I'm working on my first "real" Arduino project using an Uno, a GPS module, LCD screen, and microSD card; basically I'm building a specialized GPS receiver. Everything works - I've tested the functionality and I get a valid signal, data is written to the card, info is displayed on the screen, etc. The Uno has been getting all its power so far from the USB port; to make it portable I hooked up a 9V battery to the power jack and the screen started flashing dimly; it seems like it's not getting enough power. I have it plugged into the +5 pin on the Uno (through two shields, one for the microSD card and the other for the GPS), but when hooked up to the USB port everything is fine.
Checking the schematics, it says that the power draw from the USB port is 5V, so since that's all working (and when I plugged it back into the USB port it started working again), I figured that 9V was fine, especially since I had earlier plugged in the same battery/plug into a Duemilanove I have from an earlier learning project running a simple sketch to turn an LED on and off, and it worked just fine.
This is the part that I admit to knowing next to nothing about: external power. If the USB port only supplies 5V, and the board says it can handle up to 20V, and I attach a 9V battery...I'm stuck, what am I doing wrong?
Thanks for any help; this has definitely been a learning experience and I'm having a great time working with the equipment. I just need to make it portable and then figure out how to get into a box, but that's after I have it working externally.