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LED Matrix Watch

Wrist watch with monochromatic 16X16 LED Matrix inspired by retro LED watches of 70s and 80s.

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Goal of this project is to make unique and artistic wearable device for every day carry.

Information is dispalyed on custom 16x16 monochromatic led matrix made out of 0603 leds. Device can be programmed form Arduino IDE through built in micro USB. Micro USB is also used to charge lithium-polymer battery of 300mAh. With 300mAh battery and daily active time of 30min autonomy of 10 days should be possible. There are also RTC chip with alarm function, accelerometer, ambient light sensor, piezo transducer and hall effect switches to replace mechanical switches. Small neodymium magnet on the side of the watch is used to activate hall switches and emulates the look of "Crown" of mechanical watches.

Watch assembly is made out of 3D printed body, smoked acrylic used as screen cover, back cover pcb, strap adapters for 22 mm strap, electronics and magnets.

Electronics are made out of two PCBs arranged in "sandwich" structure. Top board holds 16x16 matrix while bottom board holds electronics for driving the matrix. 

Matrix is made up out of tightly spaced 0603 LEDs. Since matrix is diagonally 1.38 inches long that means that the screen has pixel density of 16.3 ppi.

Texas Instruments TLC59282 16ch constant current LED driver in combination with two 74HC164 shift registers is used to drive LED matrix. TLC59282 can sink up to 45mA of current and has an option to set current through each channel down to 2mA.

Silicon Labs CP2102N does USB to UART translation so that se ATmega328P can be programmed via microUSB connector. ESDR0502N is located on USB data lines to provide ESD protection.

As replacement for mechanical switches AH3360 Hall effect switch IC is used. Not using mechanical switches makes enclosure sturdier(no moving parts) and eliminates the need for debouncing and makes enclosure easier to 3D print. Downside is that each Hall switch uses 7uA of current on average.

For keeping track of time and date PCF8563 RTC is used. PCF8563 has integrated capacitor on its OSCO so putting external capacitor only on OSCI pin is needed. Changing the external capacitor on OSCI pin makes it possible to compensate oscillator frequency offset due to parasitic capacitance of PCB traces.

Ambient light sensor APDS-9005 is used to adjust brightness of the dispaly depending on the availability of ambient light which in turn helps with decreasing power consumption.

Battery is charged via single-cell charge management montroller MCP73831. Lithium-Polymer cell is charged to 4.2 with set current of 300mA. Depending on the version MCP73831 supports variuos preconditioning and end of charge modes of operation. Maximum charging current can be set to 500mA. Load sharing circuit is not implemented since the circuit while inactive enters low power mode.

Led Matrix Watch - Manufacturing files.zip

Schematics, Gerbers, Assembly drawings, Boms and 3D models.

x-zip-compressed - 2.01 MB - 10/15/2017 at 22:38

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  • 1 × ATMEGA328P-MMHR Microcontroller
  • 1 × CP2102N-A01-GQFN24 USB to UART bridge
  • 1 × PCF8563BS Real-Time Clock
  • 1 × MMA8452QT Accelerometer
  • 1 × TLC59282RGER Led driver

  • Bill of materials

    Mile3 days ago 0 comments

    In link below you can find complete bill of materials for electronic modules. Bill of materials of the complete assembly can be found in attached files.

    BOM link

    Project cost (approximation):

    • Electronic components ~35$. 
    • 3D print of enclosure ~15$. 
    • Li-Po battery ~6$. 
    • PCBs  ~20$. 
    • Strap holders and straps  ~10$. 
    • Magnets  ~5$. 
    • Screen protection acrylic ~2$. 

  • Final assembly

    Mile3 days ago 0 comments

    For now here are some pictures of the assembled device. Video that showcases its features will come soon. Assembly is made out of 3D printed body, smoked acrylic used as screen cover, back cover pcb, strap adapters for 22 mm strap, electronics and magnets.

