Friday, 19 January 2018

Software and electronics for driving 5725 LEDs

Following on from my previous post about the touch controller I will now talk about the software and electronics we are using to drive the 5725 LEDs.

All software is written using Python, mostly utilizing OpenCV and Numpy for their great image manipulation cpabilities. The LEDs we will be using are designated SK9822, these are going to be spaced at 15 LEDs per meter, these are not usually made in this size so we incurred a higher than expected cost when ordering them. The triangular panels will consist of several strips of the LEDs spaced at 15 strips per meter, this gives us an LED density of 225 LEDs per square meter. This density was selected for manageability of the overall LED array and for power reasons. The image below shows a render of the LED layout, this render will be used for generating our pixel map.

We had originally thought we would go with the WS2812B LEDs as these are cheaper than the SK9822 but the latter has a global brightness control which allows for much better low brightness colour depth which is an area I have found the WS2812s to be lacking in. I have built a few LED projects using the WS2812B LEDs and have noticed serious flicker noticeable when videoing the LEDs, this is another area the SK9822s excel in as they have a PWM frequency of 4.7kHz versus the 430Hz of the WS2812. More detail on these LEDs can be found at Tim's Blog.

WS2812B left SK9822 right

Driving the panels of LEDs will be 5 Teensy micro controllers, handling 4 panels each, a single Teensy could theoretically drive all of the LEDs but this was decided against due to wiring complexity. The main computer running the majority of the software will send the RGB pixel data over USB to the micro controllers. The Teensy will be running a library called FastLED to control the LEDs, this means it is just acting as a buffer between the main computer and the LEDs.

Apart from the touch input software discussed last post there are a number of different pieces of software we have written. One is a tool to define the LED layout, you upload an image or render of the array and that is displayed on screen, using mouse clicks on either end of a string of LEDs and entering the number of LEDs between, it allows you to quickly define the layout of any shaped array. We now have our LED array defined and the coordinates from our touch input device so we needed to combine these to generate our output to be sent to the LED controllers.

The first control program is relatively simple, we take the touch coordinates from our touch dome, look for the closest corresponding pixel in our LED array and set the colour of that pixel to white (or any colour of our choosing). This pixels information gets stored in an array which will then be sent on to the LED controllers. This is just the beginning and only a basic feature set is implemented, in the future brush size and colour will be changeable on the fly using designated spots on the touch input dome as the input.

In order to see what is happening on the LED array without having to assemble the physical array we wrote a bit of software which renders an approximation of the array on the computer screen. This allows us to quickly test the software stack without the hassle of dealing with hardware.

For powering everything we will have a 240V to 48V DC 30A supply consisting of 4 server power supplies in series, these will be located on the ground in a safe, electrically insulated cabinet. The 48V from this will run to each of the LED panels and then on each panel a 48V to 5V DC 15A power supply will be used to regulate the voltage to the LED and micro controllers. The total power consumption at full white will be approximately 1700W which is slightly over what the power supplies are rated at, it will be very rare for us to display full white on all the LEDs so I have deemed this to be safe, even so there will be fuses at each power supplies output. In the photo below I am load testing a single 48V to 5V supply, in the background you can see the four 12V server power supplies.

The wiring of the dome will be quite a task by itself, I have chosen the Deutsch DTM series of connectors as they are waterproof, reliable and reasonably cheap. They will allow the wiring loom to be disconnected from each part of the system and packed up by itself, this should aid in transport and storage.

Next post I will discuss the mechanical construction of the geodesic dome. For all software information please see our github here.

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