Some Newbie Questions about LEDs

Other Hardware LEDs and Multiplexing
1

Hello there,

I am new to the forum and new to electronics, so please excuse me if these questions are too simple.

I just did the Arduino Fade example. I was wondering how many LEDs I could safely attach to a single Output Pin. Is there a way to make those calculations? Would it be better to use resistors in series or parallel?

Thank you.

2

A 5volt Arduino (not all are) outputs 5volt, and can normally deliver 20mA (40mA absolute max) per pin.
(there is also a max for the whole chip)

There are ~2volt "standard" LEDS and ~3.3volt "high-brightness" LEDs.
Two ~2volt LEDs can be connected in series, but ofcourse only one 3.3volt can work on 5volt.

LEDs need a current limiting resistor in series.
This resistor is calculated to drop the remaining voltage at the needed current.
Or current is calculated for a known LED/resistor.

Example: one 3.3volt LED with 220ohm resistor in series.
Current = (5volt pin - 3.3volt LED) / 220 ohm = 1.7/220= 7.7mA
You could connect two of these LED/resistor to one pin.

A 2volt LED would draw (5-2)/220= 13.6mA.
One is ok, but two of these per pin would draw more than 20mA.

Unless you put them in series.
Then: (5volt - (2*2volt) /220 = 1/220= 4.5mA

I could have 20 LEDs on one pin.
As long as the total current is ~ <=20mA.
Leo..

3

Sorry, I'm a little confused about something here..

If you are arranging them in series, and you have 20 LEDs...that's 5v - (20*2v)/220...how do I use -40 to do the next equation?

Also, if the LED is only operating at 4.5mA of current, won't it be very dim?

4

julieCabazon:
If you are arranging them in series, and you have 20 LEDs...that's 5v - (20*2v)/220...how do I use -40 to do the next equation?

Also, if the LED is only operating at 4.5mA of current, won't it be very dim?

  1. You can only have two ~2volt LEDs (= ~4volt) and one current limiting resistor in series on a 5volt pin.

But you can have more of those "two LEDs and one resistor" strings on one pin.

  1. The LEDs on your Arduino have a 1k resistor in series, so the run on 3mA.
    Bright enough (too bright) as indicator LED.
    Leo..
5

But you can have more of those "two LEDs and one resistor" strings on one pin.

So are you saying you'd mix series and parallel here? Very interesting. Thank you.

6

I am trying this arrangement for two LEDs right now, but it only seems to light up when I short to one LED. Any idea why?

7

If Vled is 2V you can have a maximum of two in series.
Your LEDs are shown backwards.
.

8

Oh, so if these are 3.3v LEDs, series won't cut it?

9

Correct.
.