Voltage regulation issues on Arduino?

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!


usually the 7805 needs at least around 7 or 8 V input voltage. with too low input voltage the output voltage won't reach 5V. i guess what you experiencing ist just too low input voltage.

Ah, that could well be it. I figured if you put in 6 VDC it would be just fine, but apparently the device is a bit hungrier than that. :-? I'll try putting 7.5 VDC into that port and see what happens...

(Mark goes off and puts 7.5 VDC into his Arduino...)

OK, so now the Backpack unit is getting sufficient power, but the Arduino is not executing its downloaded program. I've seen this behavior before - as I've tried running 7.5 VDC into the regulated input. Does anyone know why this might be? It's very odd. Is something floating that shouldn't be?