from machine import Pin
from time import sleep

Set up onboard LED (GPIO 25) as output
led = Pin(25, Pin.OUT)

# Infinite loop to blink the LED
while True:
    led.value(1)      # Turn ON LED
    sleep(1)          # Wait 1 second
    led.value(0)      # Turn OFF LED
    sleep(1)          # Wait 1 second