import RPi.GPIO as GPIO
import time

# Set the GPIO mode
GPIO.setmode(GPIO.BCM)

# Define the GPIO pin for the LED
led_pin = 21

# Set the GPIO pin as output
GPIO.setup(led_pin, GPIO.OUT)

# Create PWM object with frequency 100 Hz
pwm = GPIO.PWM(led_pin, 100)

# Start PWM with 0% duty cycle
pwm.start(0)

try:
    # Gradually increase brightness
    for duty_cycle in range(0, 101, 5):
        pwm.ChangeDutyCycle(duty_cycle)
        time.sleep(0.1)

    # Gradually decrease brightness
    for duty_cycle in range(100, -1, -5):
        pwm.ChangeDutyCycle(duty_cycle)
        time.sleep(0.1)

finally:
    # Clean up GPIO
    pwm.stop()
    GPIO.cleanup()