Figure: The 555 internal circuit

The 555 circuit is consisted by two comparators, one ohmic ladder one flip-flop and a discharging transistor, as it is shown in figure

Figure: The 555 modes of operation a) monostable b) astable (multivibrator).

This circuit can be connected as a monostable multivibrator or an astable multivibrator.

The 555, connected as a monostable is shown in figure 1.1 a. In this mode of operation the trigger input sets the flip flop which drives the output to high. The discharge transistor is turned off and therefore the capacitor Ct is charged via Rt. When the voltage on the capacitor ( Ct) reaches the control voltage, which is defined by the three resistor voltage divider ( Vcont=2/3 Vcc ), the flip-flop is resetted. This turns the discharge transistor on, which discharge the capacitor. Thereafter the circuit can be charged again by a new pulse at the trigger input. The timed period is given by the

equation: T=1.1 Rt*Ct

Were T is the output pulse high period, Ct the charging capacitance measured in Farads and Rt the charging resistor in Ohms. If the circuit is connected as an astable multivibrator (figure 1.1 b), the comparator 2 of figure 1.1 sets the flip-flop, when the voltage on the capacitor Ct falls below 1/3Vcc, while the comparator 1 resets the flip-flop when the voltage on the capacitor becomes bigger than 2/3Vcc. In this case the discharging transistor is turned on, which discharge the time capacitor Ct via Rb. This allows the use of the 555 as an oscillator (figure 1.1 b) The time at the high (or charging) period is given by the

equation: Th=0.7(Ra+Rb)Ct

While the time for the low period is given by the

equation: Tl=0.7 Rb*Ct

The obvious observation from the above equations is that the duty cycle of the oscillator is always bigger than 50%. Or in other words the charging time is always bigger that the discharging period, since Ra+Rb>Rb taken in account that Ra>0. Yet if Ra>>Rb then a 50% duty cycle can be approximated.