Unless you’ve been paying very close attention to the retro-display scene, you likely haven’t come across Numitrons. These devices were invented in the 1970s, and use actual filaments (as is what’s used, or was used in light bulbs) to form 7-segment displays. While this makes driving them quite simple, just an on or off signal for each digit at a few volts, they also take a relatively massive amount of current to power.
Power concerns aside, hardware hacker “Dycus” decided to build his own Numitron watch, complete with four segments and the requisite colon in the middle. The watch has gone through three revisions, with the original drawing an impressive 450mA with all segments lit up (subsequent versions cut this number in half). An ATmega328 microcontroller is used for control, along with an RTC module, while a 100mAh lithium-ion battery provides power.
To save power, the display doesn’t come on until you tilt the watch up or press one of the buttons, and shines for five seconds. This means that, in theory, the watch can be read about 150 times before it dies, though there are still some issues with idle power draw. Nevertheless, t’s a very unique wearable timepiece that can be read well even in bright sunlight.
You can see more images of the first and third iterations of the watch here. Perhaps it will provide inspiration for your next display project!