How to build a circuit that switches between battery or AC, like a boom box (for lack of another example)?
I am designing something which will be lit by led's, and I want the device to be able to have the option of operating by way of either batteries(for the led's), or by plugging it in to 120V AC(the led's are the only things that will be powered). I know how do what I need to change the AC to DC(correct drivers etc.), however, I don't know how to design what I need to simply make it switch over to batteries if you unplug it from AC(automatically, as a boom box does if you remove the AC cord), or vice versa, switch to being able to plugging it in if batteries are dying/ are dead. Thanks.