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I have an RGB LED (datasheet here) that I want to control using an Arduino R3 (datasheet here). How can I do this? The LED has a common anode, not a common cathode. However, the arduino's I/O pins can only be +5 volts. This is a problem, because I need to somehow hook up the LED's cathodes to the arduino's I/O pins.

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This is only a problem if you think in terms of SOURCING a current from an output. Outputs also SINK current.

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When the output is set LOW current will flow INTO the pin to ground (SINKING).

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The Arduino IO pins can "output" high or low (+5v or 0v) which means you can drive a LED either direction. Generic schematic (couldn't find RGB LED in the editor):

schematic

simulate this circuit – Schematic created using CircuitLab

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