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Will I need a transistor to power 8 LEDs from one Arduino pin? Answered

I am looking into building a project where I will have 25 sets of 8 3v 20ma LEDs.  I am wanting to control each set from one output pin.  I am almost certain that the Arduino card itself would not be able to power all 200 LEDs at once.  Because of this I believe I will need to set up some kind of transistor array?  I am new to all of the building aspects of such circuits and what not.  So my question is what type of transistor will I need to get the desired power to light up all LEDs at once without burning out the card?

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chrisgward (author)2010-06-08

You do not *need* a transistor to do this, but the LEDS will have to be wired in parallel, are not individually addressable (because of 1 pin). Also the brightness of the LED will be *slightly* dimmer than if you used an alternate power source + transistor.

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maewert (author)2010-05-27

25 sets of 8 LEDs will require 25 output pins, which the duecimila doesn't have, but the mega does.  You'll need to use a mega if you want to drive the LEDs with a single output pin for each of the 25 banks. 

Or you may want to use a set of cascading shift register chips like this: http://www.arduino.cc/en/Tutorial/LEDDriver

Here is a basic circuit if you want to use a transistor and one showing the basics of using the uln2803A.

Best of luck.

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scribbleswd (author)maewert2010-05-27

I already have the mega card so I'm on the right track there.  I don't see any links to the sample circuits your talking about it ?

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steveastrouk (author)2010-05-25

The simplest method, and one which will drive up to 8 sets of 8 LEDS, would be to use a ULN2803 darlington array, rather than a transistor.

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scribbleswd (author)steveastrouk2010-05-26

Would be be the part you are talking about:

  http://www.alliedelec.com/search/productdetail.aspx?SKU=2489280#tab=Overview

I was looking at the spec sheet for that chip and saw that the input voltage was 30v . That is a max voltage right?  For the output voltage on the Arduino pins would only be 5v correct?

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steveastrouk (author)scribbleswd2010-05-26

It will work on 5V from the arduino, and switch up to 50 V, so you can easily run 8 3V leds in series on its pins, you can run 64 leds with this chip.

Steve

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scribbleswd (author)steveastrouk2010-05-27

Thanks for the help. So the 30v input was just a max voltage then correct?  My LEDs are going to be wired in parallel but that should not make a difference though correct?  Since I am new to all of this I'm just trying to understand all of it. Again thanks for the help so far.

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steveastrouk (author)scribbleswd2010-05-27

Unless you put a series resistor on each LED, I really don't recommend parallel connections. I would have a 35 volt supply and run all 8 in series personally.

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