Column: Electronics design


Creating an “infi nite” RGB LED cube with Flowcode


By Martin Whitlock, Applications Engineer, Matrix TSL


each with three internal LEDs, requires many control signals to switch them on and off as well as a method to share control signals between them (multiplexing). T e LEDs are cathode-grounded with a current limiter resistor on the anode.


Multiplexing tool 1 T e LED cube uses two types of multiplexing to allow all 512 RGB LEDs to be driven independently. T e fi rst uses octal fl ip-fl op buff er ICs to create enough control signals to drive a full 8×8 segment, i.e., 1/8th of the cube. In an 8×8 section there are 64 LEDs, which require 192 control signals to drive all the individual colour channels. To do this, we need to chain 24 buff er ICs together so the output of the fi rst buff er drives the input of the second buff er, the second drives the third, and so on. Figure 2 shows a simple schematic with two buff ers chained together and connected to drive LEDs. T e clock signal is driven by the


F


or all those electronics hobbyists who love building cool stuff here’s how to design a fully-programmable LED light cube, the so-called


“infi nite RGB LED cube”. T is article will show how I built an 8 x 8 x 8 LED cube and the fi rmware and simulation I created using Flowcode 6. I used 512 RGB LEDs, which required


some 2200 solder joints. I came across some great infi nity mirror projects, which are basically LEDs sandwiched between a mirror and a pane of glass with a one-way mirrored surface to create an eff ect that the LEDs go on forever. I decided to apply some of these simple techniques to my cube to extend the display beyond the bounds of the actual LEDs. With 512 RGB LEDs in the design,


controller and used to pass the output signals from one buff er IC on to the next. I made the buff er driver fairly modular to allow me to scale up or down the size of the cube, depending on how many LEDs I wanted to use. To do this I created a PCB with three buff er ICs, allowing me to control a single row of eight RGB LEDs. With eight of these driver PCBs chained together I can control an entire 8×8 display segment. T e smaller 16-way ribbon cable passes the 10 control signals plus power and ground between the PCBs, whereas the wider ribbon cable passes the signals on to the LEDs via the on-board resistors. Each board is fi tted with capacitors to allow current to be sourced locally as and when needed by the LEDs. In Figure 3, each of the 8×8 LEDs


in a single segment is driven via the chain of buff er ICs. To control the other seven 8×8 display segments, we need a second method of multiplexing. For this I connected together the cathode pins of all the LEDs within the 8×8 segment, and then connected them to ground via an NPN transistor. T is transistor allows us to enable and disable all the LEDs in the 8×8 segment at once, using a single control signal to the transistor’s base; see Figure 4.


12 February 2022 www.electronicsworld.co.uk


Figure 3: Infi nite RGB LED cube By connecting all the LED cathodes


together for each horizontal 8×8 section, we can use eight NPN transistors to enable or disable each of the 8×8 sections and thus individually control each LED in each section. T is is great, but obviously means we can only light up 1/8th of the display at any one time. By repeatedly activating 8×8 sections at fairly high speed the display can be made to appear constantly lit.


The build T ere are two fairly laborious tasks required to build this LED cube: bending LED legs and soldering them together. Each step requires a jig to make the process more manageable. To bend all 512 LEDs correctly, I drilled a 5mm hole in a piece of wood and drew guide lines to help with the bending; see Figure 6. To assemble one 8×8 section of LEDs


and hold the LEDs in place, I created a jig with a 3D printer. T e LEDs were soldered together by inserting each LED one at a time into the jig before soldering the anodes together on each column and the cathodes together on each row as shown in Figure 8. Each solder joint must be nice and


strong to avoid future troubleshooting. I tested each complete panel whilst still


in the jig, to ensure that every LED lights correctly and that there are no breaks or


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