🛠 Variation d'intensité d'une LED avec le PWM matériel (GPIO18)

Objectives

Get to grips with Pulse Width Modulation (PWM): make an LED "breathe" by varying the duty cycle. The BCM2835 on the Model B+ exposes a hardware PWM output on GPIO18 (pin 12) — the only one on the 40-pin header by default — guaranteed glitch-free even under CPU load.

Hardware

• Raspberry Pi Model B+ running 32-bit legacy Raspberry Pi OS
• 1 LED + 330 Ω resistor
• Dupont wires: pin 12 (GPIO18) → resistor → LED → pin 14 (GND)
• Optionally an NPN transistor (BC547) to drive an LED strip, with GPIO18 connected to the base via a 1 kΩ resistor

Complete code

sudo apt install -y python3-rpi.gpio
sudo python3 fade.py

#!/usr/bin/env python3
"""Respiration LED sur GPIO18 (PWM matérielle) — Model B+."""
import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
p = GPIO.PWM(18, 200)                 # 200 Hz : inodore pour l'œil
p.start(0)
try:
    while True:
        up = range(0, 101, 5)         # rampe montante
        down = range(95, -1, -5)      # rampe descendante
        for dc in list(up) + list(down):
            p.ChangeDutyCycle(dc)     # 0 % = éteinte, 100 % = pleine
            time.sleep(0.05)
finally:
    p.stop
    GPIO.cleanup

Expected verification

The LED brightness rises and falls in a loop, without perceptible flickering. An oscilloscope on GPIO18 shows a 200 Hz square wave whose duty cycle varies. Ctrl+C stops the program cleanly.

Common errors

• Software PWM on another pin: possible with RPi.GPIO, but the single-core BCM2835 produces glitches under load — prefer GPIO18.
• Frequency too low (a few Hz): the LED blinks instead of varying in brightness.
• Forgetting p.stop: signal residue remains after Ctrl+C if outside the finally block.
• Confusing PWM with analog output: the B+ has no analog-to-digital converter or DAC; PWM is just rapid on/off signals.