Here (see fig) the IC works in a monostable or "single-shot" mode. Initially the attached external capacitor stays discharged internally (through a transistor). A negative trigger less than 1/3 VCC applied at pin #2 initiates a flip-flop action, quickly discharging the capacitor and subsequently driving the output high. This results in an exponential increase in the voltage across the capacitor. The time period of this change can be expressed through the formula, t = 1.1Ra.C. The capacitor is fully charged after this period when the voltage across it finally reaches 2/3 VCC.
The flip-flop is then reset back by the internal comparator stage of the IC, resulting in an immediate discharge of the capacitor and ultimately the output shifts to logic low state.
Another important configuration is the astable mode, which is basically formed by joining pin #2 and 6 of the IC together. This attributes the circuit with the property of running freely as a multivibrator through self-triggering. The charging of the external timing capacitor takes place with the help of the resistors Ra/Rb, and the discharging process completes through Rb. Therefore the duty cycle of the circuit can be accurately confirmed by adjusting the ratio of resistors Ra/Rb.
Here the capacitor undergoes charging and discharging between 1/3 and 2/3 VCC; these periods are well regulated and are not disturbed by supply voltage variations.
The above operational mode of the IC 555 identifies the chip to be basically a time interval generator with many variations. By varying the time interval ranges, circuits using this chip can be implemented for many different applications: we’ll study some of them through the following simple 555 timer circuit examples.
Images - Courtesy National Semiconductor Datasheet