I've been interfacing my Arduino with the SparkFun LED Matrix Backpack unit, and I've discovered - after doing some hardware debugging - that when connected to an external battery pack (4 D or AA cells, so I'm bringing in around 6 VDC), the regulated +5 output presented on the header never actually goes above around 4 volts - which is too little to power the Backpack unit. I get full voltage when I go to the VIN (9v) header, so I can get my project to work, but I'm wondering what's going on with the 7805 or associated power circuitry that seems to be knocking about 3v off of my DC source... It would be safer to power the backpack unit with regulated +5 VDC from the header.
I have two Arduinos and I get the same behavior on both of them.
Any input would be greatly appreciated. Thanks!