You need to start with a 12v power supply to be able to step down to 5v. The only way to get 12v from a 5v supply is to use a DC to DC converter chip as I mentioned before.
I have a couple of units I've made from a basic 12v @ 2A supply which I put in a slightly larger case with a LM7805 and a USB socket to give me a 12v and 5v output.
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