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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?

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    Can you please give us more than "it doesn't work?" That's really hard to diagnose. Please edit with circuit diagrams. Thanks! – Anonymous Penguin Jan 19 '15 at 15:59
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    OK. First of all, have you tried adjusting the contrast? I don't know why that would change between power sources but it might. This image labels the thing you need to turn to adjust the contrast. Turn it slowly from one side to the other until you can see something. Also: the AAs can't provide enough voltage- even when completely full. (They're in series, right?) Add two more in series to achieve 9V at full power. Also, try replacing the 9V if needed since they drain quickly. Also, connecting to VIN and GND for sure? – Anonymous Penguin Jan 19 '15 at 16:29
  • Sorry. I plug the LCD Keypad Shield to my Arduino Uno R3. I connected Arduino to computer by USB cable and load example cod from this page: dfrobot.com/wiki/… Everything worked perfectly. When I disconnected Arduino, and instead of USB cable I used DC cable display didn't worked. The power led and LCD backlight was on but there where no letters or any other mark on display. When I changed cable back to USB it worked normal again. – wena2k432 Jan 19 '15 at 16:38
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    You originally mentioned a battery, so I was giving advice on a batter. Can you please post a picture of your wiring for the battery to the Arduino? Thanks! – Anonymous Penguin Jan 19 '15 at 16:40
  • I tried to change the contrast and to change the power supplier but it dosen't work. With 9V display only blinks from time to time. I'm affraid that something isn't right with my Arduino. – wena2k432 Jan 19 '15 at 16:45
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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.

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