from machine import Pin
import time

led = Pin(11, Pin.OUT)
button = Pin(18, Pin.IN, Pin.PULL_UP)  # button to GND

while True:
    if button.value() == 0:  # held down (pulled to GND)
        led.value(0)
    else:
        led.value(1)
        time.sleep(0.25)
        led.value(0)
        time.sleep(0.25)