Add Wifi LED driver post

content/posts/wifi-led-driver/index.rst

@@ -3,3 +3,160 @@ 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/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>
+
+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/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>
+
+.. raw:: html
+    
+    <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>
+
+
+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.
+
+.. FIXME openscad screenshot
+.. FIXME enclosure parts
+.. FIXME finished enclosure with board inside
+
+.. raw:: html
+
+    <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>
+
+.. _`multichannel LED driver`: {{<ref "posts/multichannel-led-driver/index.rst">}}
+.. _`LED characterization`: {{<ref "posts/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
+