    Strap adapters fit snugly and don't wiggle. They can be removed by pushing back locking pin on the bottom of the case. Customized pcb that acts as improvised cover is attached by 4 magnets and due to two cavities on each side of the case can be easily taken off. Smoked acrylic is glued on front face of the watch to protect electronics inside.

  • Making new version - part 2.

    Mile10/02/2017 at 20:55 0 comments

    I have finished testing assembled modules. Only thing left is to connect them together and mount then in enclosure. I am planning to finish hardware part of the project this weekend. Here is picture of Led matrix PCB running test software.


    Here are also some pictures of the enclosure. I have tested few types of adapters and straps (apple watch compatible off eBay) and they all fit perfectly and snugly. Magnets work and fit great although they might be tiny bit too strong.

  • Making new version - part 1.

    Mile09/25/2017 at 22:58 0 comments

    Here are two gifs showing working first prototype of Led Matrix watch. Gif on the right shows led matrix under neutral density filter (ND16) that is used in photography (got it from eBay). Plan is to cut filter into shape and put it over led matrix when in enclosure. Filter is 1.8mm thick.

    Next up, you can see pictures of the new PCBs. PCBs are made by OSHPark. Some changes introduced from the first version are: 

    • Optimization of components - capacitors (some 0402 caps changed to 0603 to reduce cost)
    • TH connections replaced with SMD pads
    • Reduced size so that smaller and more ergonomic enclosure is possible
    • Accelerometer changed from LIS3DHTR to MMA8452QT
    • Shift registers changed from 74HC595 to 74HC164
    • Led driver changed from TLC5928 to TLC59282 (more current to drive leds)
    • NCP170 LDO repleced by high efficiency buck converter TPS62743 (WCSP package)
            Led Matrix      Motherboard       PCB spacer         Backplate

                                                   

    There were no major problems with PCB assembly. All components reflowed nicely, except U2 that was shorted. Short was due to stencil being too thick so there was too much solder paste on the footprint. Also there were two mistakes on the PCB so they need to be corrected. PCBs on OSHPark link have all those mistakes corrected.

    1. VSEL pins of the TPS62743 were tied to the ground which gave output voltage of 1.2V and I needed them to be tied to input voltage to give output voltage of 3.3V. I disconnected traces from ground plane and connected them to input decoupling capacitor via thin enameled copper wire.
    2. MISO pin was not connected to ISP header, therefore jumper wire is needed from MCU pin 14 to ISP header pin 1. Jumper is need only while programming via ISP. After programming bootloader all programming is done via USB.


    • Reworking shorted U2 (WCSP-8, 0.4mm) - Desoldering shorted component, cleaning pads with soldring iron and soldering component with hot air station (with addition of flux).

    Enclosure was modeled in solidworks and ordered from i.materialise.com (price was 15EUR for one piece). Enclosure was printed using SLS printer in black polyamide and polished after. Few pictures to test if all fits together:

  • Soldering - Part 2.

    Mile07/02/2017 at 19:01 0 comments

    After some time off it's time to continue this project. Since last update i was left with soldering led matrix. Plan was to use custom ss stencil to hold LEDs in place while heating them with hot air station, but it didn't work(kind off). Since stencil was chemically made(etched) holes at the edges of stencil weren't big enough to fit LEDs so i had to improvise. Basically i mounted only one LED for each segment and shorted other one. This means that there are now 256 LEDs on PCB. Next step is to assemble the module, see how matrix looks under diffuser, test and improve the design. Here is the video and some pictures of the process:





  • Soldering - Part 1.

    Mile06/05/2017 at 22:15 1 comment

    Here are some picture and video of soldered led matrix driver board. All components were placed by hand using fine tip tweezers. It took me around 45 min to setup and 15 min to place all of the components. There were no solder bridges after reflow, only few unsoldered pads(U13 last picture) that needed to be reheated with soldering iron. Next up is writing test code while I wait for stencil for led matrix PCB(It takes long becouse it is free). Soldered led matrix PCB will be shown in "Soldering - Part 2." project log.

