🛠 Variateur de lumière : PWM matériel sur GPIO18

Objectives

• Understand PWM: frequency and duty cycle to simulate an analog voltage.
• Use the hardware PWM output (GPIO18, pin 12 of the 26-pin connector).
• Create a breathing LED effect in Python.

Hardware

• Raspberry Pi Model A, LED + 330 Ω resistor on GPIO18.
• Raspberry Pi OS legacy; hardware PWM may require dtoverlay=pwm in config.txt depending on the image.
• Stable power supply: the polyfuse on the Model A does not tolerate overconsumption.

Complete code

#!/usr/bin/env python3
"""Fondu PWM matériel sur GPIO18 (broche 12)."""
import RPi.GPIO as GPIO, time

PIN = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(PIN, GPIO.OUT)
p = GPIO.PWM(PIN, 500)          # 500 Hz
p.start(0)
try:
    while True:
        for d in list(range(0, 101, 5)) + list(range(95, -5, -5)):
            p.ChangeDutyCycle(d) # rapport cyclique 0..100 %
            time.sleep(0.03)
except KeyboardInterrupt:
    pass
finally:
    p.stop
    GPIO.cleanup

# Vérifier le brochage PWM matériel
pinout  # ou consulter https://pinout.xyz

Expected verification

The LED gradually brightens and dims without perceptible flickering. At 0% it is off, at 100% it is at full intensity; only pin 12 (GPIO18) provides hardware PWM on the 26-pin connector.

Common errors

• Applying PWM to a pin without hardware capability: RPi.GPIO then switches to software PWM, which is less stable.
• Forgetting p.stop / GPIO.cleanup: the pin remains occupied when rerunning the script.
• Frequency too low (a few Hz): visible flickering, the LED appears to blink.
• Confusing pin 12 and GPIO12: it is indeed physical pin 12 that carries GPIO18/PWM.