The monostable, or one-shot multivibrator has only one stable state and one nonstable state. The one-shot circuit is used to establish a fixed time interval, the beginning of which is triggered by a sharp positive going change in voltage. At the end of a predetermined period, determined by the value of circuit components (especially C9 and R4), the circuit returns to its original state.
During the stable state of operation, Q1 is cut off and Q2 is conducting. This conduction of Q2 is due to the positive voltage on the base of Q2 determined by the resistor R4. With a positive signal applied to the trigger input and coupled through D10, Q1 goes into conduction. The drop in voltage is coupled through C9 to the base of Q2; this cuts off Q2. The rise in voltage at the collector of Q2 is coupled through R6 to the base of Q1 to keep it conducting. As the discharge of C9 becomes less intense, the bias on Q2 again rises above cutoff. When Q2 returns to the conducting state, the drop in voltage at its collector is coupled to the base of Q1, and cuts it off.