loading

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

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?

sort by: active | newest | oldest
chrisgward6 years ago
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.
maewert6 years ago
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.
scribbleswd (author)  maewert6 years ago
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 ?
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.
scribbleswd (author)  steveastrouk6 years ago
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?
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
scribbleswd (author)  steveastrouk6 years ago
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.
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.