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Project Roadmap

In this log, we want to share our experience of bringing our FunKey Project from its early-prototype stage to a “Ready For Manufacturing” / “Community-Ready” state necessary to launch a successful crowdfunding campaign that in turn should provide us with the lever required to launch a product in mass production.

Where Do We Come From

People following the Funkey Project certainly remember that the original motivation came from @Sprite_tm‘s 2016 Hackaday superconference project of a pocket retro gaming console, that lead @c.Invent to develop the #Keymu – open source keychain-sized gaming console as a Proof-of-Concept that entered the 2017 Hackaday Prize. The corresponding Youtube video is now peaking at 4.8M views and #6 in Youtube’s Popular Videos – Game Consoles & Mobile Phones category!

Unfortunately, the tiny (and expensive) Intel Edison CPU module at the heart of the console was abruptly discontinued, which forced him to find a replacement module. Given the tiny dimensions, this prove to be very difficult, but @Squonk42 joined him to develop a new board around the LicheePi Zero module. Eventually, a new #Funkey Zero console which was used to validate the new CPU choice took part in the 2018 Hackaday Prize:

At this point, it became clear that the main feature of the #Keymu – open source keychain-sized gaming console was its foldable design, providing a maximum size for both the screen and keypads in the smallest form factor. It also became clear that such a hinged design required the help of a professional mechanical designer, and this challenge motivated @David.Larbi who joined the team at the beginning of this year. Read More

Layout: Screen FPC Extender

LCD Screen

The chosen LCD screen for the FunKey console is awesome:

Hinge

The FunKey console uses a foldable design in order to reduce the device size when not in use, and maximize both the screen and keypad size when playing. The screen flat cable must then go into the hinge, and in order to avoid too much stress that would eventually lead to broken cables, it must be “rolled” into it like a flypaper in order to divide the stress over the longest possible length.

Unfortunately, the stock LCD screen ribbon cable is not long enough. This is of course something that the manufacturer can customize, but this costs a fixed tooling fee of $800… We plan to go this way for mass production, but this sounds like a lot of money for the prototyping phase only.

FPC Extender

So, we decided to use cheaper alternatives for our prototypes:

  • for the Revision A, we used individually soldered thin enameled copper wires and an ultra small PCB placed into the hinge: definitely not something to use for more than 2 boards!
  • for the Revision B, we decided to invest some money into a customized FPC (Flat Printed Cable) as we can get 5x FPC prototypes for ~$100 at PCBWay

The problem is: I never designed FPCs before 😉

I got some basic hints from one of my colleague:

  • the copper density must be as constant as possible
  • the traces must use smooth curves instead of sharp angles to avoid tearing during flexion

However, a few questions remained unanswered:

  1. what material and thickness to use for the stiffeners?
  2. what material and thickness for the flex itself?
  3. which stack-up?

I decided to crawl the Web for more information, and here is what I found, I hope this may help some other PCB designers: Read More

Layout: Screen FPC Extender Update

The previous log detailed the screen FPC extender, used to adapt the stock LCD screen to the FunKey main PCB.

Before launching the FPC fabrication which is quite expensive (~ $100 / 5 pieces), we decided to have a dry run using a paper mock-up. Here is the result:

We covered the old Rev. A PCB with a paper print of the new Rev. B one, and created a paper version of the flex, gluing the connectors on them at the right positions. As you can see, the result is not too bad.

This was until we tried to open the lid flat:

Well, the FPC is a little bit too long, causing a “wave” that totally defeats the original purpose of the “flypaper design”: the small bend radius would certainly cause a failure sooner or later.

Moreover, the FPC is a little bit too wide, too, so it interferes with the internal ribs somehow.

So, back to the drawing board, we decreased the length by 2 mm, and the width to 4 mm, here is the result:

Much better! And the behavior when opening / closing the lid is also improved:

Here are the corresponding layouts:

Meet the team

I tried different angles to approach this log, and writing about us from a third person’s point of view simply wouldn’t work. It was too detached for my taste and it felt hypocritical writing things about myself as another’s POV. Instead I chose the 1st person narrative to talk about our team, “I” meaning “Vincent” in this case. It feels more genuine, and I get to really “tell” our story this way.

