I am having a strange problem and need guidance on what may be causing this. I have a project powered by an Arduino Mega; when I supply power via USB from my computer, it works great (even my very old Mac which I believe has lower voltage USB). When I power via battery through DC barrel jack, the LCD is hard to read and the ping sensors don't operate properly. Clearly there is a power issue, but I don't know what or why. For batteries, I am using fresh AA's in a 6 x AA = 9v which is well within the input range listed for the Mega. (I've also tried a single 9v battery through the DC barrel jack, same bad result).
I am using completely separate power supplies for the rest of the project...I run a separate supply to the Adafruit Motor Shield, and a third power supply to power the servo (common ground on that one).
Grumpy_Mike -- I am measuring 9.12 volts. I wasn't positive where to measure, please let me know if this isn't the correct method: I popped the top off of the barrel jack connector so I could connect my multimeter problems to the +/- terminals. I assume anywhere I measure on the Arduino will give me 5v regulated voltage?
Edit in response to @Grumpy_MIke's comment in the next post ....
My question relates to whether the power plug and the Arduino socket make a good connection with each other. There are several different versions of concentric power connectors - some are for very fat centre pins and others for thin centre pins so there may be a mismatch. Also, in my experience they can suffer from corrosion when unused for a while.