(1) constitutes a Schmitt trigger, which is used for the interface of TTL system, shaping circuit or pulse amplitude discrimination.

(2) forming a multivibrator and a signal generating circuit;

As shown on the right,

Oscillation period: t = 0.7 (r1+2R2) c.

(3) A monostable trigger is constructed for timing delay shaping and some timing switches.

555 application circuit can be combined with 1 or above to form various practical electronic circuits, such as timer, frequency divider, pulse signal generator, component parameter and circuit detection circuit, toy game machine circuit, audio alarm circuit, power exchange circuit, frequency conversion circuit, automatic control circuit, etc.