FunKey’s project is currently composed of three people which, by order or arrival in the team, are: Vincent (myself) Michel and David. Since we are a small team, the best way to present all of us is simply through FunKey’s story, so here it goes:

Vincent

As some of you may know Funkey started out as a project submitted during the 2017 Hackaday Prize called Keymu. Before explaining why I undertook such a project, let me quickly go over some of my background.

Originally, I studied telecommunications during my master’s in France. Telecommunications is a vast topic that encompasses many areas such as informatics, electronics, networking, signal processing… I specialized in the latter, and during my last year abroad, I quickly drifted towards computer vision, a topic that never ceases to amaze me. After the master’s, I actually continued in this branch during my PhD.

So, yeah, I was a research scholar, in Computer Vision and Machine Learning at the time, pretty far from electronics…

Something always felt missing though, all this high-level knowledge felt shallow without the low-level one. Computer vision is often used on embedded products, allowing them to “see” the world and understand it for us, faster than us. I simply felt the need to know how to build the machines for which we developed these algorithms, in order to understand the whole chain. After my PhD then, I started working as an embedded engineer for a company in France. This is where I met Michel and David for the first time.

When, a few months later, Hackaday published an article about SpriteTM’s tiny console, it was love at first sight. I wanted very badly to build one, not just to own it but because it called to my inner retro-gamer instincts and it was a great way to learn about electronics and embedded software. At the time SpriteTM did not release yet any of the code or designs, hence Keymu was born.

I quickly realized, I undertook something way bigger than expected for a first “side project”. It was completely different than SpriteTM’s design: it was based on a computer module, I needed to develop a custom linux distribution (very different code than for microprocessors, especially the drivers), the mechanical hinge was harder than expected…This is when Michel entered into play.

Michel

It is safe to say that Keymu might have never hatched without him. Apart from helping me with the electronical design, every roadblock I met seemed only small bumps before his incredible experience. He followed the design and all the new ideas from the start, started himself installing the environment for developing the Linux distribution with Yocto, proposed solutions to many encountered problems… All in all, he was already part of the team from the start, he apeared only “officially” in the team during our next project: FunKey Zero but it is simply formalities.

For the purpose of this log, I have asked my partners to present themselves through small bios. Since I started this first narrative point of view, it feels only logical to keep it throughout the log, that is why I prefer to restranscribe their bios as they’ve been provided.

Here is Michel’s:

I started my engineering experience at age 2 (back in 1968) by inserting metallic knitting needles into a live wall socket.

I stumbled upon my first computer (Goupil 2) in January 1980. Shortly after that (and for quite a long time), I got my second computer: an Apple II, still in love with it! I developed my first expansion board for it to hook a Teletype Model 33 from 1963 as a daisy wheel printer over 20mA current loop. I have been developing both hardware and software at the same time since then.

My first Unix steps were in 1984, using a CU connection to the university PDP11. I switched to Amiga 1000 in 1986, and worked mainly on 68k-based custom-made computers until 1996, with lots of business travels worldwide (spent 1 1/2 year in Atlanta, GA, USA during the Olympic Games in 1996).

I discovered Linux 0.9.x in 1994 (Thx to Pierre Ficheux). In 2002 I wrote the iPaq Linux Howto: http://mstempin.free.fr/linux-ipaq/html_nochunks/Linux-iPAQ-HOWTO-1.1.html and since then have used mostly Linux-based systems for my personal work.

I Reverse-engineered the TL-WR703 pocket router hardware in 2012: http://squonk42.github.io/TL-WR703N/ and https://github.com/Squonk42/TL-WR703N

In April 2014 I started the Domino.io project (https://www.kickstarter.com/projects/706167548/dominoio-an-open-hardware-wifi-platform-for-things/description “An Open Hardware WiFi Platform for Things”). We launched a successful Kickstarter campaign in April 2015, with all products delivered to users (almost) on time. They are still available from GL-Inet https://store.gl-inet.com/collections/other-products/products/domino-core-board shop, eBay and Amazon

A lot of experience indeed. At this point I left back FunKey’s story at FunKey Zero. For those who do not know this project, we hit a real roadblock with Keymu, one that could not be overcome this time: the main computer module (Intel Edison) had been discontinuited. In order to improve Keymu, we then developped a board with everything new: processor, storage, display, sound, power management,…

This board was part of our FunKey Zero project submitted in 2018 and was meant to focus on software and electrical development only. We knew Keymu’s hinged design was the only way to go to optimize screen size, buttons thickness, and battery’s autonomy but Michel and I simply did not have enough mechanical design experience to build something fit for production, especially with an active hinge.

