I have recently received 3x CD4021BE shift registers. I am wondering if they can be used for output. e.g arduino to led's. what i have seen so far online they are only input.
It is a Parallel input to Serial output, so no, i don't think so.
It is meant for the opposite, reading a series of buttons.
For controlling multiple LED's one can use a 4015 (you can read buttons with that one as well)
For such applications check the intended LED current and the maximum output current of the chips. In detail the very old CD40xx series can not supply much current. For modern versatile chips with easy control look for port expanders.
Yeah but a 4021 is a parallel input top Serial output.