LED Matrix Watch

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

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This project is inspired by retro led watches of 70s and 80s. This watch is more of a artistic expression than something practical.

As display watch uses 16x16 monochromatic led matrix made up of 0603 leds. 8 bit (255 steps) gray scale control of matrix is supported. MCU is flashed with Arduino bootloader so that it can be programmed form Arduino IDE through built in micro USB. Micro USB is also used to charge lithium-polymer battery of some 100mAh (probably more). With 100mAh battery and 30min active time per day autonomy should be around 10 days. There is also RTC chip with alarm function, accelerometer, ambient light sensor, hall sensors to replace mechanical switches, battery monitor circuit and piezo transducer. Small neodymium magnet is stored in watch enclosure to activate switches. Since magnet is cylindrical in shape and stored on watch side it emulates the look of "Crown" used to wind mechanical watches.

Watch is made of two PCBs arranged in "sandwich" structure. Top board holds 16x16 LED matrix while bottom board holds electronics for driving the matrix. Boards are connected with pin headers and spaced 0.8mm(thickness of spacer PCB) .

LED Matrix is made up of tightly spaced 0603 Vishay TLMS1000 Low Current LEDs. This is possible due to shape of the lens which covers entire LED body and pads that are located beneath the LED. Each matrix segment is actually two LEDs so that it looks like a single 1.6x1.6mm square.

Texas Instruments TLC59281 16ch constant current LED driver in combination with two 74HC595 shift registers is used to drive LED matrix. TLC59281 can sink up to 35mA of current and has an option to set current through each channel from 2mA to 35mA.

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 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 100mA. 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.


Schematics in PDF format

Adobe Portable Document Format - 1.94 MB - 04/01/2017 at 20:29

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  • 1 × ATMEGA328-MMHR AVR 8-bit MCU
  • 1 × CP2102N-A01-GQFN24 USB to UART bridge
  • 1 × PCF8563BS RTC
  • 1 × LIS3DHTR Accelerometer
  • 1 × AH3360-FA-7 Hall effect switch
  • 1 × APDS-9005 Ambient light sensor
  • 1 × MCP73831-2ATI/MC Battery charger
  • 1 × NCP170AXV330T2G LDO
  • 1 × TLC59281RGER LED driver
  • 1 × 74HC595BQ Shift register

  • 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:

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Enjoy this project?



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.

  Are you sure? yes | no

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).

  Are you sure? yes | no

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

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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