This is when David entered the team.

David

To be completely fair, David already knew about Keymu since he helped giving advice when I worked on the mechanical design. After FunKey Zero, we again asked for help reviewing our design and I remember David being instantly on boad (he told me at the time that the project looked really “funky” without even knowing the name was “Funkey”). He was in search of interesting projects to work with, and we were in search for a professional mechanical engineer: it was a perfect match.

As before, here is his bio in his own words:

I have found my calling at age 14 when my parents offered me an electronic discovery kit (before that I wanted to be a road train driver).

To this end I have studied electronic for 9 years. During my last three years of studies, I have discovered electro manufacturing services (EMS) for the first time and had the chance to program my first SMT machine: the CP6 from FUJI: CHIP SHOOTER.
I then worked for 14 years for a company selling cameras designed for industrial applications and scanners for ancient manuscripts.

It is during this time that I have made the switch from electronic design to mechanical design as I self-taught myself how to use SOLIDWORKS. After many hours, I learned to master it and started developing professional mechanical designs.

In 2017, I started my own company (www.novatech-engineering.fr): a mechanical design office specializing in the integration of electronics, the design of plastic injection parts and additive technologies. Projects I designed with my clients are mainly IoT products as well as manual tools (plastic cutters, ZAMAC or magnesium).

I remain passionate and curious about technology in general. The FunKey Project is an adventure full of opportunities to discover and learn. An occasion to conceive, build and sell a very fun product among a pluridisciplinar team, all revolving around a great topic: the retro-gaming.

Back to the story: since then we are equally working on FunKey’s project, as well regarding our respective abilities (software, electrical design, mechanical design) as regarding all other aspects of the project (website, social networks, newsletter, this hackaday page, publicity, emails, commands, production…)

We all have day-to-day jobs but we are working hard during our free time to try and make FunKey a reality.

FunKey Revision B is out!

Here it is: the FunKey Revision B is eventually out!

You can find the corresponding design file ZIP and schematics in PDF in the “File” section.

FunKey Revision A Board

We consider FunKey Revision A board as an “alpha test” board. We built 2 units using manual pick & place and a small T962 reflow oven.

Except for a few bad solder joints, the only real problem we encountered is a wrong value for the R11 DDR RAM equalization resistor (was 240k instead of 240R). Another minor problem was a wrong footprint for transistor Q1 (was SOT323 instead of SOT23)

We can consider it as a success, as the boards worked (almost) out of the box and we were able to mount them into a 3D-printed case and make some nice videos out of them.

However, assembling this board manually doesn’t scale up very nicely: as these are dual-sided PCBs, the boards needs to go twice in the oven, using solder paste with different reflow temperatures, and just placing the 175 components on each board takes hours and a lot of patience…

We thus decided to move to…

FunKey Revision B Board

The goals for this revision were:

  • correct the Revision A bugs
  • make some mechanical adjustments required to ease the FunKey assembly into the case
  • replace some components to reduce the BOM cost
  • add test pads for a better testability
  • outsource the PCB Assembly (PCBA)
  • avoid over-engineering a working board!

Bug Corrections

As discussed above, the only physical changes were to replace the value for the R11 DDR RAM equalization resistor (was 240k instead of 240R) in the BOM and to change the footprint for transistor Q1 (was SOT323 instead of SOT23).

