Ben Eater and his 555-timer-based clock module (https://www.youtube.com/watch?v=kRlSFm519Bo) was instrumental in my early understanding of electronics and embedded system design.

I purchased the clock module, which was a sub-kit of Ben's 6502 based computer (https://eater.net/6502). The module utilizes the bistable mode (debounced switch between modes), a bistable and and astable mode.

In astable mode, you can use a potentiometer to adjust the clock frequency. In monostable mode, you can use a push button to pulse the clock.