As the name suggests, these watches use analog panel voltmeters instead of traditional watch faces to display the time. The idea didn’t occur to me, so I never really blogged about design; I just made one and have it on my office desk.
The idea is enduring, but most of the designs I see on the internet are unnecessarily complicated and not that pretty, so when I decided to create a revised design, I thought it might be good to document it better. The process started with a rough mockup in a 3D design program:
For this version of the meter clock, I opted to use three common, 90° panel voltmeters from Amazon (AddAbout $9). I took them apart, took careful measurements of the faces, and then printed replacement decals on adhesive paper. Printable PDF templates can be found Here.
Note that the new hour gauge has 13 divisions from 0 to 12, while the minute and second templates have 61 divisions from 00 to 60. This is because I wanted to apply a consistent motion to each arm; This meant that at 11:30, the hour dial could not simply be stuck at 11; It needed to move towards the twelfth division, even if it was never to reach it.
In addition to many other problems, the cheap “Baomen 65C5” meter I am using has a terrible plastic flange. I decided to use a hidden decorative pattern to hide this flange from view and keep the front panel interesting. This made it more expedient to cut the front and back on a CNC mill rather than building the enclosure by hand (as I did for version 1):
The rounded sidewall presented a very different challenge. For a seamless appearance, I had to bend a flat piece of wood using a shaped template. To pull it off without a steam bending jig, I had to cut a series of internal notches on the side wall. This allowed the wood to bend more easily:
The wood had to be wetted, clamped and then allowed to dry. After a few days, I glued the curved side wall to the front and back faces, relying on another template cut from scrap plywood to get the exact fit without any more gymnastics with clamps and ratchet straps:
Anyway – here’s the assembled piece after sanding and a coat of nitrocellulose lacquer:
Not bad, right?
The circuit is much less interesting and took about an hour: I caught the venerable AVR128DB28 The MCU powered it from a wall wart, connecting it to an 8 MHz crystal (ECS-80-18-4X-CKM). A 32.768 kHz crystal would also be suitable. The panels are connected to three digital output pins (PC0, PC1, PC2). Finally, two input pins (PD6, PD7) are connected to two small pushbuttons on the back and are used to set the time.
Note that the circuit does not require a digital-to-analog converter or any other additional components to drive the meter; Instead, I am using a relatively high frequency digital pulse train. The meter’s inertia does the rest, settling into an intermediate position based on the software-controlled signal duty cycle.
code can be seen Here; This is a short and good comment. The basic idea is to advance a 10Hz counter using timer interrupts synchronized with the crystal. With that out of the way, the main event loop calculates the appropriate duty cycle and then manually toggles the output pin. Although the chip has a hardware PWM module, the application is so simple that using the PWM circuitry won’t really gain us anything.
Here’s the obligatory “rollover” video captured around 11:59:59:
peace out.
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