A few quirks in the schematics were corrected:

  • the “START” and “SELECT” signal labels were swapped (now also renamed to “START” and “FUN”)
  • the comment for R8 was referencing the wrong chip (U2, now U3)
  • some signal labels around U5 were not the same size as all other labels

Mechanical Adjustments

Although we were able to mount the Revision A PCB into the 3D-case, we found some minor assembly problems:

  • the 1.5 mm headroom over the PCB button side was too low over some components (the power inductor L6 and the transistor Q1): L6 has been moved to the other PCB side with all components for the DRAM Power, and the Q1 transistor was moved away from the buttons
  • the screen connector has been moved to be in front of the hinge flex opening and turned 180° to expose all the active pins to on the PCB edge side
  • consequently, the UART and battery connectors have been shuffled around, and the battery connector moved away from the screw well to provide more room to bend the battery wires
  • the 2x Omron B3U-3000P(M) rear buttons have been replaced because they were too fragile and did not provide a good feedback when pressed. We now use 2x Panasonic EVP-AVAA1A, which are the right-angle equivalent of the EVP-BB2A9B000 we have for the top buttons. These buttons are backed by the PCB edge so they have no chance to break if pushed too hard, and their haptic feedback is really good
  • the speaker mounting was tedious: we tried to avoid having to solder wires, but the PCB thickness of 0.8 mm was too important to provide a way to bridge the gap between the trace and the speaker pad with a solder blob, we had to use some TH resistor wire to do the connection:

    We changed the design to use castellated pads (“half-moon” plated holes on the PCB edge) positioned farther just over the speaker pads, so the distance between the PCB copper pad and the speaker pad is now zero and allows soldering the speaker with just a small solder bridge

Component Sourcing

The FunKey Revision A board used mostly components available from major online distributors, ( we ordered them from Mouser, as they are today significantly cheaper than Radiospares, Farnell or even Digi-Key). OTOH, some exotic components (the Allwinner V3s CPU, the AXP209 PMIC and the microUSB connector) had to be sourced from AliExpress, as they were not available anywhere else at a cheaper price.

But mostly, the FunKey Revision A board should be considered as a “western” board, whose components are not optimized to be produced in China, which will be our final production country for obvious cost reasons.

We took the opportunity to roll out the FunKey Revision B to switch to mostly Chinese suppliers whenever possible. We found most of the components at www.lcsc.com and its Chinese sister website www.szlcsc.com.

The price difference there is significant, as passive components are 10x cheaper, connectors are 5x to 6x cheaper, and you can find some equivalent DC/DC or PMIC chips at a fraction of their western price. Another way of saving money is to avoid crossing borders: in this case, you’d better buy the components to mount on the PCB in China from China, if possible.

However, there are some components we still had to provision from global online distributors:

  • the L3/L4/L5 power inductors
  • the 0603 current measurement resistor R21
  • all the EVP-BB2A9B000 and EVP-AVAA1A (R/A) tactile switches as the required quantity was not in stock from LCSC
  • the S14 MEDER MK24 Reed switch (more on this later in this article!)
  • the SP1 CUI CDM-10008 speaker
  • the PCAL6416AHF,128 I2C GPIO expander U1
  • the PAM8301AAF audio amplifier U3

All other passive components were replaced by some available at LCSC, but not necessarily the cheapest ones: we chose the “cheapest available in quantity” ones instead, to make sure we don’t have to switch to another reference later.

We found equivalent parts for the MicroUSB and MicroSD card connectors and for the DRAM DC/DC U4, as well as for the crystals Y1/Y2. the AXP209 PMIC U5 was directly available there, too.

Only the Allwinner V3s CPU U3 had still to be sourced from Alibaba.

With these component changes, we are now confident that we can reach our target electronic BOM cost for MP (Mass Production).

PCBA Tests

Another very important point to consider in order to get a product that is RFM (Ready For Manufacturing) is to make sure to have a good test plan for the PCB/PCBA: you cannot count on everything working as expected without a glitch on thousands of pieces!

Of course you may ask the PCB manufacturer to perform 100% electrical test on the naked PCB using “flying probes”, but another important issue is to make sure the final PCBA (PCB Assembly) is good too.

The technique most suitable given our expected production quantities and board characteristics (small size and dual-sided) is to use test jigs with some retractable interfaces featuring either mating connectors and/or spring-loaded “Po-Go” pins that will make contacts with corresponding test pads on the PCB.

On the FunKey Revision A PCB, we already had some test pads, but they prove too small to be useful. For Revision B, we increased their size to 1 mm diameter, and we made sure to have all important signals available on the least populated PCB side (the button side).

