DIY Split-Flap Display

This is a DIY split-flap display. Prototype four-character display: video.

The goal is to make a low-cost display that's easy to fabricate at home in small/single quantities (e.g. custom materials can be ordered from Ponoko or similar, and other hardware is generally available).

You can view an interactive 3d model of the design on the project website.

The splitflap community Discord server is the best place to keep up with the latest changes or ask questions about the project!

Want to help support development or just say "thanks"? Consider a one-time or monthly sponsorship:

❤️ Sponsor scottbez1 on GitHub

Using this project in a commercial setting or for paid client work? Go right ahead - it's open source (just make sure to follow the terms of the Apache License)! I would, however, ask that you consider sponsoring the project. I've been developing and maintaining this project in my free time for over 6 years, and I'd love to continue working on it. Sponsorships allow me to pay for prototypes and development tools that make this project possible. Unlike pure software projects, every iteration has real hardware costs; sponsorships allow me to keep iterating and improving the project faster. Thank you!

Current Status

You can download the latest stable release from the official 'releases' page.

Releases have been tested and used to produce working units, but as this is a continuously evolving open-source project, there may always be minor issues and/or incomplete documentation from time to time.

Component	Status	Notes
Enclosure/Mechanics	Stable
Classic Controller Electronics	Stable
Firmware	Stable
Control Software Example	Stable	Example python code for driving the display is in the software directory

💡 There's a new electronics system, called the Chainlink system (more on this below), which makes assembly easier and supports larger displays (100+ modules), using an ESP32 for the controller. It's stable and recommended over the older "Classic" controller for new builds.

Here's a video of a large 108-module display powered by 18 Chainlink Driver boards and a Chainlink Base:

I'd love to hear your thoughts and questions about this project, and happy to incorporate any feedback you might have into these designs! Please feel free (and encouraged) to open GitHub issues, email me directly, reach out on Twitter, and get involved in the open source development and let's keep chatting and building together!

Build Your Own

Please note, updated documentation for the Chainlink system is still in progress; you may see outdated references to the "Classic" electronics in some documentation in the meantime. If you have any questions, please don't hesitate to ask in the community Discord server!

• Ordering guide (the "easy" route) - updated for Chainlink
• [Chainlink Driver Electronics User Guide]https://paper.dropbox.com/doc/Chainlink-Driver-v1.1-Electronics-User-Guide--BsVuP9OHVo952_ZdJDBomCCzAg-U0DAXrSxEoOhgSoRU39hq
• Latest stable release
• Assembly instructions (v0.5/0.6/0.7)

Table of Contents

• Design Overview
  • Mechanical
  • Electronics
    • Module Electronics
    • Chainlink Electronics
      • Chainlink Driver
      • Chainlink Buddy [T-Display]
      • Chainlink Buddy [Breadboard]
      • Chainlink Base
    • Miscellaneous Tools
      • Flaps and Fonts
      • 3D Printed Tools
      • Chainlink Driver Tester
    • Classic Controller Electronics (deprecated)
  • Code
    • Firmware
    • Computer Control Software
• Contributing/Modifying
  • 3D Design
  • Electronics Design

Design Overview

Mechanical

The mechanical/structural components are made from laser-cut 3mm MDF or acrylic, and held together with M4 bolts and nuts. The design is parametric and built using OpenSCAD. See below for more info on rendering/modifying the design.

Latest auto-generated (untested!) artifacts^⚠️:

• For Ponoko 3mm MDF (1x svg, 4x svg)
• For Ponoko 3mm acrylic (1x svg, 4x svg,)
• For generic material (0.18mm kerf correction) (1x svg)
• For Elecrow 3mm Wood (1x zipped pdf, note that Elecrow orders are automatically 5x)
• For Elecrow 3mm Acrylic (1x zipped pdf, note that Elecrow orders are automatically 5x)

^⚠️For tested/stable/recommended artifacts, use the latest release instead

Electronics

For small displays (up to 3 modules), you can skip the custom controller boards and use off-the-shelf ULN2003A driver modules plugged into an Arduino Uno. This is partially documented in the wiki but may require some additional tinkering to get it to work. Help wanted: if you'd like to help improve these instructions, please reach out in the Discord server, thanks!

