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Topic: Arduino Pro 3.3v with Transistor/Super Bright LED Problem (Read 2803 times) previous topic - next topic

wgarrido




So I converted my robot to 3.3v by using a Arduino Pro 3.3v version from Sparkfun.

Problem:
Trying to run a Super Bright led from a pin, or course it worked fine on the 5v arduino but not so much on 3.3v.

Part: http://www.sparkfun.com/products/531

Problem at first LED wouldn't turn on. realized not enough voltage.

Solution:
Get a transistor and power directly from battery (Lipo, or borrow from step-up at 6v)

Results:
Not so bright LED.

I am confused why the LED isn't bright and I can barely get 3mA reading on my multimeter when the LED is on.

My problem is what to use for R1 and R2.

I have tired many combinations and best I could get is 3mA.


I guess I don't understand transistors correctly.

I bought a variety pack at RS, so I have 2n2222, 2n3904, and 2n4401. All very similar. I can get better brightness with the 2n2222 but the 2n3904 should be fine. at 200ma rating verses 600ma. All have a hfe of 200 according to the package but the datasheet varies from <100 to 300.

So what am I doing wrong?

wgarrido

One thing I forgot to add was when I measure the voltage from the Arduino pin when the transistor is on I get < 1v. Is that normal? Also the Arduino pin pies out less than 3.3v. Around 3.0v. Is that okay?


Thanks, really, confused. Just when I think I understand things this happens lol.

CrossRoads

You do have D5 set as an output, right?

What do you have at R1?
(3.3V-0.7)/.035 = 75 ohm, use 82 for R1

Then (6V - 3.4 - 0.7)/.020 = 95, use 100 for R2
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

wgarrido

OMG!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I can't believe the pins weren't set as output!!!!!!!!!!!!!!!!!!!!!!!!!!!

I should of stopped and post sooner lol!!!!!!!


I don't even need the transistors now.

But still might be a good idea to get maximum brightness? Since the 3.4v drop?


wgarrido

In that case would I better using the 6v as the power source instead of the battery to get max brightness?

CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

wgarrido

Thanks so much!!!! Now can't wait to get home from work tomorrow.  :)

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