Electronic Circuits and Projects Forum

I have been creating various analog circuits for the past few months, and have decided to try digital circuits. I am looking for a circuit that is challenging but not too hard, if is not too much to ask i would prefer it be somewhat useful, e.x. a circuit that stops charging a capacitor after a certain amount of time. I dont care how long it takes just as long as it is somewhat challenging and useful. Thank you ahead of time.