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diff --git a/content/blog/wifi-led-driver/index.rst b/content/blog/wifi-led-driver/index.rst new file mode 100644 index 0000000..f1e53a9 --- /dev/null +++ b/content/blog/wifi-led-driver/index.rst @@ -0,0 +1,145 @@ +--- +title: "Wifi Led Driver" +date: 2018-05-02T11:31:03+02:00 +--- + +Project motivation +================== + +.. FIXME finished project picture with LED tape +.. raw:: html + + <figure> + <img src="images/board_in_case.small.jpg"> + <figcaption>The completed driver board installed in the 3D-printed case. This device can now be connected to + 12V and two segments of LED tape that can then be controlled trough Wifi. The ESP8266 module goes on the pin + header on the left and was removed for this picture. + </figcaption> + </figure> + +After the `multichannel LED driver`_ was completed, I was just getting used to controlling LEDs at 14-bit resolution. +I liked the board we designed in this project, but at 32 channels it was a bit large for most use cases. Sometimes I +just want to pop a piece of LED tape or two somewhere, but I don't need a full 32 channels of control. I ended up +thinking that a smaller version of the 32-channel driver that didn't require a separate control computer would be +handy. So I sat down and designed a variant of the design with only 8 channels instead of 32 and an on-board ESP8266_ +module instead of the RS485_ transceiver for WiFi connectivity. + +The Electronics +=============== + +The schematic was mostly copy-pasted from the 32-channel design. The PCB was designed from scratch. This time, I went +for a 5x7cm form factor to allow for enough room for all connectors and to give the ESP8266_'s WiFi antenna enough +space. The board has two 5-pin Phoenix-style_ for two RGB-White (RGBW) tapes and one 2-pin Phoenix-style_ connector for +12V power input. The control circuitry and the serial protocol are unchanged, but the STM32_ now talks to an ESP-01_ +module running custom firmware. + +The LEDs are driven using a 74HC595_ shift register controlling a bunch of AO3400_ MOSFETs_, with resistors in front of +the MOSFETs_' gates to slow down the transitions a bit to reduce brighntess nonlinearities and EMI_ resulting from +ringing of the LED tape's wiring inductance. + +The board has two spots for either `self-resettable fuses (polyfuses) <polyfuse_>`__ or regular melting-wire fuses_ in +a small SMD_ package, one for each RGBW output. For low currents the self-resettable fuses should be okay but at higher +currents their `trip times get long enough that they become unlikely to trip in time to save anything +<littlefuse-16r-datasheet_>`__, so plain old non-resettable fuses would be the way to go there. + +.. FIXME finished board photos +.. FIXME board with test tape picture + +.. raw:: html + + <figure> + <figure class="side-by-side"> + <img src="images/schematic.png"> + <figcaption> + The schematic of the driver board, with the ESP8266 on the top left, the STM32 microcontroller for LED + modulation below, the shift register in the middle and the LED drivers and outputs on the right. + <a href="resource/schematic_and_pcb.pdf">Download PDF</a> + </figcaption> + </figure><figure class="side-by-side"> + <img src="images/layout.png"> + <figcaption> + The board layout with the top side being visible. The top side contains the footprint for the ESP8266, the + microcontroller, fuses, filter cap, connectors and the shift register. The LEDs are connected on the left, + with one connector per LED tape segment. The power input connector is on the bottom right. The LED driver + MOSFETs are in small SOT-23 packages on the back of the board. Since this board is not intended for + super-high currents, the MOSFETs are adequately cooled just through the board's copper planes. + <a href="resource/schematic_and_pcb.pdf">Download PDF</a> + </figcaption> + </figure> + </figure> + +.. raw:: html + + <figure> + <img src="images/boards.small.jpg"> + <figcaption>The completed PCBs of this project (front) and the `multichannel LED driver`_ project the driver + circuitry was derived from (back). + </figcaption> + </figure> + + +The Firmware +============ + +The STM32_ firmware only had to be slightly modified to accomodate the reduced