For further information go to this “Colour Addition Tutorial” by the Physics Classroom

Varying Intensity

Sending intensity data each RGB LED in a chain

The intensity (i.e. the level of brightness) of each of the Red, Green, and Blue LED’s making up an RGB module determines its colour. But how does each LED know what level to shine at; and how can they be controlled over only three wires?

Each RGB module contains a tiny microcontroller chip that receives data from the Kookaberry over a signal wire. The other two wires (3.3v and Ground) provide the power to light the diodes and run the LED’s microcontroller.

Each LED microcontroller receives three bytes (8 bits to each byte) of data on the signal wire, each byte being associated with one of the three LEDs. The data in the byte is a number with range 0 to 255 inclusive which is interpreted as the desired intensity of the associated LED.

As the RGB LED modules are in cascade, the three bytes of data are sent downstream by each RGB LED module when new data is received by the module attached directly to the Kookaberry (just like the pass the parcel game).

When the Kookaberry finishes sending data bytes it sets its data line to 1 for a longer period (like when the game music stops) which is interpreted by the RGB LEDs as a signal to update the LED intensities with the three bytes of data currently received.

To the Kookaberry, a chain of RGB LEDs is treated as a long array of 3 byte integers [(RGB0),(RGB1),(RGB2),(RGB3), etc ,(RGBn)].

When only one RGB LED is used, the process is exactly the same but with only the RGBO bytes populated.

Varying the intensity of each LED in a RGB LED

So now each RGB LED module has the intensity data for its three LEDs. The microcontroller in the module now can set the LED intensities using pulse width modulation.

A digital signal can only be either ON (“1”) or OFF (“0”). So if a LED is controlled by a simple digital signal it can only be fully ON (maximum intensity) or fully OFF (zero intensity).

However, if the digital signal is switched (pulsed) rapidly ON and OFF – with the width of the OFF time greater than the width of the ON time – the average power to an individual LED will be less than maximum, and the intensity will be proportional to the width of the ON time.

The percentage of time that a digital signal is ON over an interval or period of time is called the Duty Cycle. This period is the inverse of the frequency of the waveform.

If a digital signal spends half of the time on and the other half off, we would say the digital signal has a duty cycle of 50% and resembles an ideal square wave. In this case the intensity of the LED would be half its maximum value.

If the percentage is higher than 50%, the digital signal spends more time in the ON state than the OFF state and vice versa if the duty cycle is less than 50%. Here is a graph that illustrates these three scenarios:

If the frequency is fast enough (typically >50Hz) the eye’s persistence of vision will make the LED appear to be on all the time without any flickering

This process is called Pulse Width Modulation (PWM)

Where can I find them in the real world?

RGB LED’s are the individual pixel elements in colour screens on your devices and on TV’s. Individually addressable RGB LED’s are linked together in columns and rows and number in the millions in standard 4k LED TV’s.

8,294,400 to be precise, because, in a 4K TV, there are 4,096 pixels across the horizontal dimension and 2,160 in the vertical.

For further information go to “What pixels are and what they mean for TV viewing” by Robert Silva for Lifewire