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Step 5Further Development
1. Evaluate FET switch supply: A charge pump will be tested. There is a certain je nous se quoi about the single battery design that almost out weighs the complexity. On the other hand, the simplicity of dual button cells (and space savings) is elegant in itself.
2. Design Interface: The watch needs to be activated by a button if the battery is to last more than an hour or two. A touch sensor based on a Darlington transistor would be cool. The interrupt pin of the uC is brought out to a pin on the development board so that various interfaces can be tested.
3. Move to surface mount: There is a lot of space on the PCB under the battery. A .150 SOIC PIC 16F684 and TC4427A fit nicely. The SOIC package seems like a reasonable size, its not a tiny TQFN or anything. All other components will still be through hole. Most parts will be readily available, the others would likely have to be ordered anyways. Moving these two ICs under the battery saves enough space that we can really start to call this a watch (perhaps a pocket watch?).
4. Add watch crystal: An external crystal (specifically the 32.whatever kHz watch crystal) will be more accurate than the internal 32kHz crystal, and use less power than the calibrated internal 8MHz crystal. It might also be used on a counter pin to increment the PIC timer while it is in sleep mode. The timer interrupt could wake the PIC to increment the seconds, but otherwise be in a VERY low power state.
Project files:
These files are very draft-y, the archive includes:
Development board in Cadsoft Eagle format.
A ton of footprint libraries that go with the PCB (socket, inductor)
Bare-bones SMPS firmware. Enough to run the power and display a digit on the nixie.
2. Design Interface: The watch needs to be activated by a button if the battery is to last more than an hour or two. A touch sensor based on a Darlington transistor would be cool. The interrupt pin of the uC is brought out to a pin on the development board so that various interfaces can be tested.
3. Move to surface mount: There is a lot of space on the PCB under the battery. A .150 SOIC PIC 16F684 and TC4427A fit nicely. The SOIC package seems like a reasonable size, its not a tiny TQFN or anything. All other components will still be through hole. Most parts will be readily available, the others would likely have to be ordered anyways. Moving these two ICs under the battery saves enough space that we can really start to call this a watch (perhaps a pocket watch?).
4. Add watch crystal: An external crystal (specifically the 32.whatever kHz watch crystal) will be more accurate than the internal 32kHz crystal, and use less power than the calibrated internal 8MHz crystal. It might also be used on a counter pin to increment the PIC timer while it is in sleep mode. The timer interrupt could wake the PIC to increment the seconds, but otherwise be in a VERY low power state.
Project files:
These files are very draft-y, the archive includes:
Development board in Cadsoft Eagle format.
A ton of footprint libraries that go with the PCB (socket, inductor)
Bare-bones SMPS firmware. Enough to run the power and display a digit on the nixie.
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