Simple Scrolling RGB LED

A simple RGB (Red, Green, Blue) moving or scrolling LED display can be made using a few 4017 ICs. Let's learn the procedure in detail.

Understanding RGB LED

RGB LEDs have become quite popular these days due to its three-in-one color feature, and because these can be driven independently using three distinct supply sources.

I have already discussed one interesting RGB color mixer circuit, which can be used to manually set the color intensities of the LEDs for producing unique color combinations through gradual transitions.

In the proposed RGB scrolling LED circuit we incorporate the same LED for implementing the effect.

The following image shows a standard RGB LED with independent pinouts for controlling the three embedded RGB LEDs.

We'll require 24 of these LEDs for producing intended scrolling effect, once procured these may be assembled serially as shown in the following image:

As can be seen , the cathodes are all made common and grounded via individual 100 ohm resistors (connected to the negative supply f the circuit).

The anode ends can be seen designated with some relevant numbers which need to be appropriately connected with the respective output pinouts of the IC 4017 circuit as shown in the following figure:

How the Circuit Functions

The circuit functioning may be understood with the help of the following points:

We can see four IC 4017, 10 stage Johnson's decade counter/divider device which are cascaded in a special way such that the intended scrolling effect is achieved from the design.

Pin#14 which is the clock input of the ICs are all hooked up together and integrated with a clock source, which can be easily achieved from any standard astable circuit such as a IC 555 atable, transistor astable, a 4060 circuit or simply a NAND gate oscillator circuit.

The speed of the frequency set on the astable circuit decides the speed of the scrolling effect of the LEDs.

When power is switched ON, C1 instantly forces pin#15 of IC1 to go high momentarily. This pulls pin#3 of IC1 to a high while the remaining pinouts of IC1 are all set to zero logic.

With pin#3 of IC1 going high causes pin#15 of IC2 to also go high, which similarly puts pin#3 of IC2 at a high logic and all its other pinouts at logic zero......this in turn forces IC3 and IC4 to go through an identical set of pinout orientation .

So during power switch ON all the 4017 ICs attain the above condition and stay disabled making sure that initially all the RGB LEDs are kept switched OFF.

However the moment C1 charges fully, pin#15 of IC1 is relieved from the high created by C1, and now it's able to respond to the clocks, and in the process the high logic sequence from its pin#3 moves to the next pin#2....the first RGB string now lights up (first RED string lights up).

With pin#3 of IC1 becoming low, IC2 too now becomes enabled and quite similarly gets ready to respond to the subsequent clock at its pin#14.

Therefore the moment IC1 logic sequence shifts further from its pin2 to pin4, IC2 corresponds by pushing the pinout high from its pin#3 to pin#4....the next RGB string now lights up (green string lights up and replaces the previous red LED string, the red being moved to the next RGB string).

With the subsequent clocks at pin#14 of the ICs the same is followed by IC 3 and IC4, such that the RGB string now appears to be moving or scrolling across the given 8 subsequent LED strips.

As the sequencing proceeds across the 4 cascaded 4017 ICs, at some point of time the last logic pulse reaches pin#11 of IC4, as soon as this happens the high logic at this pin instantly "pokes" pin#15 of IC1 and forces it to reset and return to its initial position, and the cycle starts afresh....

The above RGB scrolling effect may not be too impressive, since the moving pattern would be in the manner R>G>B......, that is one color appearing behind the other. 

In order to achieve a more interesting looking pattern in the manner R>R>R>R>G>G>G>G>B>B>B>B.....and so on, we need to implement the following circuit, it shows a 4 channel design, for more number of channels, you may simply go on adding the IC 4017 ICs in the identical, fashion as explained in this article.