Monostable Multivibrator

A multivibrator is an electronic circuit which is characterized by two-amplifying devices cross-coupled by resistors and capacitors.  It is used to implement oscillators, timers flip-flops as well. Multivibrators are used to produce square waves whether synchronous or asynchronous depending on the need. It consists of basically two-transistors which are cross-coupled in such a manner that output of one transistor is taken as an input for the second transistor. There are three types of multivibrators:

Astable, monostable and bistable.
Astable- In which the circuit is not stable in either state. It continually switches from one state to another. No clock pulse or any other input is needed in case of astable multivibrator.
Monostable- in this kind of multivibrator, one of the states is stable while the other is unstable. What happens actually is that we provide a trigger. But after some time the circuit itself becomes stable. Monostable circuits are used to create timing pulses of a fixed duration.
Bistable-In bistable circuits, both states are stable. In this state, flipping between the states takes place by trigger.

Lets throw some more light on these different types of multivibrators.
Astable multivibrator- This is a regenerative circuit which consists of two amplifying states connected in a positive feedback loop. It is connected by coupling of capacitors and resistors. Now we will emphasize on its working. For this we need to take a look of its circuit once. The astable circuit consists of two switching transistors, a cross-coupled feedback network, and two time delay capacitors which allows oscillation between the two states with no external trigger signal to produce the change in state.

Monostable multivibrators have only one state which is stable. It produces a single output pulse. This output is produced when an external input in the form of trigger is provided to it. This is also a RC coupled circuit provided with a time constant. The circuit returns back to its stable state after a certain period of time which is determined by the time constant. Monostable multivibrators are also known as one-shot multivibrators. The capacitor first charges itself with the help of a resistor and then it is discharged and brings the circuit's state to its stable state. Monostable type has the capability to produce a very short pulse and long pulse as well. It is to be noted that the time constant can be varied by changing the values of capacitors. The leading edge rises according to trigger applied while its declining edge depends on the time-period for which the trigger is applied.



As we have already covered, one fact is that a bistable has two stable states.

The bistable multivibrator can be switched over from one stable state to the other by the application of an external trigger pulse thus, it requires two external trigger pulses before it returns back to its original state. As bistable multivibrators have two stable states they are more commonly known as Latches and Flip-flops for use in sequential type circuits.

The discrete Bistable Multivibrator is a two state non-regenerative device constructed from two cross-coupled transistor switches. In each of the two states, one of the transistors is cut-off while the other transistor is in saturation, so as to fulfill the criteria of being stable all the time.

Working: Now we will see how a two transistor astable multivibrator works. We can understand its proper function with the help of the pictures below. For a better understanding lets take an example of any of the multivibrators. I am taking an example of astable multivibrator.

Assume that the transistor connected to a green LED is turned off and its collector voltage is rising towards the power supply; at that time second transistor is turned on. The other side of the capacitor is connected to the base terminal of the second transistor  is at 0.6v as transistor T₂ is conducting. Thus capacitor has potential difference of 5.4v. The first Capacitor starts its charging in opposite direction through resister R₃.

thus the base of transistor TR₂ is moving upwards in a positive direction towards Vcc with a time constant equal to the C1-R3 combination. However, it never reaches the value of Vcc because as soon as it gets to 0.6 volts positive, transistor TR₂ turns fully "ON" into saturation

         

As we can see in the two pictures shown above, one of the LEDs is turned on at a time keeping another LED off for a time until the first LED is turned on. Both of the LEDs won't work simultaneously.