM)
GPIO.setup(PIN, GPIO.OUT)
pwm = GPIO.PWM(PIN, 1000)  # frequence 1 kHz
pwm.start(0)
try:
    while True:
        for dc in range(0, 101, 5):    # monte doucement
            pwm.ChangeDutyCycle(dc)
            sleep(0.05)
        for dc in range(100, -1, -5):  # descend doucement
            pwm.ChangeDutyCycle(dc)
            sleep(0.05)
except KeyboardInterrupt:
    pass
pwm.stop
GPIO.cleanup

Expected Verification

The LED brightens and then dims in a continuous loop (breathing effect). At 0 % it is off, at 100 % it is at full intensity. For hardware PWM, add dtoverlay=pwm (or pwm-2chan) to /boot/config.txt, reboot, then control /sys/class/pwm: the signal remains accurate even under load, unlike software PWM.

Common Errors

• Confusion between hardware and software PWM: the software PWM from RPi.GPIO works on any pin but degrades under heavy CPU load; the hardware PWM (GPIO18) remains precise.
• Forgetting GPIO.cleanup: the pin remains occupied and the next script fails.
• Frequency too low: below ~100 Hz the LED flickers visibly; stay at 1 kHz.
• Using pin 12 for hardware PWM without editing config.txt or rebooting: nothing appears in /sys/class/pwm.