The function of the 555 timer is mainly determined by two comparators. The output voltage of the them controls the state of the RS flip-flop and the discharge tube. It also contains 3 working states.

This circuit shows how a 4017 CMOS decade counter can be used to build a timer circuit. Push-button S1 will discharge capacitor C1 through resistor R2. This circuit shows how a 4017 CMOS decade ...

Over on YouTube [Andrew Neal] has a Function Generator Build for Beginners. As beginner videos go this one is fairly ...