Monostable Multivibrator

Monostable Multivibrator:

Monostable multivibrator has one stable state and one quasi stable state (astable state). When an external trigger applied to the circuit, the multivibrator will jump to quasi stable state from stable state.

After the period of time it will automatically set back to the stable state, for returning to the stable state multivibrator does not require any external trigger. The time period to returning to stable state circuit is always depends on the passive elements in the circuit (resistor and capacitor values

Circuit diagram:

Circuit Operation:

• When there is no external trigger to the circuit the one transistor will be in saturation state and other will be in cutoff state. Q1 is in cutoff mode and put at negative potential until the external trigger to operate, Q2 is in saturation mode.

        In this case, charging of C 1 through RL1 can be seen which is shown in 

         Below diagram

• Once the external trigger is given to the input Q1 will get turn on and when the Q1 reaches the saturation the capacitor which is connected to the collector of Q1 and base of Q2 will make transistor Q2 to turn off. This is state of turn off Q2 transistor is called astable stable or quasi state.

       In this case charge capacitor discharges through R1 as shown in below 

       Below diagram         

• When capacitor charges to VCC the Q2 will turn on again and automatically Q1 is turn off. So the time period for charging of capacitor through the resistor is directly proportional to the quasi or astable state of multivibrator when a external trigger occurred (t=0.69RC).

Uses of Monostable Multivibrator:

The monostable multivibrators are used as timers, delay circuits, gated circuits etc.