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authorjaseg <git@jaseg.de>2021-08-15 13:43:27 +0200
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+<div class="article-meta">
+<h1><span class="title">Wifi Led Driver</span></h1>
+
+<h2 class="date">2018/05/02</h2>
+<p class="terms">
+
+
+
+
+
+</p>
+</div>
+
+
+
+<main>
+<div class="document">
+
+
+<div class="section" id="project-motivation">
+<h2>Project motivation</h2>
+<!-- FIXME finished project picture with LED tape -->
+<figure>
+ <img src="images/lyza_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><p>After the <a class="reference external" href="https://blog.jaseg.de/posts/multichannel-led-driver/">multichannel LED driver</a> 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 <a class="reference external" href="https://en.wikipedia.org/wiki/ESP8266">ESP8266</a>
+module instead of the <a class="reference external" href="https://en.wikipedia.org/wiki/RS-485">RS485</a> transceiver for WiFi connectivity.</p>
+</div>
+<div class="section" id="the-electronics">
+<h2>The Electronics</h2>
+<p>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 <a class="reference external" href="https://en.wikipedia.org/wiki/ESP8266">ESP8266</a>'s WiFi antenna enough
+space. The board has two 5-pin <a class="reference external" href="https://www.phoenixcontact.com/online/portal/de?uri=pxc-oc-itemdetail:pid=1757019&amp;library=dede&amp;tab=1">Phoenix-style</a> for two RGB-White (RGBW) tapes and one 2-pin <a class="reference external" href="https://www.phoenixcontact.com/online/portal/de?uri=pxc-oc-itemdetail:pid=1757019&amp;library=dede&amp;tab=1">Phoenix-style</a> connector for
+12V power input. The control circuitry and the serial protocol are unchanged, but the <a class="reference external" href="http://www.st.com/resource/en/datasheet/stm32f030f4.pdf">STM32</a> now talks to an <a class="reference external" href="http://www.watterott.com/de/ESP8266-WiFi-Serial-Transceiver-Modul">ESP-01</a>
+module running custom firmware.</p>
+<p>The LEDs are driven using a <a class="reference external" href="http://www.ti.com/lit/ds/symlink/sn74hc595.pdf">74HC595</a> shift register controlling a bunch of <a class="reference external" href="http://aosmd.com/pdfs/datasheet/AO3400.pdf">AO3400</a> <a class="reference external" href="https://en.wikipedia.org/wiki/MOSFET">MOSFETs</a>, with resistors in front of
+the <a class="reference external" href="https://en.wikipedia.org/wiki/MOSFET">MOSFETs</a>' gates to slow down the transitions a bit to reduce brighntess nonlinearities and <a class="reference external" href="https://en.wikipedia.org/wiki/Electromagnetic_interference">EMI</a> resulting from
+ringing of the LED tape's wiring inductance.</p>
+<p>The board has two spots for either <a class="reference external" href="https://en.wikipedia.org/wiki/Resettable_fuse">self-resettable fuses (polyfuses)</a> or regular melting-wire <a class="reference external" href="https://en.wikipedia.org/wiki/Fuse_(electrical)">fuses</a> in
+a small <a class="reference external" href="https://en.wikipedia.org/wiki/Surface-mount_technology">SMD</a> package, one for each RGBW output. For low currents the self-resettable fuses should be okay but at higher
+currents their <a class="reference external" href="http://m.littelfuse.com/~/media/electronics/datasheets/resettable_ptcs/littelfuse_ptc_16r_datasheet.pdf.pdf">trip times get long enough that they become unlikely to trip in time to save anything</a>, so plain old non-resettable fuses would be the way to go there.</p>
+<!-- FIXME finished board photos -->
+<!-- FIXME board with test tape picture -->
+<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/lyza_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/lyza_schematic_and_pcb.pdf">Download PDF</a>
+ </figcaption>
+ </figure>
+</figure><figure>
+ <img src="images/lyza_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></div>
+<div class="section" id="the-firmware">
+<h2>The Firmware</h2>
+<p>The <a class="reference external" href="http://www.st.com/resource/en/datasheet/stm32f030f4.pdf">STM32</a> firmware only had to be slightly modified to accomodate the reduced channel count since the protocol remains
+unchanged. The ESP firmware is based on <a class="reference external" href="https://github.com/Spritetm/esphttpd">esphttpd</a> by <a class="reference external" href="http://spritesmods.com/">Spritetm</a>. 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 <a class="reference external" href="http://www.st.com/resource/en/datasheet/stm32f030f4.pdf">STM32</a> to do anything.</p>
+<p>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
+<a class="reference external" href="https://en.wikipedia.org/wiki/HSL_and_HSV">HSV</a> 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 <a class="reference external" href="https://blog.jaseg.de/posts/led-characterization/">LED characterization</a>
+article.</p>
+<p>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.</p>
+<!-- FIXME screenshot of firmware website -->
+</div>
+<div class="section" id="making-an-enclosure">
+<h2>Making an enclosure</h2>
+<p>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.</p>
+<p>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 <a class="reference external" href="http://www.openscad.org/">OpenSCAD</a> 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 <a class="reference external" href="https://en.wikipedia.org/wiki/STL_(file_format)">STL</a> 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 <a class="reference external" href="https://en.wikipedia.org/wiki/Portable_Network_Graphics">PNG</a> file.
+This depth map is then imported as a triangle mesh into <a class="reference external" href="http://www.openscad.org/">OpenSCAD</a>.</p>
+<p>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 <a class="reference external" href="http://www.openscad.org/">OpenSCAD</a>, but since I have access to a sufficiently fast machine that is
+not a problem. Just be careful if you try opening the <a class="reference external" href="http://www.openscad.org/">OpenSCAD</a> file on your machine, <a class="reference external" href="http://www.openscad.org/">OpenSCAD</a> will probably crash
+unless you're on a beefy machine or interrupt it when it starts auto-rendering the file.</p>
+<p>The board is mounted into the enclosure using knurled insert nuts that are pressed into a 3D-printed hole using a bit of
+violence.</p>
+<!-- FIXME openscad screenshot -->
+<!-- FIXME enclosure parts -->
+<!-- FIXME finished enclosure with board inside -->
+<figure>
+ <figure class="side-by-side">
+ <img src="images/lyza_case_raw.small.jpg">
+ <figcaption>The 3D-printed case with threaded inserts before painting.</figcaption>
+ </figure><figure class="side-by-side">
+ <img src="images/lyza_case_complete.small.jpg">
+ <figcaption>The 3D-printed case with the board installed after painting. This was my first attempt at
+ painting a 3D-printed case so it looks pretty bad.</figcaption>
+ </figure>
+</figure></div>
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