The Chainlink electronics system is the current recommended design, which allows for larger-scale displays by chaining more modules together and uses a more powerful microcontroller (ESP32) compared to the older "Classic" controller. It also adds the possibility of wifi and BLE control, though firmware support for this is very limited for now.

The older design ("Classic" controller board), which plugs into an Arduino Uno as a shield and can control up to 4 modules is no longer recommended and will not receive firmware updates going forward. New builds are recommended to use the Chainlink ESP32-based system instead.

Module Electronics

Each module needs a hall-effect sensor for start-up calibration and fault monitoring. This can be mounted in different ways but the recommended approach is to use a small PCB, with an AH3391Q (or similar) sensor and connector, which mounts to the side of the module with a single screw and can easily be adjusted for precise calibration.

These boards are small (about 16mm x 16 mm) and the designs are available as a panelized PCB, which can be snapped apart. The panelization is configurable and is optimized for production at low-cost PCB fabricators like SeeedStudio, JLCPCB, or PCBWay.

Latest auto-generated (untested!) artifacts^⚠️:

• Sensor PCB, single (gerbers / pdf)
• Sensor PCB, panelized (gerbers / pdf)

^⚠️For tested/stable/recommended artifacts, use the latest release instead

Chainlink Electronics

The Chainlink system replaces the older "Classic" controller electronics and is designed to support long chains of driver boards to control large displays (think 100+ split-flap modules). It's also designed to be easy and cheap to build, especially in higher quantities, due to its simple BOM and surface-mount components.

The Chainlink system requires 2 core elements:

• One or more Chainlink Driver board. This is what interfaces with the motors and sensors of each module. Each Driver board supports up to 6 individual split-flap modules. Chainlink Driver boards can be chained together to construct a large display.
• Some kind of ESP32 microcontroller board. There are a few options:
  • For small/medium displays, one of the Chainlink Buddy boards are recommended
    • Chainlink Buddy [T-Display] holds a Lilygo T-Display ESP32 module which includes a built-in LCD and 2 buttons
    • Chainlink Buddy [Breadboard] makes it easy to connect a Chainlink Driver to a breadboard for prototyping, though you can also easily connect a Chainlink Driver to a breadboard with a few dupont wires.
  • For large displays, the Chainlink Base provides a number of advanced features: central power management/distribution and fault monitoring, UART and RS-485 connections, configuration switches, and status LEDs.

Chainlink Driver

Key features (compared to the older "Classic" Controller):

• Controls 6 split-flap modules per board instead of just 4 - fewer PCBs means lower cost and less wiring
• Longer PCB (200mm) gets connectors closer to each module so wires have more slack
• Primarily SMD and all components (except the pin headers and motor connectors) are available in JLCPCB's parts library for easy SMD/THT assembly
• Clock and latch lines are buffered on each board with a 74HC125 to support longer chains
• Neopixels replaced by shift-register-driven single-color LED per module (one fewer data line needed in the chain, lower current draw, and 3.3v IO friendly)
• 2 bits of loopback (connecting 2 spare output bits on output shift registers to 2 spare inputs) allows the controller to validate data integrity up and down the whole chain
• TPL7407L mosfet low-side drivers instead of ULN2003A/MIC5842 (lower on-resistance means less voltage drop in the driver = more torque, less waste heat, and less total current consumption) Due to the chip shortage, TPL7407L ICs are not currently available and will likely not be available again until mid-2022, so the auto-generated JLC component/bom files below currently use ULN2003A darlington drivers as a suitable drop-in replacement.
• Module order goes from right-to-left since this is intended to be installed and accessed from behind the modules

Chainlink Driver boards are available mostly-assembled in the Bezek Labs store, and come with the additional connectors and ribbon cables you'll need. Purchases support continued development of this project.

More information on building and using Chainlink Drivers is available in the Chainlink Driver User Guide.

Or if you'd like to order these directly from a fab, this design is optimized for assembly at JLCPCB, and files are automatically generated for ordering assembled PCBs there. Or if you wish to assemble this board yourself instead of paying for assembly, you can view the interactive BOM/placement tool

Depending on available stock at JLCPCB, you may need to manually modify the BOM file to use alternative components, or regenerate the files yourself using export_jlcpcb.py and specifying one or more LCSC_ALT_* field names to use a pre-selected alternative part number. See the schematic for available pre-selected alternatives (check the symbol's properties/fields).

