BPI inside

Tag: Building Stuff

  • LED Cube: The heart and concept of the driver circuitry

    It is pretty clear that it’s very hard to control 512 LEDs without some tricks. Most already built LED Cubes rely on two realization concepts to bring the cube to life:

    • some sort of integrated LED driver circuit (read: using chips)
    • use multiplexing / POV to lower the number of LEDs to be controlled individually

    Using an LED driver IC to controll the large number of LEDs is almost a no-brainer. Although there are solutions possible to control that many LEDs without resorting to integrated circuits (e.g. Charlieplexing logic) they all come with one or more drawbacks like complicated wiring schematics, lots of additional required components or being only able to light a single LED at any time. Using a LED driver IC avoids all those problems and makes it possible to control many LEDs even for non-professionals.

    Multiplexing takes advantage of the human eyes incapability to detect light pulses or variations in brightness if they happen just fast enough. It’s the same principle which is used in TV sets and in the cinema. In a LED cube this principle is realized by only lighting up one layer of LEDs at any time and switching between the layers fast enough so that the human eye gets the impression that the whole cube is active. For an 8x8x8 cube this means that at most 8×8=64 LEDs (within one layer) are active at any given moment and the currently active 64 LEDs are switched within the 8 layers several times per second. This reduces the required logic to drive 64 LEDs and a simple selection logic to choose one of eight layers resulting in 72 wires leading to the cube.

    The heart of my LED cube will be the STMicroelectronics STP16CPS05 (datasheet). I got the inspiration for this driver from this project and decided to use it because:

    • serial input possible using 2 pins (+1 for latch)
    • serial input accepts 3.3v, perfectly fitting for the Raspberry Pi GPIO
    • high maximum frequency of 30MHz, great for fast multiplexing (and probably software-driven PWM)
    • 16 output pins
    • simple integration and control (just clock in active/open LEDs and signal latch)
    • chainable (like done in HTNE’s cube), but in my circuit this won’t be required
    • cheap

    One drawback of this IC is that it is not offered in a DIP-package which could be immediately used on a breadboard. Therefore I will have to solder the ICs onto adapters to use them on my breadboards and such adapters themselves are not that easy to come by.

    Some friends suggested alternate ICs, eg. TLC5940, MM5450 or the MAX6962ATH but either are those much more difficult to interface with, have a smaller range of applicable voltages or I would have to use them in the same way as the STP16CPS05 without the ability to take advantage of their additional functionality. So I settled with the, in my opinion, simplest solution.

    Update 2013-05-11 Found another possible alternative, the TI [TLC5925][9]. This looks like a possible replacement if I have problems with the STP15CPS05.

    [9]: http://www.ti.com/product/tlc5925 "TLC5925 16bit LED Sink Driver]

  • Upcoming project: LED Cube 8x8x8

    As already hinted last time my next electronics project will be a monochrome 8x8x8 LED Cube.

    I chose this from a long list of possible interesting projects because its construction involves many of the areas in which I intended to work on something at some time.

    • Raspberry Pi programming
    • enhancing my basic electronics and soldering skills
    • beef up my electronics prototyping set
    • interfacing an uC/SBC with external hardware/components
    • creating an electronic circuit without step-by-step instructions
    • understanding and using an IC
    • designing and creating a PCB, maybe even etching it myself

    LED cubes in particular striked my attention a few years ago but at that time Ialways repelled by the sheer complexity and number of components in most of the LED cube building instructions. But when I saw some another building plan for a LED cube I realized that I could use my Raspberry Pi to create a much simpler control circuit. And so the decision was set.

    My plan for the next steps is to build a prototype (3x3x3) to test my whole concept, get comfortable with all the stuff and then scale it to 8x8x8. At first I’ll set up everything on breadboard and maybe, if I’m sure that everything is finished on the hardware-side, make a permanent and smaller control PCB version. Probably as a Raspberry Pi shield.

    The progress of this project will of course be documented on this blog.

  • Building Adafruits Ice Tube Clock

    As I’ve already hinted on in the past weeks I’ve tackled and completed one of my planned mini-projects.

    I assembled the Ice Tube Clock from Adafruit Industries:

    From Building Ice Tube Clock

    This kit has been on my list for some time now, I stumbled over it on the internet two or three years ago and since that time I regularly checked if it was still available. After I finished my university degree the time was due and I ordered it the instant when there was one available at one of the distributors. When it arrived a few weeks later I prepared my soldering stuff and equipment and built it during two nights.

    As my last soldering has been already quite some while I decided to practice a bit with the Pi Cobbler which also arrived with the same order.

    From Building Ice Tube Clock

    When I was comfortable with soldering again it was time to unpack the parts of the clock and check if everything was complete according to the parts list.

    From Building Ice Tube Clock

    The soldering and building of the clock was pretty straightforward and easy. I carefully read the build instructions and had no trouble with the bending, soldering and checking the intermediate results. So here are just a few impressions from the build process. I also tried the suggestion to cut the wires of the VFD tube in different lengths, every wire a bit shorter than the preceeding one, before trying to thread them into the connector PCB. I think that this really helped a lot and made it much easier to finish.

    From Building Ice Tube Clock

    The last final steps could be completed similar to the previous ones. I only had some slight difficulties with the casing because I initially did not recognize the seperate page for the enclosure-assembling on the webpage but puzzled around a bit and eventually built it correctly without instructions. Manuals are for wimps 😉

    From Building Ice Tube Clock

    Setting the time and date finished off this neat project. Nice 🙂

    A few mini-statistics:

    • 6h effort in two nights
    • 43 parts assembled
    • 1.5m solder needed
    • 1 solder-accident (although easy to fix)

    Some possible improvements are still possible. Although I ordered the kit from a distributor in Italy it still contained only an american 9VDC power adapter. In my opinion shippings within Europe could already contain a proper european power adapter to spare the need for a US/EUR converter. Furthermore the assembly instructions on the webpage do not reflect the process 100% accurately but seems a slight bit outdated. For example there has been a change on the layout of the PCB as the capacitors C5 and C6 have a slightly different placement on the board than on the images in the solder instructions. Also the screws and nuts are not made of metal but of black nylon.

    In the end I enjoyed this mini-project very much. It’s comparable to the experience I had some time ago during finishing my Master thesis, where I had to code some stuff and had the feeling that I missed real deep code-writing at work. I also missed tinkering with stuff and crafting small things. Since finishing the clock I did also some mods for my mother. Enhancing an electronic candle which ran on a single CR 2032 for ~5 days to use two AA’s instead, running now the second week on the first cheap pair of batteries and still pretty bright.

    Currently I’m already planning my next electronics project. This time I will not take a kit which can be assembled by following instructions. Instead it will be something where I will have to deal with electronics, ICs and integration with my Raspberry Pi. And it will be not something which is build after instructions but will be designed by myself.

    But more on that in a later post…