  • PCBs

    Mile06/03/2017 at 21:17 1 comment

    Here are the pictures of PCBs that arrived few days ago. Since these are the first PCBs that I ordered from OSH Park, I decided to inspect them in detail under microscope. Gold plating on copper is smooth, evenly distributed and nice to solder on (tried test soldering few components on one PCB). Silkscreen is smooth, "high resolution" and looks way better than on Chinese PCBs (Chinese mostly use dot matrix printers so theirs silkscreen looks "pixelated"). Only downside is that solder mask isn't perfectly aligned with pads, but that wont be a problem.

  • Modeling enclosure

    Mile05/18/2017 at 22:02 0 comments

    While waiting for PCBs to arrive I designed enclosure for the watch. I made few printed watch enclosures in the past and from my experience, I don't think one can be made to look good and be reliable. Moving plastic parts like holes that hold strap springs just wear out after a month of use. Therefore I designed enclosure that is compatible with apple watch accessories (straps, strap adapters...). Since strap adapters are metal and made for commercial watch they will be robust and available cheaply on ebay.

    Main parts of the enclosure are:

    • Clear cover - 1 mm thick acrlyic sheet covered with tinted vinyl wrap.
    • Main enclosure body - With slits for straps or strap adapters.
    • Back cover - 1 mm thick aluminium sheet with attached magnets and custom etched logo.
    • External magnet - Used to activate four hall effect sensors.
    • Sound port - Small hole next to external magnet for piezo sounder.
    • Magnetic USB adapter - Looks cool.

  • Ordering PCBs / Components

    Mile04/30/2017 at 21:00 1 comment

    I have ordered PCBs from OSHPark. They are expected to be done by May 19th, adding shipping time I except them by the end of May. For now only gerber files are available in project repository. I will upload entire project file with embedded step model of enclosure when model is. Finished gerbers look something like this:

    All of the components were ordered from Mouser except LEDs used for matrix (Vishay TLMS1000), connectors and battery. LEDs were ordered from TME since they were cheaper (0.0715$ @ 500pcs compared to 0.138$ @ 500pcs). Here are some pictures of lit LEDs soldered on soe random PCB (I wanted to see how well epoxy diffuses light):

  • Designing PCBs

    Mile04/19/2017 at 20:21 0 comments

    Since the last update I was working on driver PCB for led matrix but there are still few touch ups to do. PCBs will be ordered from PCBWay or OSH Park. PCBWay is dirt cheap (10 PCBs are 5$ + shipping), while PCBs from OSH Park have nice ENIG surface finish.

    Changes: I have replaced original peizo buzzer with new one that can be reverse mounted on PCB (more space efficient) and replaced 100mAh battery with 300mAh one (writing on silk is wrong. It is 300 not 350mAh). New battery is 4 mm thick(3 mm old) and more space efficient(covers more free space on PCB).

    PCB Characteristics:

    • 0.8 mm PCB thickness
    • 0.15 mm signal, 0,3 mm power trace width
    • 0.15 mm trace clearance
    • 0.3 / 0.6 mm vias

    Watch thickness should be something like this(top to bottom):

    • 2 mm Plexiglas (don't think I can get thinner plexiglas)
    • 0.2 mm of clearance
    • 0.6 mm component height on LED matrix PCB
    • 0.8 mm LED matrix board thickness
    • 1 mm of clearance for components on driver board
    • 0.8 mm driver board thickness
    • 4 mm battery thickness
    • 0.5 mm of clearance
    • 1 mm for removable aluminium back cover

    = 10.9 mm total thickens. For comparison Apple watch (42 mm version) is 12.46 mm thick.