We started to define a progressive test plan to check that all the parts on the assembled PCB perform as desired, using as little as possible steps and test vectors to make the test procedure as fast as possible: time is money on a production line!

Having a good test plan defined early in the design phase is a key point to reduce defects during MP.

Outsourced PCBA

As said earlier, the FunKey Revision A boards were assembled by us using a small reflow oven.

One major goal of the FunKey Revision B is to make sure that the board assembly can be outsourced, meaning that we are able to provide all the required information for this task, yet another key step towards a successful MP.

We took the opportunity of the limited-time (now over since July 31st) offer from SeeedStudio for free assembly for 5 pieces.

Conforming to SeedStudio requirements for gerbers files, pick and place files, assembly drawing and BOM file took us some considerable amount of time, but eventually, this will certainly help us to formalize the PCBA procedure for MP too.

We are still waiting to receive the boards that were approved for manufacturing, we cross our fingers and toes, hoping everything will go as expected!

New Features!

We tried very hard to avoid adding more features to the existing FunKey Revision A design that is working, but hey, we are engineers, after all!

But in order to avoid adding bugs by over-engineering the board, we limited them to 2 low-risk changes:

  • we added a separate LCD_RESET line for the LCD (just a single wire with a pull-up resistor R29, so we can reset the LCD without having to reset the whole board in case something goes bad with the display
  • we added a magnetic Reed switch S14 to the AXP209 PMIC’s N_OE (Negative Output Enable) input, in order to suspend the FunKey console after a delay when the lid is closed, and resume it back to where it stopped when the lid is opened again:

Conclusion

Given its limited design changes and large improvements in terms of electrical BOM cost optimization, testability and assembly outsourcing capabilities, the FunKey Revision B board must be considered as a “beta test” board, which is an important milestone towards our goal to produce the FunKey retro gaming console in large quantities in China.

Schematics: Power

Capacitor Usage

So far, we already encountered capacitors for many different usages:

Load Capacitors

We have seen load capacitors used with the 2 crystals in the discussion about CPU.

A quartz crystal always provides both series and parallel resonance, the series resonance being a few kilohertz lower than the parallel one.

Crystals below 30 MHz like ours are generally operated between series and parallel resonance, which means that the crystal appears as an inductive reactance in operation, this inductance forming a parallel resonant circuit with externally connected parallel “load” capacitance. Any small additional capacitance added in parallel with the crystal pulls the frequency lower in the range between the series and parallel resonance frequencies, insuring crystal startup and stable operation.

For modern circuits, these load capacitors have a typical small value < 20 pF.

Bulk Capacitors

Bulk capacitors are used to prevent a power supply from dropping too far during the periods when current is not available. At the same time, they help to reduce the power supply voltage ripples by smoothing their output voltage.

Many such capacitors are used at both the input and output of the numerous linear and switched mode power supplies in the PMIC discussion

The main bulk capacitor value is generally high (some µF), but there may be smaller parallel capacitors added for stability.

Coupling Capacitors

As you probably know, capacitors are made of 2 parallel conductive electrodes separated by a (thin) isolating dielectric material (even if these electrodes are rolled or layered to reduce the component size). Thus by construction, no DC (Direct Current) can flow from one electrode to the other, but by influence using the electric field, AC (Alternative Current) still can go through. This is how coupling capacitors are used to link 2 circuits while removing any DC bias voltage on one side or the other of the capacitor.

We use such a coupling capacitor in the Audio schematic description for feeding the audio power amplifier from the CPU audio output.

Filter Capacitors

We have seen many examples where capacitors are used within passive filter circuits along with resistors or inductors, mainly to remove unwanted frequencies from a power supply or a signal.

Decoupling (Bypass) Capacitors

We have already seen some decoupling capacitors when looking at the buttons circuit.

Active components such as transistors and chips are connected to their power supplies through conductors featuring a (small) common impedance made up of complex (resistive, capacitive and inductive) value. Because of these parasitic components, a device that suddenly draws some current in spikes will generate a drop in its voltage power supply. If many devices are sharing the same power supply and impedance, the state of one device will be coupled to the other ones through the common impedance of the power supply conductors and may affect thir operation.

