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I am new to electronics and Arduino.

What is the difference between power supply ground and Arduino's ground. If I connect my LED cathode to power ground, it doesn't work. But if I connect it to Arduino ground, it works. Aren't all ground the same?

Please see picture below.

enter image description here

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    Could the power supply / ground bus on your breadboard be split? One difference that I see is that the LED ground to the power supply is more than half way down the breadboard. I've got some that are split in the middle.
    – dlu
    Nov 20 '15 at 6:33
  • dlu, you are right! The breadboard was split in half. Silly me.
    – maomaopop
    Nov 22 '15 at 2:57
  • I'll bet it never fools you again! You're certainly not the first person to wonder about this…
    – dlu
    Nov 22 '15 at 3:41
  • I had long forgotten about the split in the breadboard power rails. Even when @dlu mentioned it, I thought, that couldn't be it. But then when memory serves, my breadboard have little connectors between the two rails top and bottom. I kinda see the benefit of it, but I would expect the power rail indicator lines to be split (I noticed my boards are). ie, the blue and red lines on the outer edge of the breadboard, are they continuous from one end to the other (as in your diagrams above) or are they themselves broken in the middle? As is mentioned, that won't trip you up again :)
    – Madivad
    Dec 7 '15 at 9:23
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I would expect there to be no difference between the grounds in your circuit, and after looking over the schematic for the board, it appears that all of the grounds are common.

I think the most likely explanation for what you're seeing is a quirk in the breadboard. It may be that the side busses are split at the middle of the board, or perhaps there was just a bad connection somewhere on the ground bus on that side of the board.

One way you could troubleshoot it further would be by jumping the ground side of the LED to the power supply ground. Start with the ground on the regulator and then work out until you find the spot where you loose continuity.

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I agree with the other answers. On rare occasions, TO-xxx packages have bizarre pinouts, but the 78xx series does, in fact, have ground on the middle terminal. This is surely a split power supply in your breadboard power busses, or else a bad breadboard. Many breadboards have splits in the power rails halfway down. I've also seen some with splits more often than that, perhaps as much as disconnects every 5 (or 5 pairs of) contacts. I have a breadboard, for example, which allows for 6 different voltages, all in a single line, from various connections to a battery pack. I'd recommend buzzing the breadboard before starting to use it.

I've also had breadboards whose contacts fail after using larger AWG wires in particular holes (or forcing odd connections, such as machined contacts on IC's or sockets, or larger stamped rectangular pins on a TO package such as this) into breadboard holes. The last thing you want to worry about when building a prototype of a new design is whether or not the wiring under the breadboard works.

Many multimeters have a continuity test capability for diagnosing problems such as these, BUT make sure no power is connected (or you could damage the meter on this setting).

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