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How do multi colored led lights work?

I was wondering how do multi colored led lights light up the different colors, and if it would be hard to make.

A colour changing LED isn't one LED in a package but three LEDs along with a small computer to drive them. The LED is made up of red, green and blue LEDs each of which can be controlled by a microcontroller. Since the two legs on the LED that supply the power are connected to the microcontroller and not the LED elements a current limit resistor is not required.

The microcontroller is able to turn each of the colours on or off, so if the red LED is turned on then the output from the colour changing LED is red. When the blue LED is turned on it is blue, if both the blue and red LEDs are turned on then the colour changing LED is a shade of purple (called magenta). Similarly combining red with green gives yellow and blue & green gives cyan.

Although the colour changing LED uses the six colours mentioned above, it slowly changes from one to another. This is still done using the three basic red, green & blue elements. If the red LED is combined with the blue LED, but the blue LED is only driven at 50% of its normal brightness then a colour half way between red and magenta is generated.

Whilst the red LED is left turned on, if the blue LED is slowly taken from 0% brightness to 100% brightness then the colour will gradually change from red to magenta.

If a standard LED is turned on and off very quickly, say 100 times every second then as far as the human eye is concerned it looks like it is constantly on. If the amount of time the LED is on for is the same as the time it is off for then it will be on for 50% of the time and 50% of its full brightness.

This same method can be done with the three LED elements inside the colour changing LED. This means it is possible to combine any amount of the red, green and blue to give the desired colour. Looking once again at the change from red to magenta, if the blue LED starts mainly turned off, goes to being on and off in even amounts and then to mainly being on.

kelseymh4 years ago
Each individual "RGB LED" light has three separate LED chips embedded (red, green, and blue), each with a pair of leads (some just use four leads, with common ground), all behind a diffuser. 

By running independent PWM to each chip, you can mix different intensities of red, green, and blue to get any color.
+1 (although one *could use analog control as well)
Really?  How do you use analog (voltage or current variations) to change the intensity of an LED?  Or do the RGBs come with a built-in ADC and PWM circuit?!?
Err. This will do it, three of 'em. light output varies nicely with current.
trimmable current source.JPG
Thank you, Steve!  There's a lot about practical (as opposed to theoretical :-) electronics I still have to learn.
You're welcome. Incidentally, I have no clue why the picture now says PLOKI on it ?
EdurusFas (author)  kelseymh4 years ago
WOW!  Soo much info :) COOL:) 
The LEDs that I've seen that change color - only have two leads coming from them - which is why I thought that maybe they used varying voltage to change the color - but would that even make sense? 

BTW- Sorry for the late reply- I asked the question, and then I became very busy with classes.

Thanks everyone! :)
The intensity of a single LED P/N junction in the RGB is governed by the forward voltage/forward current (of which there are three). If you drive the forward voltage using an analog source, it results in the same behavior as the effect that a PWM produces (which is in actuality little more than an analog voltage due to intrinsic RC..you could look at it like calculating the average value of a non-symmetric square wave of freq f, remembering that the square wave is 0-V rather than -1/2V-1/2V) ) . Also, as a reminder (although you know this already) RGB leds are just three leds with a common cathode. can be driven using constant current, voltage, or PWM modulated sources...)

But I definitely agree it's much easier and afaik more efficient to accomplish using PWM, or three channels of  D/A. Color matching makes it a bit more complicated, since one needs to do more footwork but that's abit outside the scope here.

Here's a simple experiment (and fun!) you can do with a single LED...

Use a 10-turn pot (one of those little 3-pin bourns trimmers for instance) as the limit resistor in a simple LED circuit (power supply, resistor, led and back to common on power supply). I deally one would use a trimmer and a fixed resistor to ensure that the led doesn't experience an overvoltage situation, but if you're careful it's not necessary

Now, set the pot by using calculations before applying power to ensure that you don't overdrive the led. (note: that's the "if you're careful")

Now, when you power-up the supply, play with the pot. You'll actually see a variance in the color with most leds...not talking the significant change that occurs when you're  at overdrive and they go orange, but the variation within the "useable" range...We'd consider it dim vs bright at first glance, but if you put your thinking cap on and consider that an RGB led is blending three leds power independently (with a common return/cathode)...

That's all that's going on with RGB leds... you go from
off thru bright on each color, which is mixed with one or more of the other leds to produce the effect of the color that our eye dithers out.

It's not quite so simple to reproduce known colors consistently with multiple "arrays" in which additional bias can be applied, sets of RGBs are selected, etc...but the basic situation is right there.
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