I have a problem with powering my LCD Keypad Shield from a DC cable (4 AA batteries or 9V battery). Everything looks good, but the LCD screen doesn't display anything. When I used a USB cable, the screen works as it should.
What is the problem?
I do not have an answer... but it might be power consumption problem, ou voltage stability problem. I use a LCD shield with an RFID card reader (contactless card reader like you can find in the subway). When I plug the thing to a 9v battery the contrast is higher than when I connect it to an USB cable.