In order to decouple the devices, capacitors placed as close as possible to the device power supply input pins are used, which act as local energy storage. These capacitors are also named “bypass capacitors” as they shunt transient energy from the power supplies past the device to be decoupled, right to the GND return path.

There may be different capacitors values placed on the same power supply pins in order to filter transients at different frequencies: the bigger the capacitor value, the lower the frequency. A typical value is 100 nF, and values from 1 µF to 10 µF are used for lower frequencies and / or higher current draws, while lower values of a few nF are used for filtering higher frequencies.

In essence, decoupling capacitors are not very different in their function from bulk capacitors: the only difference is one of scale, both of current and of transient duration. Bulk capacitors deal with large currents and periods of 10s of ms, whereas decoupling capacitors are used for much lower currents and much briefer periods (typically 10s of ns for TTL or CMOS devices) .

Schematics

The last part of the FunKey schematics merely contains only decoupling capacitors:

One exception is the Allwinner V3s CPU HPR/HPL circuit which features an RC-to-ground circuit between the amplifier and the preamplifier input with the resistor R27 and capacitors C79 and C81, as recommended in the V3s hardware design guide.

The only other remarkable point left in this schematic is the resistor divider R25 / R28 which provides a reference voltage at half the DRAM power supply voltage level, which is used for the integrated DDR2 DRAM merged drivers and dynamic on-chip termination already discussed at the end of the previous CPU schematic description.

Conclusion

This concludes the description of the FunKey gaming console electronic schematics. The full schematics is available in the design Zip file and in PDF format too.

As we have seen, the design is not overwhelmingly complex, but it contains many details that are all important to make sure the device works as expected.

Schematics: SD Card

The FunKey game console uses the SD Card both as its boot device and its only storage device, so a good operation of this interface is absolutely mandatory.

The Allwinner V3s provides 2x 4-bit MMC / SD Card / SDIO interfaces. In the FunKey, only interface #0 is used.

If you look on the Web, you will find many contradictory SD Card interface designs, with a combination of pull-up / pull-down resistors, ESD devices and power supply filtering, with all pins wired or not, such that it is very difficult to know what is really required. To better understand the situation, we need to go back to the specifications.

Specifications

The SD Card physical interface is provided in the “SD specifications, part 1, Physical Layer Specification version 2.00, May 9, 2006”, for which a simplified version is available here.

The MMC phyiscal interface can be found in the “Multi Media Card System Specification version 4.3, JESD84-A43, November 2007”, available here (registration required).

But a good summary of the requirements is given in the “AN10911 SD(HC)-memory card and MMC  Interface conditioning” application note from NXP, from which this schematic is taken:

Note: This schematic does not include details concerning card-supply and typical power-supply decoupling capacitors.

Write Protect (WP)

A write protect mechanical switch is provided in the full-size SD Card, but not in the mini or micro SD Card form factor. As we plan to use a micro SD Card only, it is not used for the FunKey, along with its pull-up resistor and ESD protection.

Card Detection (CD)

As the SD Card is required to boot the FunKey, is always inserted and opening the device is requried for its removal, we don’t need the optional card detect mechanical switch feature (even if the chosen connector provides it) and its related pull-up resistor and ESD protection.

The SD Card specification provides another mean to detect the card using a card built-in pull-up resistor on its DAT3 signal, that can be later disconnected during normal operation using he SET_CLR_CARD_DETECT (ACMD42) command. In order to correctly detect if the card is inserted, a high value external pull-down resistor (> 270 kohms) is required to drive the detect signal low when no card is inserted, while the card built-in 10-50 kohms resistor will drive this signal high when inserted.

However, this feature is not compatible with MMC cards, so its usage should be avoided and the mechanical detection is preferred.

Pull-Up Resistors

Both the SD Card and MMC specifications require not to leave the interface signals floating, except for the CLK signal, where a pull resistor would cause significant signal distortion because of the required high speed and short rise/fall times. OTOH, it is recommended to add a series resistor on this CLK signal as close as possible to the clock source (the CPU) to avoid ringing, as we already discussed it in the log about the CPU.

Hopefully, the Allwinner V3s CPU provides internal pull-up resistors for all these signals, so we don’t have to add external pull-up resistors. These resistors are given with a typical value of 100 kohms (50 min, 150 max). Unfortunately, the CMD signal for MMC card features an open-drain output mode, and its value should be undercut (down to 4.7 kohms) to guarantee a sufficiently short rise time in this mode.

