Somebody told me the top diagram is the correct way, by adding 150ohm resistor leading to LED. Zener should give 5.1V. He said this is how you design correctly using 'blocks', so previous stage doesn't get unwanted side effects. If current is limited by 300ohm resistor anyway, why cant I connect output of Zener direct to LED like in bottom diagram? Why is 150 ohm resistor required? Also, why in bottom diagram zener not working?
