from machine import Pin
import time

led1 = Pin(2, Pin.OUT)
led2 = Pin(4, Pin.OUT)
led3 = Pin(15, Pin.OUT)
counter = 0

while True:
    led1.on()
    time.sleep(0.5)
    led1.off()
    time.sleep(0.5)
    led2.on()
    time.sleep(0.5)
    led2.off()
    time.sleep(0.5)
    led3.on()
    time.sleep(0.5)
    led3.off()
    time.sleep(0.5)
    counter += 1
    print('Counter : ',counter)