Schematics

The FunKey SD Card interface schematic is the following:

Even if in the FunKey device the SD Card and its connector are not accessible without opening the enclosure, there may be some situations where the user may decide to do so. We thus attach an ESD protection TVS diode (D16, D17, D18, D25, D26, D27, D28) on each signal to avoid any ESD hazard.

As discussed above, a single pull-up resistor R10 is used on the CMD signal for MMC compatibility.

The micro SD Card connector built-in card detection switch is not used, since the card must always be inserted for the FunKey to boot, and the corresponding pins are thus connected to GND.

The SD Card power supply is done through an RC low-pass filter to provide a soft-start operation, as the card built-in large bulk capacitor on its power rail may collapse the supply voltage when initially powered up.

Photo shoot side by side with PocketSprite

If you are on this page you might already know about the PocketSprite. For those who don’t it is the tiniest keychain-sized console, born from a crazy tiny gameboy project introduced in 2016 by SpriteTM. It was this project that inspired our first Keymu gaming console. We encourage you to go on Keymu’s page if you want to know more about how FunKey started.

A few years later SpriteTM eventually launched a successful crowdfunding campaign and from there went to production to build and sell the PocketSprite. As for the PocketSprite’s initial prototype that inspired our first Keymu prototype, it is now PocketSprite’s production success that inspires us to do the same with FunKey.

Keymu’s idea – then forwarded to FunKey – was to build our own version of SpriteTM’s first prototype from scratch while improving where we thought it could be done. As for mechanical design this translates mainly in FunKey’s clamshell design which allows to fit a wider screen, larger buttons to play comfortably, a bigger battery and all this while keeping a very tiny form factor. 

But photos are worth a thousand words so here they are:


3D Printed prototypes before injection molding

Introduction

FunKey’s design has been completely rethought. It is now sturdier, slimmer, with an active hinge that puts no stress on the cables and most importantly thought for production. We have designed every component while thinking about the injection molding process, this means clearance angles, drawers for the mold and other things that will be explained in a dedicated log about the design.

Here instead we would like to take an unconventional path and start talking about the in-between process that is after the design and before the injection. Let’s say your design for injection molding parts is done, how do you test it to assert its validity before investing thousands of dollars in a mold and go into production?

Nowadays, we are lucky to live in a world where 3D printing not only exists but has been vastly democratized to the masses. Just between our three group members we own about five different 3D printers (not all in the best of shapes but they still help making the point). In order to validate our design, then 3D printing is the right solution, but FunKey being what it is – that is to say very tiny – not all 3D printing technologies are adapted to our needs.

The question is, then, which type of 3D printing technology is the most adapted to our needs? We have tested the three principal ones: FDM, SLA and SLS and wanted to report our results in this log so that other makers can use this knowledge.

Brief description of the different 3D printing technologies we used

  • FDM (Fused deposition modeling) which consists in building parts layer-by-layer from the bottom up by heating and extruding plastic filaments. It is by far the most common technology among hobbyists since great quality printers are now available for a few hundred dollards and the filament is relatively cheap (). 
  • SLS (Selective laser sintering) which sinters a powder by using a laser. Thiese printers are still way too expensive to be accessible to the mass market but it is possible to contract 3D printing companies to get prints for an affordable price.
  • SLA (Stereolithography) and/or DLP (Direct Light Processing) which solidifies a liquid resin layer-by-layer by photopolymerisation. Formlabs has democratized this technology which was previously reserved to professionals (even if the printers are still relatively expensive compared to FDM ones as well as the material). 

Comparison of the print results

The mechanical properties to consider for our prototype are the following:

  • resistance
  • precision
  • surface finish

For the FunKey, we need a great precision for all the small details, a good surface finish and a sturdy case to resist the efforts on the hinge.

FDM

For this technology we have tried two prototypes, one printed with a common simple extrusion printer and the other with a double extrusion printer with a water-soluble material. The water-soluble material is used as support material and life-changing during the post processing of prints. You can see below the prints realised with an Ultimaker 2+, nozzle 0.6mm, layer height 0.15mm and PLA filament.