channel count since the protocol remains +unchanged. The ESP firmware is based on esphttpd_ by Spritetm_. The modifications to the webserver firmware are pretty +basic. First, the UART console has been disabled since I use the UART to talk to the STM32. The few bootloader messages +popping out the UART on boot are not an issue, since they're unlikely to contain the fixed 32-bit address prefix the +serial protocol requires for the STM32_ to do anything. + +Second, I added LED control by adding drivers for the serial protocol and a bunch of colorspace conversion functions. +When I first tested the prototype software, I noticed that color reproduction was extremely poor. When I just sent a +HSV_ rainbow fade from a python command line, the result looked totally wrong. The fade did not seem to go at a constant +speed and some colors, in particular yellow, orange and greens, were not visible at all. The problem turned out to be a +stark mismatch of the red, green and blue channels of the LED tape and less-than-optimal color reproduction of the pure +colors. I decided to properly measure the LED tape's color reproduction so I could compensate for it in software. This +turned out to be an extremely interesting project, the details of which you can read in my `LED characterization`_ +article. + +Third, I updated the built-in websites with some ad-hoc documentation on how to use the thing and a basic interface for +LED control. + +.. FIXME screenshot of firmware website + +Making an enclosure +=================== + +To be actually useful, the driver needed a robust enclosure. Bare PCBs are nice for prototyping, but for actually +putting the thing anywhere it needs a case to protect it against random destruction. + +The board has four mounting holes with comfortable spacing in its corners to allow easy mounting inside a 3D-printed +case. The case itself is described in an OpenSCAD_ script. To make it look a little nicer, a little 3D relief is laid +into the lid. The 3D relief is generated with a bit of blender magic. The source STL_ model is loaded into blender, then +blender's amazingly flexible rendering system is used to export a depth map of a projection of the model as a PNG_ file. +This depth map is then imported as a triangle mesh into OpenSCAD_. + +For the relief to look good, I chose a rather high resolution for the depth map. This unfortunately leads to extreme +memory use and processing time on the part of OpenSCAD_, but since I have access to a sufficiently fast machine that is +not a problem. Just be careful if you try opening the OpenSCAD_ file on your machine, OpenSCAD_ will probably crash +unless you're on a beefy machine or interrupt it when it starts auto-rendering the file. + +The board is mounted into the enclosure using knurled insert nuts that are pressed into a 3D-printed hole using a bit of +violence. + +.. _`multichannel LED driver`: {{<ref "blog/multichannel-led-driver/index.rst">}} +.. _`LED characterization`: {{<ref "blog/led-characterization/index.rst">}} +.. _ESP8266: https://en.wikipedia.org/wiki/ESP8266 +.. _RS485: https://en.wikipedia.org/wiki/RS-485 +.. _Phoenix-style: https://www.phoenixcontact.com/online/portal/de?uri=pxc-oc-itemdetail:pid=1757019&library=dede&tab=1 +.. _STM32: http://www.st.com/resource/en/datasheet/stm32f030f4.pdf +.. _ESP-01: http://www.watterott.com/de/ESP8266-WiFi-Serial-Transceiver-Modul +.. _74HC595: http://www.ti.com/lit/ds/symlink/sn74hc595.pdf +.. _AO3400: http://aosmd.com/pdfs/datasheet/AO3400.pdf +.. _MOSFETs: https://en.wikipedia.org/wiki/MOSFET +.. _EMI: https://en.wikipedia.org/wiki/Electromagnetic_interference +.. _polyfuse: https://en.wikipedia.org/wiki/Resettable_fuse +.. _SMD: https://en.wikipedia.org/wiki/Surface-mount_technology +.. _fuses: https://en.wikipedia.org/wiki/Fuse_(electrical) +.. _littlefuse-16r-datasheet: http://m.littelfuse.com/~/media/electronics/datasheets/resettable_ptcs/littelfuse_ptc_16r_datasheet.pdf.pdf +.. _OpenSCAD: http://www.openscad.org/ +.. _STL: https://en.wikipedia.org/wiki/STL_(file_format) +.. _PNG: https://en.wikipedia.org/wiki/Portable_Network_Graphics +.. _esphttpd: https://github.com/Spritetm/esphttpd +.. _Spritetm: http://spritesmods.com/ +.. _`HSV`: https://en.wikipedia.org/wiki/HSL_and_HSV + |