Prevent mains plug from "charging" battery

[staze]

All,

So, my son has a vibrating lounger (he's 3 months) that runs on 3xC (1.5V) batteries... and obviously it chews through them pretty fast (Fischer-Price must own a big share of Duracell and Energizer). I'd like to add a DC jack, which is simple if you just wire it in, or use dummy batteries, but I think that's a hack. I'd like to add a jack, and make it "safe" incase there's also batteries installed in the unit, not have them blow up from being "charged" by the wall connection, or have the device get twice the voltage it should. Obviously the latter could be fixed with an LM317 set for 4.5V, but what about the former? Would guess just a diode between the batteries and the DC jack, but I thought I'd ask if anyone has a solution I'm not thinking of.

Or, maybe I'm tired (read, 3 month old) and it can be done with just a diode and a properly sized wall wart. I think I still need a regulator, or zener, or something to prevent accidentally giving the unit 9V if it had both batteries and the wall wart connected.

Thanks!

[AmmoJammo]

Most DC jacks are switched.

http://www.ebay.com.au/itm/5-5-mm-x-2-1mm-DC-Power-Jack-Socket-Female-Panel-Mount-Connector-HP-/161112814814?pt=AU_CablesConnectors&hash=item25831274de

you plug something in, it disconnects the batteries...

[staze]

oh, fine, be that way. 

 

[AmmoJammo]

Go to bed!