my guess is the LED is always on or you just see a blip once. try moving the serin outside of the loop. The way you have it it will be waiting for a serin every time through the loop. next problem is you are turning the led on, waiting "x" time, then turning it off. THEN you go right back to the top of the loop and turn it right back on.so if it doesn't hang on the serin, it turns on almost instantly. try adding a pause after you trun it off also. That is assuming GPIO.3 is your LED.