I added a question related to digital circuits and cryptography. How can I know whether this question should be posted to information security, crypto or eletric engineering communities?
Well the really simple way is to try one and see if it gets closed or migrated :-)
The reason that question was closed on Security is that as it stands it has nothing to do with information security. It also has nothing to do with cryptography (despite the fact you are probably wanting to use the random numbers in a crypto application)
Your question appears to be mostly related to electronics, so that is where I'd suggest you post it, after closely looking at their How to Ask page, as all SE sites differ in terms of detail required in a question.