🛠 Variateur d'intensité : faire varier une LED en PWM matériel

Objectives

Use the BCM2711 hardware PWM output (the SoC has a hardware PWM) to vary the brightness of an LED: duty cycle, frequency, breathing. You will enable the device tree overlay and then control it via the command line and Python.

Hardware

• Raspberry Pi 4B, LED, 330 ohm resistor
• GPIO18 = physical pin 12 (hardware PWM channel 0)
• sudo apt install gpiod python3-gpiozero

Complete code

# 1) Activer le PWM : sudo raspi-config -> Interface Options -> PWM
#    ou ajouter dtoverlay=pwm-2chan dans /boot/firmware/config.txt
sudo reboot
gpioset --pwm gpiochip0 18=100=0     # 0 % de rapport cyclique
for d in 100 75 50 25 0; do echo "Duty $d %"; gpioset --pwm gpiochip0 18=$d=0; sleep 1; done

# breathing.py - respiration de la LED via gpiozero
from gpiozero import PWMLED
from signal import pause
led = PWMLED(18)          # GPIO18, PWM materiel
led.pwm_frequency = 1000  # Hz
led.source = "pulse"      # oscillation continue du rapport cyclique
pause

Expected verification

The LED fades progressively at each step of the shell script, then breathes continuously with the Python script. At 0% it is off, at 100% it is fully on; the eye perceives the variation thanks to a frequency of a few hundred Hz.

Common errors

• Choosing a pin without hardware PWM: the overlay is only routed to GPIO12/13/18/19.
• Forgetting dtoverlay=pwm then gpioset --pwm: the line rejects the pwm flag.
• Confusing frequency (speed of the cycle) and duty cycle (proportion of high state): it is the duty that controls brightness.
• Frequency too low (< 100 Hz): the LED flickers visibly.
• Physical/BCM confusion: pin 12 = GPIO18.