Monostable Multivibrator

Team members

Sivani Varma

S. Sneha

U. Dhana laxmi

Project Guide

V. Arun

Assistant Professor

Department of ECE

Multivibrator

A multivibrator is an electronic circuit used to

implement a variety of sample two-state systems such

as oscillators, timers and flip-flops. It is characterized by two amplifying devices

(transistors, electron tubes or other devices ) cross –coupled by resistors or capacitors.

The name “multivibrator” was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics

AIM

This is a touch switch version of a BC547 invertor or transistor connected as a mono stable multivibrator. The transistor shall act as an amplifier by which it will be able to sense slight loads on the plate such as, static electricity from the human body.

Circuit Diagram

Types of multivibrators

There are three types: Bistable Monostable Astable

Each of the multivibrator has two states.Stable state: it is a state in which the device can stay permanently until a proper external triggering signal is applied, it wont change its state.

Quasi state: It is the temporary state and the device cannot stay permanently and after a pre determined time, the device will automatically come out of quasi stable

A multivibrator has two cross coupled inverters i.e.., the output of the first stage is coupled to the input of the second stage and output of second stage is coupled to input of first stage.In monostable the one coupling is resistor and the other is capacitor.

Monostable Multivibrator In a monostable multivibrator, one of the state is absolutely

permanent i.e, stable and the other one is temporary i.e, quasi-stable.

When an external trigger pulse is applied to the mono-stable at appropriate point, the mono-stable changes it state from stabe state to quasi-stable state.

It stays in the quasi-stable state for a predetermined length of certain interval remains there until another pulse is applied.

Thus a mono stable multivibrator can not generate square wave of its own like an astable multivibrator.

Only external pulse will cause if to generate the square wave.

In other words, a multi vibrator  in which one transistor  is always conducting (i.e. in the ON state) and the other is non conducting (i.e. in the OFF state) is called mono stable multivibrator. It is also called a single shot or single swing or a one shot multi vibrator. Other names are delay multi-vibrator and univibrator.

Collector – coupled monostable multivibrator

Components of circuit: circuit of a monostable multivibrator using NPN transistor.

two similar transistor Q1 and Q2 with equal collector loads i.e. RL1 = RL2 the values of -VBB and R3 are such as to reverse bias Q1 and keep it at cut off.

The collector supply Vcc and R2 forward bias Q2 and keep it at saturation.

A trigger pulse is given through C2 to obtain the square wave.

Circuit Diagram

Procedure

Initial Conditions:

Let us suppose that in the absense of a trigger pulse and with S closed, initially the circuit is in its stable state i.e. Q1 is OFF and Q2 is ON (at saturation).

When Trigger Pulse is applied:Let us see as what happens when the trigger is applied.

If positive trigger pulse is off sufficient amplitude, it will override the reverse bias of the E/B junction of Q1 and give it a forward bias, Hence Q1 will start conducting.

As Q1  conducts, its collector voltage falls due to voltage drop across RL1. It means that potential  of point A falls (negative going signal). This negative going voltage is fed to Q2 VIA C1where it decreases its forward bias.

As collector current of Q2 start decreasing, potential of point B increases (positive going signal) due to lesser drop over RL2. Soon, Q2 comes out of conduction.

The positive going signal at B is fed VIA R1 to the base of Q1 where it increases its forward bias further. As Q1 conductors more potential of point A approaches 0V.

This action is cumulative and ends with Q1 conducting at saturation and Q2 cut-off.

Return to initial Stable State: As point A is at almost OV, C2 starts to discharge through saturated Q1 to ground. As C1 discharges, the negative potential at the base of Q2 is decrease. As C1 discharges

further Q2 is pulled out of cut-off. As Q2 conducts further, a negative going signal from point B VIA R1 drives

Q1 into cut-off. Hence, the circuit reverts to its original state with

Q2 conducting at saturation and Q1 cut off. It remains in this state till

another trigger pulse comes along when the entire cycle repeats itself. The width of duration of the pulse obtained at the collector or output of

either transistor (Q1 or Q2) of the monostable multivibrator is given by the

expression T = 0.69 R2 C1

Uses of monostable multivibrator

The falling part of the output pulse from MMV is often used to trigger another pulse generator circuit thus producing a pulse delayed by a time T with respect to the input pulse.

MMV is used for regenerating old and worn out pulses. Various pulses used in computers and telecommunication systems become somewhat distorted during use. An MMV can be used to generate new, clean and sharp pulses from these distorted and used ones.

Thank you!