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[on:]

Hello I'm currently planning on using 4 LEDs for my project, which I will hook up to an Arduino Uno. Being new to all this I want to make sure that I've gotten the numbers correctly.

Do I need 3 resistors for each LED?

According to the Datasheet for the LEDs https://www.sparkfun.com/products/10820 this is what I've calculated:

Forward Voltage: 2.2V (Red), 3.4V (Green & Blue)
Forward Current: 20mA (For Red, Green and Blue)

I'm assuming that since I'll be using a wall DC charger for the Arduino (9V DC), that the source voltage is 9V.

I'm using this calculator to calculate the ohms: http://led.linear1.org/1led.wiz

Based off of my calculations Am I correct?

- 4x 390ohms, 1/2W (The wizard recommends a 1/2W or greater 390 ohm resistor. The color code for 390 ohms is orange white brown). <-- That's for the red value.

- 8x  330ohms, 1/4W (The wizard recommends a 1/4W or greater 330 ohm resistor. The color code for 330 ohms is orange orange brown.) <-- That's for the green and blue values. Would these work: https://www.sparkfun.com/products/8377

I've looked for them on sparkfun but I can't find any 390ohm resistors. Does anyone know where to find them or a better alternative? Thanks! 

[Reply #1 on:]

If you are controlling an LED with an Arduino pin they will be getting 5V, not 9V.  Recalculate for 5V.

Those RGB LED's are "Common Anode" which means the + side of all three colors are connected together.  To control them you will need to connect the common Anode to +5V and the individual Cathodes, through resistors, each to a different Arduino pin.  If you use the 6 pins on an Arduino UNO that support PWM/analogWrite() it will be easy to adjust the brightness.

[Reply #2 on:]

Thanks John!

I still don't have my arduino, I'm planning on ordering tonight! So I totally see what you're saying with the whole source voltage thing.

Source Voltage: 9v
Forward Voltage: I was using the max before, but I heard it's not recommended so the average is (Red = 2.0mA, Blue & Green = 3.2mA)
Forward Current: 20mA
Datasheet: http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/YSL-R596AR3G4B5C-C10.pdf

So based on that, is 150 ohms for red, and 100 ohms for green and blue about right? Someone recommended to go with 180omhs for the red, to balance the brightness off. Would you recommend that? Should I order 12 resistors or do I need less for these 4 leds?

[Reply #3 on:]

So based on that, is 150 ohms for red, and 100 ohms for green and blue about right? Someone recommended to go with 180omhs for the red, to balance the brightness off. Would you recommend that? Should I order 12 resistors or do I need less for these 4 leds?

180 for Red sounds reasonable.  As does 100 for Blue and Green.

You need one resistor per LED and there are 3 LEDs in each RGB package.  At a minimum you will need 12 for four RGB packages. 

Resistors are incredibly inexpensive in bulk and very handy.  You might want to buy an assortment or at least get enough of each value you order that you will have plenty of spares. Perhaps 25 of each value.

[Reply #4 on:]

if you put the leds in parallel you would need three 100ohm and one 150ohm i think.
or replace the 150ohm with this to adjust the brightness http://www.ebay.com/itm/250848525880

[Reply #5 on:]

Thanks John!

Last thing,

Is there any difference between 1/2W & 1/8W, the site calculated 1/8W for both red and blue, but I can only find 1/2W. Would that be cool?

UPDATE: I found these (Would these work?):

100 ohms 1/8w: http://www.radioshack.com/product/index.jsp?productId=12554192&filterName=Type&filterValue=1%26amp%3B%23047%3B8-watt+resistors

180 ohms 1/8w: http://www.radioshack.com/product/index.jsp?productId=12554195&filterName=Type&filterValue=1%26amp%3B%23047%3B8-watt+resistors

[Reply #6 on:]

If you expect the current through LED is around 20ma, 1/8 should be fine. Formula for wattage rating required for you situation is:

P = I^2 * R

so P = 0.0004 * 100 = 0.04W and P = 0.0004 * 150 = 0.06W much less than 1/8W

[Reply #7 on:]

the site calculated 1/8W for both red and blue, but I can only find 1/2W. Would that be cool?

Yep... a resistor's wattage is the power it can handle, so by going up to a bigger one you're safe. You might waste a few pennies- which would matter if you were in factory production and buying zillions. Higher the wattage also means slightly larger, so if you're building a circuit in a restricted space, they do take up more room.

John said:

You might want to buy an assortment or at least get enough of each value you order that you will have plenty of spares. Perhaps 25 of each value.

Have a look at this "book" of 500x 1/4W resistors at Sparkfun

[Reply #8 on:]

Does anyone know where to find them or a better alternative?

use constant current diode,  20 ma LED use 20 ma constant current diode, if LED is not bright enough,  parallel one more 5 ma constant current diode. no 20 ma constant current diode on hand, then parallel  2 pcs  10 ma one.

Pro. support much wider power supply  3-100 V.
Con. a little pricy. I paid 0.40 USD if I recall right.


I made 100/full member! 

[Reply #9 on:]

In the future, consider an online LED current calculator such as this one:
http://ledcalculator.net/

[Reply #10 on:]

In the future, consider an online LED current calculator such as this one:
http://ledcalculator.net/

He did.... mentioned this one in first post.....