Latest auto-generated (untested!) artifacts^⚠️:

• Schematic pdf
• PCB overview pdf
• PCB gerbers zip
• PCB bom (for JLCPCB assembly) csv
• PCB CPL (for JLCPCB assembly) csv
• PCB bom (for manual assembly) interactive

^⚠️For tested/stable/recommended artifacts, use the latest release instead

Chainlink Buddy [T-Display]

The Chainlink Buddy [T-Display] is a convenient way to connect a T-Display ESP32 board (recommended microcontroller) to a chain of Chainlink Drivers.

Key features:

• TTGO T-Display ESP32 module as the controller, which includes USB-C, color IPS LCD display and buttons
• Extra terminals for every pin of the T-Display allow you to connect any other peripherals to the ESP32 (the connection to the Chainlink Driver requires only 4 of the GPIOs)
• Optional barrel jack makes it easy to use a "wall wart" AC adapter/power-supply (since the Chainlink Driver only has screw terminals for power) -- plug in a 12V supply and then run a wire from the onboard screw terminals to the Chainlink Driver's motor power screw terminals.
• Optional 5V regulator allows for powering the ESP32 without a USB connection, using the 12V motor power supply

Chainlink Buddy [T-Display] boards are available in the Bezek Labs store, and come with the additional connectors you'll need. Purchases support continued development of this project.

Latest auto-generated (untested!) artifacts^⚠️:

• Schematic pdf
• PCB (gerbers / pdf)
• Panelized PCB (gerbers / pdf)
• PCB bom (for manual assembly) interactive

^⚠️For tested/stable/recommended artifacts, use the latest release instead

Chainlink Buddy [Breadboard]

The Chainlink Buddy [Breadboard] makes it easy to connect a Chainlink Driver to a breadboard for prototyping. You could use 5 dupont wires and have a messy rats nest, or you could use a single ribbon cable and this slick breakout board.

Chainlink Buddy [Breadboard] boards are available in the Bezek Labs store, and come with the additional connectors you'll need. Purchases support continued development of this project.

Latest auto-generated (untested!) artifacts^⚠️:

• Schematic pdf
• PCB (gerbers / pdf)
• Panelized PCB (gerbers / pdf)
• PCB bom (for manual assembly) interactive

^⚠️For tested/stable/recommended artifacts, use the latest release instead

Chainlink Base

This has been tested and appears to work, but is not yet recommended for general use. Firmware support is in progress.

The Chainlink Base PCB is an optional alternative to a Chainink Buddym, designed for particularly large displays. It hosts the ESP32 and adds additional connectivity options (terminals for UART and RS485 serial) and power distribution (independently-monitored power channels for multiple "zones" of Driver boards).

Key features:

• TTGO T-Display ESP32 module as the controller, which includes USB-C, color IPS LCD display and buttons
• Optional master relay output for 12V PSU control (5V relay, up to ~500mA coil current)
  • Future firmware will power on the 12V PSU after a startup self-test, and power off PSU in case of any faults
• 5 channels of independently monitored 12V switches for powering groups of Chainlink Driver boards (6-10A max per channel)
  • Depending on the motors you use, each channel may be able to power about 6 Chainlink Driver boards which is 36 splitflap modules
  • Each channel includes an automotive fuse holder for additional over-current protection
  • INA219 and shunt resistor provide high fidelity voltage and current monitoring
  • Firmware will power on each channel after a startup self-test, and power off the channel in case of any faults
  • 3.3V output for powering many Chainlink Driver boards
• Flexible controller input power
  • USB power from the T-Display works by default, though external power is recommended for larger displays
  • Regulated 5V can be connected directly to the screw terminals, or
  • if you are using an always-on 12V PSU without a master relay, you can install a buck module and power the board from 12V using the 7-28V screw terminals

Latest auto-generated (untested!) artifacts^⚠️:

• Schematic pdf
• PCB overview pdf
• PCB gerbers zip
• PCB bom (for manual assembly) interactive

^⚠️There are currently no stable releases of this board. Once the board design is tested and recommended, a stable release will be created on the Releases dashboard

Classic Controller Electronics (deprecated)

The Classic driver board is not recommended for new designs.

