I have an uno with a bunch of stuff connected and when plugged into the computer via USB it works just fine. I have an external 9.6V battery that I connect to VIN and GND and it still works fine.
I then plug an MP3 Shield into the UNO. If I use the USB to power it it continues to work fine. However if I use the external battery to power it the external devices stop working.
I've narrowed it down to the fact that if I plug the external battery GND into the UNO GND then all the pins seem to suddenly stop understanding HIGH and LOW. I've checked and re-checked all the solders, cleaned the board,etc. but always this is the case.
I'm kinda stuck here. I'm thinking of trying to use the barrel jack instead of VIN but I need to get a jack to attach to the battery. In the mean time, anyone have any thoughts? My only guess is that the MP3 (Music Maker Shield) isn't properly handling the higher voltage input (ie no regulator?).
UPDATE: I've tried an external power source in both the barrel jack and the USB(I have an adapter you plug into an outlet and can then plug a USB cable into) and both have the same result where the external devices connected to the UNO stop working. The SAME USB plugged into the computer, however, continues to work just fine. So I have to guess it has something to do with the regulator a