Let's Make Robots!

8x common anode RGB LED + resisters choices

Hi all,

I am planning to have 4 pair of common anode RGB LEDs and have each pair in Front-Left(FL), Front-Right(FR), Rear-Left(RL) and Real-Right(RR). Now, I am going to wire them up each pair so in the end to Arduino I will have R,G,B and common anode on each side.

Here is my question, where and which resister I should use? Should I connect both common anode in 5V or 3v3? Should I use resisters in each pin in RGB or common RGB after I connect them all together or common anode?

If you have better suggestion please let me know how to wire such setting up. Thanks a lot!

 

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.
I would use 5V, since the Blue and Green LED's may require something close to 3.6V for power. If you figure 25mA for each LED: 5V/0.025mA = 200 Ohms. A 220 Ohm resistor is the closest common value.

Thanks for your input. If thats the case, can I connect 4 Red(or G,B) pins by using one "220" since they all light up or off at the same time(parallel with one resistor), or I have to use "220" in each single pin? I heard poeple saying connect LED in parallel is not a good idea? But for some reason I need to connect them in parallel, is there any better way to do that?

Say I have a car with 4 signal lights in side FR, FL, RR, RL. Each signal light has 2 RGB LEDs in it so it can light up in 3 different colors. Each side of signal(Left or Right) always light up the same color(4 LEDs in each side light up at same time). WHat's proper why to wire them up with resistor?

(I hope I am clear enough to describe this, sorry my English is really bad)

If you have 2 or more LEDs connected in parallel, all using the same current limiting resistor, then you'll have problems.

Even 2 red LEDs from the same manufacturer, produced in the same batch, will have slightly different forward voltages. If you put them in parallel they won't share the current evenly, and one will be brighter than the other. It only takes a tiny difference in the forward voltage for one LED to steal almost all of the current available through the resistor, leaving none for the other LED.

It's always best to give each LED its own resistor when you can.

I'm not sure how your LED's are connected. Maybe a diagram?

I just quickly draw one. Not an engineer, hope you can understand my strange drawing :P

the resistor that should be placed in line with one or more LEDs in series, but, I can't currently recall it. I do know a single LED is (Supply Voltage - LED Voltage)/LED forward Current or in the case of many red LEDs ( 5 - 2 ) / 0.02 = 150 ohms. While, blue and green LEDs are a bit closer to ( 5 - 3.6 ) / 0.02 = 70 ohms. Red LEDs are more efficient than blue or green LEDs. If you were to use the same sized resistor for all 3 you would notice your blues and greens to be dimmer overall.

Alright, ran across a calculator and I must admit my electronics math is corroded beyond belief :P
Two blue or green LEDs in series would require more voltage than your 5v supply can offer and therefore be dimmer yet. If that is indeed the case you should probably connect each anode in parallel so each gets enough forward voltage. 

I agree with telefox's comment. Each LED should have its own current limiting resistor on the cathode (in your case).

Thanks for all your inputs. I have add resistor in each LED pin and I try it in breadboard. They works fine and I will add those in my Tachikoma. Thanks again!

Parallel connection will require more current then an ATmega pin can supply. In ATmega328 datasheet is stated that maximum current per I/O pin is 40mA and maximum current for Vcc and GND pins is 200mA. Taking every RGB LED as three distinct LEDs and you will need 8*3*20ma = 480mA. Even with 10mA per pin you are over the chip's maximum 200mA.

If you have a higher than 6V supply you may try some parallel and series connections with multimple driving pins and limiting the current through LEDs to 10mA  you may have a chance to use only resistors, but I recommend you to use some LED driver ICs or at least some FET or NPN transistors.

Have you read this http://letsmakerobots.com/node/4948 ?