Aiyo....why susah susah one. Use a DPDT switch laa. Turn ON and OFF the 5V and 12V at the same time. No need to build fancy circuit. Very simple....one switch do it all.....

Here's a pic of the switch

You make me look so stupid by showing me that picture�

Ya, Actually I have that switch in the picture, but it was slightly different, cause the button can't stay put. It will bounce back if I push up or down.

I don't stay anywhere near electronic shop, so it won't be convenient for me to buy that kind of switch.

Therefore I use the wall switch to mod it.
Btw, what is relay? as shown in 2uk3y picture