The Classic controller board is designed to plug into an Arduino like a shield, and can control 4 stepper motors. Up to 3 driver boards can be chained together, for up to 12 modules controlled by a single Arduino. The designs for the controller can be found under electronics/splitflap.pro (KiCad 5 project). Nearly everything is a through-hole component rather than SMD, so it's very easy to hand-solder.

The driver uses 2 MIC5842 low-side shift-register drivers, with built-in transient-suppression diodes, to control the motors, and a 74HC165 shift register to read from 4 hall-effect magnetic home position sensors. There are optional WS2812B RGB LEDs which can be used to indicate the status of each of the 4 channels.

The PCB layout is 10cm x 5cm which makes it fairly cheap to produce using a low-cost PCB manufacturer (e.g. Seeed Studio).

Latest auto-generated (untested!) artifacts^⚠️:

• Bill of Materials (csv, interactive)
• PCB (gerbers / pdf)
• Panelized PCB (gerbers / pdf)

^⚠️For tested/stable/recommended artifacts, use the latest release instead

Miscellaneous Tools

Flaps and Fonts

If you'd like to print your own flaps, or cut custom vinyl letter stickers, the project includes a script to generate vector design files that is extremely configurable:

• Font for text (this is further customizable in OpenSCAD)
• Character-set - which letters/numbers/symbols/colors are included and in what order
• Bleed - extends rendering past the borders of the flaps to compensate for slight misalignment of printing and cutting operations
• Keepout areas - option to highlight keepout violations for manual review, automatically clip them, or ignore them
• Rendering options:
  • Single-sided - useful for previewing how all letters will look on flaps
  • Front/back - for batch duplex printing, generate separate front-side and back-side files (e.g. sign shop printing on a flat sheet of PVC)
  • Side-by-side - for individual flap printing, each flap's front design is laid out side-by-side with its back design

TODO: finish documenting this and render some example images...

3D Printed Tools

The project also includes a number of optional 3D printed designs to make assembly easier. These include:

• a flap scoring jig for precisely marking the cut point when splitting CR80 cards
• a flap punch jig for aligning the punch when making the pin cutouts on either side of a flap
• a flap container for storing and organizing stacks of completed flaps
• a sensor PCB holder for storing and protecting soldered sensor boards

All of these designs are parametric and customizable within OpenSCAD. To print them, open up the relevant file in OpenSCAD and use File -> Export -> Export as STL to render the design as an STL file for your slicer.

Chainlink Driver Tester

This is not likely to be useful unless you're planning to manufacture dozens to hundreds of Chainlink Driver boards, but the Chainlink Driver Tester is a complete testbed for Chainlink Driver boards as they come assembled by the PCBA fabricator.

This is currently under very active development.

Key features:

• TTGO T-Display (ESP32) controller, screen, and buttons for controlling tests and reporting results
• Pogo-pins for all connectors on the Chainlink Driver board-under-test (screw terminals, sensor pin headers, and motor connectors)
• 12V switch to supply motor power to the board-under-test, with automotive fuse and INA219 voltage/current monitoring (based on the Chainlink Base channel switch design)
• Separate 3.3V supply for the board-under-test, protected with a polyfuse, should avoid browning out the Tester's MCU in case of 3.3V short-circuits
• Motor and sensor connections are broken out from the pogo-pins for a full closed-loop hardware test
• Screw terminals to chain another Chainlink Driver (not under test) to validate that chained outputs work on the board-under-test
• MCP23017 GPIO expander with 8 GPIO pins exposed via headers for future expansion inputs
• Large cutout allows a barcode scanner or camera to be aimed at the bottom of the board-under-test for tracking serial numbers.
• Buzzer option for audible pass/fail feedback

Latest auto-generated (untested!) artifacts^⚠️:

• Schematic pdf
• PCB overview pdf
• PCB gerbers zip
• PCB bom (for manual assembly) interactive

^⚠️There are currently no stable releases of this board, and there may never be as it is a niche production tool, not an end product. If you need this tool, you are likely actively involved in development and should understand the revision history and current status of development enough to make an informed decision about which revision(s) to use.

Code

Firmware

The driver firmware is written using Arduino and is available at arduino/splitflap/Splitflap/Splitflap.ino.

