I have just finished my binary wrist watch project (well, the new revision anyway). I was surprised at how small I was able to make it compared to last time.
I chose to go with the “super-yellow” color LEDs as they fit the purple OSHpark PCB very nicely. The biggest challenge was actually making a good looking wrist band for the watch. I originally intended to use a design like this but it turned out that due to lack of enough para cord I had left, I went with a simpler design that I had done once before.
Unfortunately I used the wrong footprint for the PIC when laying out the PCB. The PIC comes in a SSOP-20 package and the footprint marked “SSOP-20″ in the KiCAD library is actually a TSSOP-20. Ridiculously enough there’s also a separate TSSOP-20 footprint in KiCAD.
I was able to get away with bending the pins of the SSOP inward and then reflow soldering it to the TSSOP footprint. It’s a bit of butchery but I couldn’t give this one to Mr. Murphy.
After getting all the hardware wrapped up, I could start coding. The watch can be woken up from sleep mode (display off, only RTC running) by holding the right button for one second. It can be put back into sleep mode by holding the right button for one second again.
In sleep mode the watch consumes about 150µW when powered from a CR2032.
I don’t know how accurate my meter is at the µA range, I might measure the consumption again after receiving one of Dave Jones’ µCurrent measurement adapters.
Somewhere in the PIC datasheet it is mentioned that floating pins should be pulled to VDD or GND when not used to prevent excess power consumption because of switching currents. I haven’t done this in this revision of the board and I don’t know how big the impact on power consumption would be.
The time setting mode can be entered by holding the left button for two seconds. After that the display is blanked and the time can be set bit by bit by switching through the bits by pressing the left button. If the bit is to be set, the right button has to be held down for one second. The watch automatically enters normal display mode after the last bit has been set (once you’ve gone through all the bits).
All there is left to do is see how well it will hold up when I wear it every day and how long the battery will last.
Altough not having worn the watch in the presence of normal humans, I can already guarantee that now everyone will be able to easily identify me as a nerd.