Monostable Circuit Multivibrator is below the picture
Monostable multivibrator contains only one stable state. The other state is quasi stable state. Hence it is also called as one shot multivibrator. It is used to generate precise time delays
When a sufficient magnitude of positive pulse is applied to its trigger input the base Q1 is forward biased and it starts conduction. The current IC1 starts flowing through the collector of Q1. The voltage of VC1 decreases.
The regenerative feedback also increases the conduction of Q1 and also decreases the conduction of Q2. Finally Q1 goes to saturation and Q2 goes to cutoff.
The voltages VC1 is low and VC2 is high. The voltage already placed at the collector of Q1 is fully dropped across the capacitor C1. Hence the voltage now occurred at the base of Q1 is less positive and the voltage occurred at the base of Q1 is less positive and the voltage occurred at the base of Q2 is ‑Vcc. This state is a quasi-stable state because it is automatically changed by the charging effect of capacitor C1.
Now the capacitor C1 is going to charge towards VCC through the resistor RB2. When the capacitor C1 attains a sufficient voltage to switch ON the transistor Q2 it starts conduction.
By the regenerative feedback actions the conduction of Q2 increases and the conduction of Q2 increases and the conduction of Q1 decreases.
Finally Q2 goes to saturation and Q1 goes to cutoff. Now the base voltage of Q1 goes to ‑VBB and the base voltage of Q2 goes to less positive.
This state is a stable state. This state is converted only by applying trigger pulse to the trigger circuit. The ON period of Q1 or the OFF period of Q2 depends upon the value of C1 and RB2