When struck, tuning forks produce a tone —an oscillation — at a certain pitch. While normally meant for musical purposes, this type of device can actually be used as a (compartively) slow electronic oscillator, and as seen in this project by Wilco Lunenburg, the basis for a clock!
The build actually started out some time ago, when Lunenburg was able to use a photo interruptor sensor to measure a fork’s vibrations at ~440Hz. After passing this signal through a dual op-amp setup, the result is a nice steady square wave that can be measured with an oscilloscope.
From here, the next logical step was to use this oscillator in a clock setup, which he did with the help of an ATtiny13 chip that keeps track of time and controls a 7-segment LED display. A coil assembly is also used, presumably to strike the tuning fork when needed to keep the tone going. Incidentally, the fork was actually about 1.5 Hz lower than its 440Hz rating, which is probably fine for tuning a guitar, but not acceptable for clock use. After some file work, he got it to a nearly perfect 440 Hz tone at 25°C (77°F).
You can see the results in the video below. Although the tone is nearly perfect, it’s also quite audible, meaning that this type of clock is very annoying without some sort of heavy insulation. It’s a fun experiment, but perhaps not one that others will be rushing to duplicate!