Below are the prints from an Ultimaker 3, nozzle 0.4mm, layer height 0.10 and double extrusion with standard PLA and water-soluble material.

Here are the results after a one hour bath in cold water and some toothbrushing to remove the residues:

This technology (and the PLA used) is sturdy enough for our needs but, even if the printing params might not have been optimal, the surface finish and level of details are far from ideal. It is especially noticeable when we start putting together the different parts. The outer parts are ok but, even with soluble material, the inner parts needing supports material lack precision and surface finish. 

SLS

With this technology we have chosen to go with PP (Polypropylene) among the variety of available materials such as PA12, PA+GF (Glass Filled), PP (Polypropylene), Alumide (Mix between PA powder & aluminum powder), TPU92 (Thermoplastic Polyurethane)…

Here is how the prints turned out with Polypropylene:

This time the inner parts of the casing were the same quality as the outer parts. As you can see this material is too elastic though. But if materials can be swiched for sturdier ones, precision still falls behind our needs. Besides, surface finish is not smooth and the porous material quickly tends to absorb dirt.

SLA

Different types of resin were tested with this technology:

  • Somos® EvoLVe 128 ( white)
  • Accura Xtreme (gray)
  • R11 from Envisiontech (red transparent)
  • ANYCUBIC resin (green transparent).

The biggest drawback with SLA printing would be that it is still very far from being as user friendly as FDM printers. Not only does the resin needs to be handled carefully in the first place but the post-processing treatments require one to not be afraid of a little chemistry. The prints must be cleaned with isopropanol alcohol, and after removal of the supports, they need to be strengthened with UVs. Also, the smell of all these products is not the greatest.

The results however are definitely worth it!

A great exemple of the kind of precision that can be achieved without a sweat by SLA technology would be the tiny plastic rib depicted in the image below.

It needs to be precisely 0.7mm wide and printed over a part that would need support with FDM technology.

As can be seen in the picture below, only SLA achieves this level of precision (gray casing) with a very neat rib while even SLS  (white casing) clearly falls behind. 

Here is another example of the kind of precision achieved with SLA: the FunKey logo is only extruded 0.1mm in the casing. As you can see it is not only clearly visible but also very neet on the SLA print. This is not the case with other technologies that printed the same STL file.

We noticed our SLA prints were quite fragile however, this material is a bit more breakable and one needs to be careful when assembling the parts, espacially with the screws. They need to be put gently not to break the shanks.

Precision: theory vs reality

Theoritically here is the precision that each technology can achive in average:

  • FDM: ± 0,15 % (lower limit with standard nozzle at ± 0,2 mm), layer height: 0.05 to 0.3mm
  • SLS: ± 0,3 % (lower limit ± 0,3 mm) / layer height: 0.12mm
  • SLA: ± 0,2 % (lower limit ± 0,2 mm) / layer height: 0,05 to 0,2 mm

Below can be seen different measurements of the casing’s width (theoritically 44,3mm)

FDM :

SLS :

SLA :

Globally all three technologies are decent, it is the level of details that lacks for SLS and especially FDM compared to SLA.

Conclusion

This log does not pretend to be an extended research on 3D printing technologies but merely an attempt at finding the one best suited for trying out our prototype before production. Keeping this in mind we might not have set the very best parameters for FDM prints and they might be greatly improved, but our conclusion would be that in order to achieve the level of details FunKey needs, SLA technology is simply one step further regarding accuracy and surface finish.

We encourage anyone however to print Funkey with their own setups and tell us about the results. All the files are available here or on thingiverse and grabcad.

Thanks to the SLA prints we were able to validate our design choices regarding the integration of the electronic components, the new screen with its flex and the strength required to open/close the hinge. It also allowed us to see where our design could be improved, for example by decreasing the led brightness, reducing the logo’s size, adding some matter to block the light from the display backlight, … and some other things that we will detail in a dedicated log.

We are now working on fixing these small issues and printing new SLA versions to validate them and then be ready for injection molding (probably in ABS). We intend to show in a future log the design rules that were applied to conceive FunKey’s plastic parts for injection molding (Draft angle, injector pin, injection gate, cavity, core…).