The firmware implements a closed-loop controller that accepts letters as input over USB serial and drives the stepper motors using a precomputed acceleration ramp for smooth control. The firmware automatically calibrates the spool position at startup, using the hall-effect magnetic sensor, and will automatically recalibrate itself if it ever detects that the spool position has gotten out of sync. If a commanded rotation is expected to bring the spool past the "home" position, it will confirm that the sensor is triggered neither too early nor too late; otherwise it will search for the "home" position to get in sync before continuing to the desired letter.

Computer Control Software

The display can be controlled by a computer connected to the Arduino over USB serial. A basic python library for interfacing with the Arduino and a demo application that displays random words can be found in the software directory.

Commands to the display are sent in a basic plain-text format, and messages from the display are single-line JSON objects, always with a type entry describing which type of message it is.

When the Arduino starts up, it sends an initialization message that looks like:

{"type":"init", "num_modules":4}

The display will automatically calibrate all modules, and when complete it will send a status update message:

{
    "type":"status",
    "modules":[
        {"state":"normal", "flap":" ", "count_missed_home":0, "count_unexpected_home":0},
        {"state":"sensor_error", "flap":"e", "count_missed_home":0, "count_unexpected_home":0},
        {"state":"sensor_error", "flap":"e", "count_missed_home":0, "count_unexpected_home":0},
        {"state":"sensor_error", "flap":"e", "count_missed_home":0, "count_unexpected_home":0}
    ]
}

(Note: this is sent as a single line, but has been reformatted for readability above)

In this case the Arduino was programmed to support 4 modules, but only 1 module is connected, so the other 3 end up in "sensor_error" state. More on status updates below.

At this point you can command the display to show some letters. To do this, send a message to the Arduino that looks like this:

=hiya\n

The = indicates a movement command, followed by any number of letters, followed by a newline. You don't have to send the exact number of modules - if you send fewer letters than modules, only the first N modules will be updated and the remainder won't move. For instance, you could send =a\n as shorthand to only set the first module (even if there are 12 modules connected). Any letters that can't be displayed are considered a no-op for that module.

Whenever ALL modules come to a stop, the Arduino will send a status update message (just like the one following initialization, shown above). Here's what the fields mean in each module's status entry:

• state - normal indicates it's working as intended, sensor_error indicates the module can't find the home position and has given up trying (it will no longer respond to movement commands until told to recalibrate - see below). panic indicates the firmware detected a programming bug and has gone into failsafe mode (it will no longer respond to movement commands and requires a full reset of the Arduino to recover - should never happen).
• flap - which letter is shown by this module
• count_missed_home - number of times the module expected to pass the home position but failed to detect it. If this is non-zero, it indicates either a flaky sensor or that the motor may have jammed up. The module automatically attempts to recalibrate whenever it misses the home position, so if this number is non-zero and the module is still in the normal state, it means the module successfully recovered from the issue(s). However, if this number keeps going up over continued use, it may indicate a recurrent transient issue that warrants investigation.
• count_unexpected_home - number of times the module detected the home position when it wasn't supposed to. This is rare, but would indicate a flaky/broken sensor that is tripping at the wrong time. Just like with missed home errors, unexpected home errors will cause the module to attempt to recalibrate itself.

If you want to make all modules recalibrate their home position, send a single @ symbol (no newline follows):

@

This recalibrates all modules, including any that were in the sensor_error state; if recalibration succeeds they will return to the normal state and start responding to movement commands again.

Contributing/Modifying

Looking to make some modifications or play around with the design on your local machine? Jump right in! Note that all of the scripts and automation are developed for Ubuntu. Mac OS support is planned, but not currently implemented (but feel free to open a PR if you want to help!).

3D Design

The main design file is 3d/splitflap.scad

You'll need a recent version of OpenSCAD (e.g. 2015-03), which may need to be installed through the PPA: sudo add-apt-repository ppa:openscad/releases

In general, solid objects such as the enclosure sides or spool components are built from 2d primitives and then extruded to the appropriate thickness for 3d rendering, rather than using 3d primitives. This simplifies the design without losing expressiveness; the perpendicular laser cut beam doesn't allow for cuts that vary in the Z dimension anyway.

Note that while the design is parameterized and many values may be tweaked, there is currently no error checking for invalid parameters or combinations of parameters. Please take care to validate the design if you change any parameters. For instance, while most of the design will adjust to a changed num_modules value, certain values may cause some elements to intersect with other elements or protrude beyond their expected dimensions.

