Power bypass circuit?

Hey everybody, Ok so here's the deal...I'm building some external speakers for my zune. I'm using some the board out of some old computer speakers to amplify the sound. Right now, since I'd like to make it portable, I've got it hooked up to a 12v regulator going to two 9v batteries. Since it seems to drain the batteries of charge pretty quickly, I'd really like a better way to keep them powered when I'm near a wall outlet (like when I'm at home). I managed to find an old 12v wall adapter that I could use as a constant source of power. Correct me if I'm wrong but I'm pretty sure that I can't have it and the batteries hooked up at the same time. So what I'd like to know is how do you build a bypass circuit (or whatever it's called) so that, when the wall adapter is plugged in, the board will draw power from it instead of the batteries? Sorry for the long read, and happy building everyone.

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Big Bwana9 years ago
most plug in jacks have a switch in them, (( they will have three leads on them )) and this is so you can do just what you want to... You just run the battery power through the switch and hook both upto the amp, and when you plug it in, it switches to mains power from battery and visa versa.... If you have any old unused or broken item that your adapter jack fits into you might even be in luck.. Or you can order them for a few bucks.....
Triblade101 (author)  Big Bwana9 years ago
Ok thanks
Umm, I'm not sure about how to make it choose the batteries or the wall power, I think a NAND gate is what you need here, have diodes running to the batteries and then to choose...

Triblade101 (author)  killerjackalope9 years ago
Thanks, except...I'm not sure what a NAND gate is, :P the only thing I know right now is I have a board for an old PSP amplifier that takes batteries, but you can also plug in a 5v adapter (usually the PSP charger) charger and it would power the amp, without using the batteries. I would just use this board but the sound quality isn't as good. Care to explain more about the NAND Gate though?
It's a logic gate, I'll dig up my old tech book and figure out what configuration you would need...
Triblade101 (author)  killerjackalope9 years ago
Ok, thanks for the help :)