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.
More undocumented applications for the 555 ‘oscillator:’ Not all applications oscillate as we will see. Most are knowledgeable about common CMOS logic inverters and Schmitt triggers such as the 74HC04 ...