Combining Arduino and LEDs on same power channel

Adafruit has a great tutorial on some square LEDs that I would like to use for a project. I have a regulated 12V 5A power supply that I would like to use. According to the instructions, it says to wire the power to the LEDs separate from the Arduino. My question is, how can I wire these things such that I do not kill either the LEDs or Arduino using a SINGLE power supply? I don't want two plugs hanging off the thing I'm making.

Thanks.

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migrated from stackoverflow.comMar 13 '12 at 13:46

It's a good idea in general to keep the power for your logic and power devices separated. Motors for instance will generate a lot of noise on your power, and to some extent this may pass straight through the voltage regulator, polluting the microcontroller's $V_{DD}$.

One way to keep power supplies separated is feed them via a diode from the common power source. Place a buffer capacitor after the diode, sized for the expected load.

If the LEDs are controlled statically this should be less of a problem than for motors.

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The LEDs take 12v. The Arduino takes 7-12v which it steps down to 5v using a voltage regulator - a 7805 I think. You DO NOT want to use the power outs of the arduino board since that will draw too much current and overheat the regulator. You COULD tap the 12v power supply before it goes into the arduino power socket and use that to power the LEDs.

You need resistors on the LEDs (unless they are built in - I didn't read the spec closely) otherwise you can blow the LEDs.

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You cannot drive LED's attached to 12V directly from Arduino, it requires the use of a transistor or other sort of driver. It is called an 'open collector' or an 'open drain'. http://en.wikipedia.org/wiki/Open_drain

Check the image at this page under Transistors: http://www.prolectron.com/elect/ which says "This is an RS232 to Open-Collector circuit." That is pretty much what you need for driving those LED-modules from 12V supply.

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