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----
-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
-
-    <div class="subfigure">
-        <figure>
-            <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>
-            <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>
-    </div>
-
-.. 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
-
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