    Assembly Video:

    Some picture of PCBs:

View all 10 project logs

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Discussions

Brain Seabird wrote 10/05/2017 at 22:09 point

Possibly stupid question: Isn't the TPS62743/U2 a WCSP-8 BGA package with solder balls already attached? I ask because you mention the short being caused by too much solder paste on the pad which strikes me as redundant and possibly the cause of the short since BGAs don't need extra solder.

I'll freely admit I don't know what I'm talking about but if I'm confused about something, please do tell. I like the idea of eventually trying some smaller BGA packages so having a better idea of what it's like to actually use them for 'home-gamers' would be great.

The project is awesome! Keep up the sweet work.

  Are you sure? yes | no

Mile wrote 10/07/2017 at 23:12 point

Solder paste is usually added on BGA footprint because it contains flux. Flux helps activate solder balls and center the package in place.

If I was soldering BGA package by hand (not using stencil) i would just add some flux and heat it with hot air station. I would stay away from liquid fluxes because they evaporate too quickly when using higher temperatures. Solid choice for flux would be Amtech NC-559 no-clean flux gel - good wettability, affordable and easy to clean with cotton swabs. 

  Are you sure? yes | no

msowa2424 wrote 09/30/2017 at 16:13 point

Neodymium magnet that activates hall switches sounds interesting as a watch crown. With 2 hall sensors and a spring mounted magnet we could sense the rotation of the magnet and lateral force applied, nice.

  Are you sure? yes | no

Mile wrote 10/02/2017 at 21:08 point

Cool idea! In that case vertically mounted sensors (ones in TH package) need to be chosen due to the magnetic field of the used magnets.

Thank you for the comment.

  Are you sure? yes | no

Wolfgang wrote 09/29/2017 at 06:48 point

Awesome! are the "order from OSH Park" badges supposed to be links? They seem to be broken!

  Are you sure? yes | no

Mile wrote 10/02/2017 at 21:00 point

They were supposed to be links bot something went wrong. Sorry for that. I got that fixed now.

  Are you sure? yes | no

oshpark wrote 09/29/2017 at 03:57 point

Awesome project!

  Are you sure? yes | no

Mile wrote 10/02/2017 at 21:02 point

Thanks, glad to hear that.

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Vikas V wrote 08/13/2017 at 19:07 point

Awesome project. Hand placing that many small LEDs so close together is no small feat. I know first hand!

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zakqwy wrote 04/10/2017 at 18:09 point

I like this project, and I _especially_ like the cover art on the project page. Great shot.

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al1 wrote 04/02/2017 at 11:53 point

do you plan to place all leds by hand? This should be quite time consuming

  Are you sure? yes | no

Mile wrote 04/02/2017 at 13:07 point

Yes. I will be using manual pick&place machine. Since leds are in 0603 package I don't expect any problems even if needle nose tweezers are used. If I can manage 5 sec per led it would take me around 40 min + 30 min for PCB with drivers and mcu + 5 min to apply solder paste. 80 min is reasonable since I plan to make 2 prototypes (one for me + one for a friend).

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K.C. Lee wrote 04/02/2017 at 07:57 point

Make sure to shoot video if you reflow them in the oven.  The self alignment for a large matrix this close is going to be interesting.

  Are you sure? yes | no

Mile wrote 04/02/2017 at 13:13 point

Since I will be using fine grain paste led should slide in place just fine also will make sure to document the entire assembly process.

  Are you sure? yes | no

Mark Jeronimus wrote 09/28/2017 at 07:47 point

I see some LED pads have large traces going inside whereas others don't. This is bound to cause subtle disalignments when reflowing. Take a look at Figure 6-4 of http://www.altronmfg.com/pcb-design-for-manufacturability/

  Are you sure? yes | no

davedarko wrote 04/02/2017 at 03:17 point

good luck with  soldering that :D

  Are you sure? yes | no

Mile wrote 04/02/2017 at 13:15 point

Thx. I quite like that part :D

  Are you sure? yes | no

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