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Arduino sketch for my program

I followed this instruction, but I found that it doesnot work (the RGB LED is off).
So, instead of connecting the long leg of RGB LED to ground (-), I connect it to the 5V source (+). And, it works.
But, it does not work as expected. Then, I figure out that the Logic (HIGH/LOW) is somehow inverted, so I invert the logic in my program and it works as expected.
My question is why does that happen?

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  • \$\begingroup\$ Probably the instruction used a common cathode LED, and you used a common anode LED. \$\endgroup\$ – Peter Bennett Jun 4 '17 at 6:18
  • \$\begingroup\$ @PeterBennett So, Arduino logic is only designed for common cathode? \$\endgroup\$ – Terry Smith Jun 4 '17 at 6:25
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    \$\begingroup\$ No. The particular example program you used was designed for common cathode. As you found out, the program can be easily changed to deal with common anode. \$\endgroup\$ – Peter Bennett Jun 4 '17 at 15:04
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In a common cathode LED, the common lead goes to the ground. The three individual colours are taken high to turn them on. That's positive logic.

In a common anode LED, the common lead is the positive, so is taken to +5V. The individual colours must now be taken low to turn them on. That's negative logic. It's not a function of the Arduino, it's a function of the LED and its connection.

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