Rendering

Laser-cut vector files

The design can be rendered to 2d for laser cutting by running 3d/scripts/generate_2d.py [--panelize <number>], which outputs to 3d/build/laser_parts/combined.svg. The optional --panelize argument allows for rendering a panel of modules in a single SVG, for bulk laser-cutting.

Internally, the design uses a projection_renderer module (3d/projection_renderer.scad), which takes a list of child elements to render, and depending on the render_index renders a single child at a time. It also adds material to each shape to account for the kerf that will be cut away by the laser.

The generate_2d.py script interacts with the projection_renderer module by first using it to determine the number of subcomponents to render, then runs OpenSCAD to export each component to an SVG file. It does some post-processing on the SVG output (notably adds "mm" to the document dimensions), and then combines all components into the single combined.svg output.

Once the combined.svg file is generated, you'll want to double-check there aren't any redundant cut lines that are shared by multiple adjacent pieces, to save time/cost when cutting. They should be detected automatically (and highlighted in red in the rendering above), but it doesn't hurt to double-check. In Inkscape, select the "Edit paths by nodes" tool and select an edge to delete - the endpoints should turn blue. Then click "Delete segment between two non-endpoint nodes", and repeat this for all other redundant cut lines.

Animated gif

The design can be rendered to a rotating 3d animated gif (seen above) by running 3d/scripts/generate_gif.py, which outputs to 3d/build/animation/animation.gif

The generate_gif.py script runs multiple OpenSCAD instances in parallel to render the design from 360 degrees to individual png frames, which are then combined into the final gif animation. As part of building the animation, generate_gif.py renders the design with multiple configurations (opaque enclosure, see-through enclosure, no-enclosure and no flaps) by setting the render_enclosure and render_flaps variables.

STL models/web viewer

The design can be rendered to a series of STL files (one per color used in the model) in order to be displayed in an interactive web-based 3d viewer. Similar to the projection_renderer used to render individual components for laser-cutting, the ColoredStlExporter detects all the colors used in the model and renders them one-by-one to separate STL files, along with a manifest that maps each STL file to its RGB color. The STL files and manifest are loaded using three.js to display an interactive model on a web site using WebGL. See this blog post for more details on how the export and three.js renderer work: OpenSCAD Rendering Tricks, Part 3: Web viewer.

Electronics Design

All of the electronics are developed using KiCad 5. Panelization is provided by KiKit and gerber/BOM generation is provided by KiBot.

Rendering

The mechanical and electrical design renderings and links above are automatically updated on every commit with the latest rendering. See this blog post for more details on how that works: Automated KiCad, OpenSCAD rendering using Travis CI.

The PCB layout can be rendered to an svg or png (seen above) by running electronics/scripts/generate_svg.py file.kicad_pcb. This uses KiCad's Python scripting API to render several layers to individual svg files, manipulates them to apply color and opacity settings, and then merges them to a single svg. For additional details, see this blog post: Scripting KiCad Pcbnew exports.

For reviewing the design, a pdf packet with copper, silkscreen, and drill info can be produced by running electronics/scripts/generate_pdf.py file.kicad_pcb.

Gerber files for fabrication can be exported by running electronics/scripts/generate_gerber.py file.kicad_pcb. This generates gerber files and an Excellon drill file with Seeed Studio's naming conventions and produces a .zip which can be sent for fabrication.

EESchema isn't easily scriptable, so to export the schematic electronics/scripts/export_schematic.py starts an X Virtual Frame Buffer (Xvfb) and open the eeschema GUI within that virtual display, and then send a series of hardcoded key presses via xdotool to interact with the GUI and click through the dialogs. This is very fragile but seems to work ok for now. For additional details, see this blog post: Using UI automation to export KiCad schematics.

License

I'd love to hear your thoughts and questions about this project, and happy to incorporate any feedback you might have into these designs! Please feel free (and encouraged) to open GitHub issues, email me directly, reach out on Twitter, and get involved in the open source development and let's keep chatting and building together!

This project is licensed under Apache v2 (see LICENSE.txt).

Copyright 2015-2021 Scott Bezek and the splitflap contributors

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.