yhan wrote:
I have here 7805 regulator ready for use. I just dont understand if a
12 volt circuit design can be operated on 5 volts.

I'm very sorry, I was quite careless. Like someones already mentioned,
it won't work (and actually the chip will burn) if the signal levels
are too high, even though u used proper dropping at the Vcc. You may
never use higher signal level than specificated...