ScrolLED watch

A different idea is to add a small solar panel/cell with some changes:

• Adding a small solar panel which mounts on top of the microcontroller and other circuitry on the top side. Some options are the following:
  • A polycrystalline small cell measuring 23mm * 20mm * 3.2mm like these
    • http://www.aliexpress.com/item/20pcs-0-5V-80MA-polycrystalline-silicon-solar-cell-panel-DIY-technology-Small-production-material/32586814132.html
    • http://www.aliexpress.com/item/20pcs-0-5V-80MA-polycrystalline-silicon-solar-cell-panel-DIY-technology-Small-production-material/32631751716.html
  • A high efficiency monocrystalline solar cell available on digikey measuring 22mm * 7mm * 1.8mm
    • http://ixapps.ixys.com/DataSheet/KXOB22-12X1L-DATA-SHEET-20130902-.pdf
    • http://www.digikey.com/product-search/en/sensors-transducers/solar-cells/1966326?k=solar panel
• A rechargeable lithium ion battery needs to be used such as the following:
  • http://www.aliexpress.com/item/high-quality-SD351530-95mah-3-7v-rechargeable-lithium-ion-battery/32531538474.html
• The solar panel provides 0.6V (max) at 50mA. A boost converter is needed to get this voltage up to charge the battery
• All the circuitry needs to be on the top side. No double sided load
• The solar panel (especially the 22mm * 7mm one) needs to be somehow mounted on the top-side circuitry

Switching over to an ARM based solution is much better than an AVR+RTC chipset solution. Been looking at the STM32F030 chip and it is a perfect fit for this application! It has the following very desirable characteristics:

• 32.768kHz oscillator on-chip. Just need to hook-up the crystal. This might not be as simple as I thought. It has OSC_IN and OSC_OUT for HSE oscillator and OSC32_IN and OSC32_OUT for LSE oscillator. The OSC32 pins are not available on STM32F030 TQFP32 package
• RTC on-chip with calendar and leap year correction
• 8MHz RC oscillator on-chip for running the actual application
• Serial Wire Debug (SWD) based programming using just 2 pins (SWDIO and SWCLK). Need to use the STLinkV2 programmers which are inexpensive ($3 delivered!) on aliexpress
• Works down to 2.4V which is the end-of-life voltage of a 3V CR2032 battery
• 20mA max I/O drive current which can directly drive display LEDs
• Available in QFP32 package. Just enough number of IO needed

Need to use the NDK crystal mentioned in the clocking application notes (page 28). The NX3215SA part is available on aliexpress.

http://www.st.com/content/ccc/resource/technical/document/application_note/c6/eb/5e/11/e3/69/43/eb/CD00221665.pdf/files/CD00221665.pdf/jcr:content/translations/en.CD00221665.pdf

All information for getting started on using STM32 bare bones is mentioned in this excellent article (we don't need no dev boards!) :

http://kevincuzner.com/2016/05/22/dev-boards-where-were-going-we-wont-need-dev-boards/

Or maybe a low power version like the STM32L031 or STM32L051. It works down to 1.8V. Its actually less expensive than the ATMEGA88V (in single quantities on digikey)

The dense LED matrix has some issues:

1. Light leakage. In traditional dot matrix displays, there are opaque barriers between adjacent LEDs which eliminate light leakage. In this project, when an isolated LED is lit, the light coming off of its sides slightly illuminate the adjacent LEDs. This reduces visibility. This effect is more pronounced in the board where I have used white LEDs and not so much in the board where red LEDs are used. I guess in case of white LEDs, the yellow phosphor coating is getting illuminated on the adjacent LEDs from the stray side light of the illuminated LED. Couple of ways to fix this problem,
   1. Use narrow beam angle LEDs. These are expensive and taller than regular LEDs
   2. Add opaque barrier between the LEDs. Couple ideas here:
      1. Add a laser cut acrylic which has cutouts for the LED placements. This can be challenging as the gap between LEDs is very small and runs into the limit of laser cutter. Also need to consider the variation in movement of LEDs after reflow soldering
      2. Use an opaque epoxy coating in the gaps between the LEDs. The commonly available black electronics potting compound can also be used for this purpose. The epoxy needs to be dispensed from a syringe between the LEDs and let to set. Because the epoxy flows when it is in the liquid phase, a barrier might be needed on all four edges of the LED matrix to make sure it does not flow and run off the edges
   3. Use transparent LEDs instead of frosted/diffused. This largely eliminates the problem because the stray sideways light will just pass through the transparent material of the adjacent LEDs and not disperse the light. This applies particularly to white LEDs
   4. Use thin LEDs. SMD 0603 LEDs are available in thicknesses of down to 0.25mm. If there is no sideways light, there would be no light leakage. Also, if it is thin, it being diffused instead of transparent would not be a problem. Some examples of thin form factor LEDs
      1. http://www.kingbrightusa.com/images/catalog/SPEC/APG1608SURKC-T.pdf
      2. http://www.sunledusa.com/products/spec/XZMDKT53W-6.pdf
2. LED orientation. The design assumes that LEDs sit flush on the PCB after reflow so that all of them orient exactly perpendicular to the PCB. But this might not always be the case as can be seen from the pictures. The surface tension of molten solder helps a lot for this but not always. I have observed that the pads which have a little more solder paste applied to them have trouble pulling the LED upright when molten. So, need to use as little solder paste as possible so that this does not happen. Also, while placing the LEDs, it might help to completely seat them on the PCB
   1. This would be less of a problem if thin LEDs are used (as pointed above)