Monostable Multivibrator

Multivibrators are non-sinusoidal relaxation oscillators which can be of three types viz., astable multivibrators, monostable multivibrators and bistable multivibrators.
It is a well-known fact that all the systems exhibit the natural tendency to be in their stable states. However one can even design the circuits which will switch-over into an unstable state when triggered externally. The time-period for which the system remains in this quasi-stable depends on the design and can be varied by changing the values of the associated circuit components. On completion of this definite time-interval, these circuits will resume their stable state spontaneously i.e. they do not need any kind of trigger for this action. These kinds of circuits can be referred to as Monostable Multivibrators as they possess just a single stable state. Due to the same reason, they are even known as One Shot Multivibrators, Single Shot Multivibrators, Single Swing Multivibrators or Delay Multivibrators and Univibrators.

Generally, monostable multivibrator circuits comprise of two set of components viz., passive (resistors and capacitors) and active (transistors or Op-Amps or 555 timer ICs). Figure 1 shows such a circuit designed using two bipolar junction transistors (BJTs) Q1 and Q2, one capacitor C and four resistors RC1, RC2, R1 and R2. The frequency of the output signal generated by them can be varied by varying the values of the capacitors and the resistors present in the circuit.
monostable multivibrator designed using bjts

Initially, the system shown will be in its stable state wherein the transistor Q1 will be cutoff while Q2 will be in saturation. As a result, the collector of Q2 will be shorted to ground due to which the output will be low. Further at this state, the right-plate of the capacitor C will be 0.7 V as it is connected to the base of Q2, while the charge on its left plate will be increasing gradually towards VCC. On applying the trigger to the base of Q1, it will turn ON, causing the flow of current through RC1. As a result, the collector terminal of Q1 as well as the left-plate of the capacitor C will be shorted to ground. This causes the capacitor to discharge, while turning OFF Q2 for the entire period of discharging cycle. This OFF state of Q2 is nothing but the astable or quasi-stable state, wherein the output of the circuit goes high.

However during this time, Q1 remains in its ON state only, as it has its base connected to the high-voltage point, the collector of Q2. Next, the cycle repeats by turning ON Q2 and by switching OFF Q1, once the capacitor fully discharges.
In these kinds of circuits, the duration for which the output remains high before the circuit reverts back to its stable state is decided by the RC time constant of the design. The mathematical expression for the pulse train generated by monostable multivibrator is given by T = 0.693 R1C. Further, it is to be noted that, in these designs, the time interval between the successive trigger pulses should be maintained greater when compared to that of the RC time constant. This is because, if it is not so, the output would be still high when the next trigger pulse is applied, leading to unsatisfactory results.

Monostable multibrators produce a perfect square wave at their output as their output is not concerned with the charging of the capacitor. These are almost half the size of the astable multivibrators and are simple in design and inexpensive. As a result, they are extensively used as timers, delay circuits, gated circuits, frequency dividers, etc. and also to generate fixed-duration pulses sensitive to some external event, to control the frequency of the analog circuit’s output, to synchronize the line and frame rate of television broadcasts, to regenerate old and worn out pulses in telecommunication and computer systems and to moderate the tunes of various octaves in the case of electronic organs.

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