The monostable circuit is modified to re-trigger itself by connecting trigger terminal and threshold terminal. As in the monostable circuit, the output is set during the charging period of a capacitor. When the voltage across capacitor tends to exceed 2/3 Vcc, the output is reset. The capacitor discharges through Rb. As the voltage tends to fall below 1/3 Vcc the timer is triggered again itself. Thus, the output is reset during the discharging period of the capacitor.
Astable Multivibrator using 555
However, the Set and Reset time durations will not be equal to this circuit. The resistors Ra and Rb are involved in charging the capacitor but, only resistor Rb is involved in discharging period. So, charging time will definitely be more than discharging time i.e.., TON is more than TOFF. Hence, this circuit can be used to generate clock pulses for digital circuits or where the frequency is the parameter rather than the duty ratio.
Astable with Equal Ton and Toff
This is called Square wave generator, which can be used to generate musical notes by setting the frequency of wave at desired musical note. For example, setting the frequency at 440 Hz and connecting speaker at the output in proper configuration gives musical note ‘A’ of musical piano. This note is used as tuning standard.