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Topic: Controlling 50+ RGB LEDS using Arduino (Read 2 times) previous topic - next topic


Nov 11, 2012, 10:32 pm Last Edit: Nov 11, 2012, 10:50 pm by Freumar Reason: 1
Alright so I've finally gotten all my parts to build my own RGB LED Strip. I am using the following schematic:

Only I am extending it for 50+. I have:

12v / 6A DC Power Supply
50 RGB LEDs (Datasheet - http://www.noodlehed.com/ebay/datasheets/HH-1000CRGBW810.pdf)
470 and 510 1/2 Watt Resistors
3 NPN Transistors (Datasheet - http://www.datasheetcatalog.org/datasheet/kec/MJE13005.pdf)

When I wire it all up, the RGB LED is so dim you can barely see it lit. I've got each color connected to Pins 4, 8 and 12 into the Base, the light wired up to the collector and the emitter to the ground.If I remove the transistors and directly connect the light to just the power supply, the light works perfectly and is as bright as can be, leading me to believe I'm not understanding something about these transistors and how to control them using the Arduino. To turn them on I am just using digitalWrite(pin, HIGH).

Also I do not have an base resistors for the transistors, I'm not sure if that could affect it. If I need the, can anyone tell me how to calculate the resistance required??

If anyone can explain to me why my LEDs are barely turning on that would be so helpful. I'm so close to being finished and this is all that's keeping me from it :(

To elaborate since I forgot a little bit...I am just testing this with one LED currently. When that one LED is powered using my power supply, it is dim. When powered using the USB from the Arduino it is slightly brighter but not fully lit. This is confusing because shouldn't it be brighter with the higher voltage and current the power supply gives??

If any more information is needed please just ask and I'll be happy to answer!!


Are you running them from the 12V supply or from some unspecified 7.2 to 9V supply?  That number is needed for calculating the current limiting resistors.

At 20 mA each * 50 LEDs you'll need 1A per color so 3A total.  Or were you planning on running them closer to their absolute maximum rating: 50 mA.

The base resistors protect the Arduino from having too much current drawn.  At 5V you want a resistor greater than 125 ohms so 200 is a good choice.
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I am using the 12v 6A external power supply to power the LED. And yes, I calculated that 3A will be needed for 50 so I bought the 6A power supply. And for the base resistor, if it's 200 ohms at 5v, what would it be at the 12v??

I am only doing this with one LED so far so this is why it's confusing me why the light it dim. I could understand if my power supply wasn't strong enough for all 50 or the arduino couldn't power all them...but I've only tested this with one LED so far. The LED is not broken so it should be full strength like it is when I directly connect it to the power supply without the transistors =/

Just remembered too which could be useful. When the arduino is powered with a 9v battery the LED doesn't even turn on, but when powered through USB it's brighter, but still pretty dim. I know the USB supplies more current, but with only one LED this shouldn't make a difference =/


There is another problem here,  the specs for the transistor you are using say that the gain ( basically collector current to base current ) can be as low as 19.

You want the transistor to switch hard on.

This means that to switch 1 amp it would need a base current of 52 mA, which is too much for the Arduino .

I would rather use a power mosfet which needs virtually no gate current, and acts as a less than 1 ohm resistor when on.

As to the illogical symptoms your test setup is displaying, are you sure you have the transistor wired correctly ?  the base is on the left when you look at the writing side , with the legs downwards.
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Nov 12, 2012, 03:05 am Last Edit: Nov 12, 2012, 03:08 am by Freumar Reason: 1
Yes, I'm sure I have it wired correctly, it's confusing the hell out of me too haha What you're saying makes sense that not enough current is being supplied to the transistor to turn it on. What is a MOSFET transistor? Does it work exactly the same way: base, collector, emitter...when base is supplied current it switches on? Would it be possible to use 6 of those transistors in total to make 3 Darlington pairs?? I'm new to using transistors so if any basic explanations could be given that would help so much.

I really appreciate the explanation regarding my transistors, very helpful in understanding them!!

Also if I do go for the MOSFET, what specs should I look for in handling 12V 6A? I'm assuming I'd need pretty good ones right?

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