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I currently have an anode RGB connected to the 11, 10, & 9 PWM pins on my arduino. However I would like to add 3 more LED's to my project, but I don't want to necessarily take up every single PWM pin (I still want to attach a shield). Is there a way for me to hook up all 4 LED's & still be able to only use a minimum number of pins? Keep in mind that I do want to use all 12 resistors for the 4 LEDS. Oh and all the LEDs will do the exact same thing (They all will be red, and all turn blue, etc) if that helps.

Here's how my board looks right now:

enter image description here

I have three NTE123AP transistors I can use for this project.

P.S. I attached the .fzz file so that if any of you would like to edit the schematic image, it would be super easy. In fact that would be better since I'm new at this, and it would be easier to follow! : )

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  • \$\begingroup\$ Individual Control or all with the same color (All Red at the same brightness, or essentially 12 individual leds?) The former could be done just with transistors, while the latter would require a shift register or i2c/spi expander or led driver ic. \$\endgroup\$
    – Passerby
    Commented Apr 6, 2013 at 0:31
  • \$\begingroup\$ Basically I want 4 RGB LEDs and they all will do the same thing. For example I want to sync sound with light. So when Track 1 plays all 4 LEDs will glow red, and when Track 2 plays they will all glow purple, If Track 3 and they'll glow blue, etc. Hope that helps! I mentioned 12 resistors, because similar to the image I posted, I want a resistor for each led pin, because I don't want to sacrifice brightness. Hope that helps! \$\endgroup\$
    – VAlexander
    Commented Apr 6, 2013 at 0:40

2 Answers 2

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Fairly simple. Just using npn transistors. Duplicate per color channel.

enter image description here

Your transistor can sink up to 600mA continuously. At 20mA per led, 4 per channel, that's only 80mA. That's more than enough.

But make sure your power supply can support that. 80 * 3, that's 240mA on the leds alone, not including the rest of the arduino, the transistor base, anything else you are using.

OP Provided a Fritzing Diagram of the answer as well: enter image description here

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  • \$\begingroup\$ @CloudyDays 3 channels (One Red, One Green, One Blue). 4 Leds. Sorry, I don't use fritzing, but simply, Battery Plus is VCC, Arduino_PWM_PIN is the same one you used, and Battery - is Gnd. \$\endgroup\$
    – Passerby
    Commented Apr 6, 2013 at 1:14
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    \$\begingroup\$ @CloudyDays In this case, yes, vcc is 5v. And that setup is mostly correct. First, the transistor, make sure that it matches the base/emmiter/collector pinout of your transistors, not all use the same pinout. And you have all four leds off one resistor (per color). It might work, but that's not the best option. But generally correct. As far as fritzing go, I still think it's a good tool, just like any, but just don't use it myself. \$\endgroup\$
    – Passerby
    Commented Apr 7, 2013 at 0:23
  • \$\begingroup\$ @Passerby thank you so just to clarify, is it best to use a total of 3 resistors overall for the 4 leds, or should I use 12 (in this scenario). Also, do I only need 1 transistor or all 3? \$\endgroup\$
    – VAlexander
    Commented Apr 7, 2013 at 1:46
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    \$\begingroup\$ @CloudyDays while 3 resistors work, it does not work well. 12 is preferable. And you need all three transistors, one for each color/channel. Otherwise, the red, blue, and green channels would all be the same, so you would be controlling them like a single led (white-ish). \$\endgroup\$
    – Passerby
    Commented Apr 7, 2013 at 3:42
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    \$\begingroup\$ @CloudyDays That works. Just make sure that the Collector/Middle Pin is connected as well (Right now, nothing connects to it). As for the transistor, the flat part where the writing is, is normally the "front". And if you had connected it ecb, like you had, you could have shorted the PWM to ground, without a load. Possibly blown the arduino port. Connecting it CBE (as in, Collector to Ground), well, not too sure. \$\endgroup\$
    – Passerby
    Commented Apr 8, 2013 at 4:19
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These diagrams/images might be better to visually see whats going on:

COMMON ANODE RGB w/ TRANSISTOR

COMMON CATHODE RGB w/ TRANSISTOR

Side note

@Passerby: Where did you get that schematic image in your post?

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  • \$\begingroup\$ Welcome to EE.SE. Thanks for posting the schematics. As a general rule, questions to other users are posted as comments, rather than answers. It doesn't take a lot of rep to get the commenting privilege. \$\endgroup\$
    – Nick Alexeev
    Commented Jun 11, 2013 at 22:14
  • \$\begingroup\$ sorry not sure how it works here yet.. but thats my image, which is why I asked, cant post images until 10 rep points or something. \$\endgroup\$
    – whispers
    Commented Jun 12, 2013 at 3:10

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