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I've been curious for quite some time about the 2-lead RGB color-cycling LED's. I know that they have 3 tiny LED's inside and an IC, but I'm really curious to understand what's going on in the IC that allows such a small amount of power (a 3v Coin Cell seems to work quite well) to not only power all 3 LED's, but also to power the transistors (I assume that's what's causing the cycling).

I'd love to make a circuit (whether or not it's exactly like the chip) that can achieve the same result: creating the cycling effect using only a coin-cell battery as power. This is mainly for educational purposes, as I feel it would help me to understand electronics all the better. Can anyone help me achieve this? Thanks for your help!

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    \$\begingroup\$ I once made a coin cell powered electronic candle using an RGB LED, some resistors, and a PIC10. 3V is actually a lot of voltage - the problem with coin cells is the lack of current, so it doesn't run for long: youtu.be/B3H0lHu6_Jw \$\endgroup\$ – Majenko Jun 20 '14 at 18:24
  • \$\begingroup\$ @Majenko Unfortunately this doesn't answer my question. Knowing that this uses a PIC doesn't help me because what I'm hoping to learn is the inner workings of an IC capable of achieving this effect, not which IC to use. \$\endgroup\$ – Hendeca Jun 20 '14 at 18:29
  • \$\begingroup\$ I know it doesn't describe the internals of the chip, but it does demonstrate one easy way of building a circuit that does a similar thing, using software PWM to control the brightnesses. You could try creating a hardware PWM circuit to change the brightness depending on a varying voltage input - that would be fun (and also kind of useful) \$\endgroup\$ – Majenko Jun 20 '14 at 18:33
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    \$\begingroup\$ As others have mentioned, this is done with a small uController. Building a circuit that does everything there, would actually end up being quite complicated relative to the complexity of the code necessary to do it on a uController. If you want to understand electronics, this may not be the best project to do so. There are simpler places to start in digital land. If you're interested in uController programming, then it could be a good project to do. \$\endgroup\$ – horta Jun 20 '14 at 19:07
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    \$\begingroup\$ The thing is that how the IC accomplishes the color fading, is not easily replicated with pure glue logic. The basics of how it works is (multiple) pwm for brightness control, and then a timer to control the fading times. Pwm is easy, a 555 timer and a pot to vary the rate. But to get dimming and fading would be a huge mess of multiple timers and counters, logic chips, etc, and has no relation to the programmed math/code involved. \$\endgroup\$ – Passerby Jul 3 '14 at 16:10
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I'm really curious to understand what's going on in the IC that allows such a small amount of power (a 3v Coin Cell seems to work quite well) to not only power all 3 LED's, but also to power the transistors (I assume that's what's causing the cycling).

It's a standard IC, capable of the same current sourcing as most modern ICs or MCUs. 20mA needed for a standard LED is a pretty small amount. Even three on the same time is only 60mA, plus 2 or 3mA for the IC itself. That's for 100% on. No transistors needed.

Then you have PWM to reduce the brightness, by pulsing the diodes, reducing the current in a given period of time. Finally you have the color fading, which is done on the order of Seconds. That's eternity for a microcontroller, so easy to do.

Some of the more specialized ICs can have current limiting to reduce the external part count as well, making external resistors unnecessary, while most rely on the High ESR of Coin Cells to work.

And the expected lifetime of these specialty LEDs are fairly short compared to regular LEDs. Regular LEDs are considered 10000+ hours at 20mA constant current. These Color Changing LEDs are built to last longer than a few batteries, as most people never replace the batteries in the color changing tea lights or whatever.

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  • \$\begingroup\$ This is a good explanation and combined with your above comment, I think it answers my question. Probably not going to try this, as it seems like more trouble than its worth just to learn the circuit side of it. Seems like it's not really practical to try to recreate what can easily be done with an IC. \$\endgroup\$ – Hendeca Feb 24 '16 at 18:23
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This is an hypothesis, but it might only be a chip which drive each LED one at-a-time using Persistance Of Vision. Therefore, only a single LED is lit at a time, but you fell like you see many colors at once, because your eyes are too slow to figure out the trick.

This causes the current consumption to go down to the one of a single LED. Also, the chip probably consumes a very little amount of current if it runs out of low power mode.

You could use any MCU with integrated hardware PWM. If you use an MSP430 (I use this one because I know the features that are available, but it would work with anything else), you could configure 1 timer as a PWM source to drive the LED with a chopped current. This dims the led and reduces the current. Then, you use another timer to switch between the led to be lit using a capture and compare register that toggle the state of a GPIO. For two color, you would use two GPIOs which would drive a small transistor (2N2222 or any small-signal FET would do the trick) which would sink current for the GPIOs alternatively.

You have many options, just choose a low power MCU and check your options.

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