Monostable Multivibrator using Transistors

In this tutorial it is explained how monostable multivibrator using transistors works. It is explained how the current flow in stable and unstable states in the transistor based monostable multibrator circuit. It is illustrated how to bias the transistors so that they operate as switches. Also explained is how a transistors is biased in cutoff and saturation region and how to calculate the biasing component values. It explains how to calculate the monostable multivibrator output pulse duration. Retriggerable monostable multivibrator circuit is then explained.

Monostable Multivibrator

A monostable multivibrator also called one-shot multivibrator is a circuit that generates pulse with fixed duration time interval. A monostable vibrator has one stable state and one quasi-stable state(unstable state). Unless it is driven by external trigger signal, the output is a stable state output and when an external trigger signal is applied, the output momentarily goes is driven into quasi-stable state for some duration and then returns back to its stable state. For example, initially when the power is applied to a monostable multivibrator, the output is low and stays low. Then if a 50ms trigger signal is applied, a pulse which last for some duration like 100ms is generated at the output. Then after 100ms the output goes back to its low output stable state. This is why monostable multivibrator is also called one shot multivibrator. Monostable Multivibrator is one type of multivibrators- the others being astable multivibrator and bistable multivibrator.

Transistor Switches in Monostable Multivibrator

When a transistor is operated between saturation region and cutoff region, the transistor operates as switches, that is the transistor is either in ON state(saturation region) or OFF state(cutoff region). The transistors in all multivibrator are operated as switches and hence to use them in multivibrator they must be biased in the cutoff and saturation region.

Saturation Region

When the biasing resistors are selected such that transistor operates in the saturation region to turn it fully ON, the collector current is at maximum(saturated). The collector current is made maximum by having base current high. The base current is high when the base resistance is low. So in order to drive the transistor into saturation we need small base resistance. When transistor is in saturation the collector to emitter is effectively shorted.

The saturation current is \(I_C(sat)=\frac{V_{CC}}{R_C}\)

Cutoff Region

A transistor is in cutoff region when the base current is zero. This causes the collector current to be nearly zero and the collector to emitter is effectively open circuit. Also when the a transistor is in cutoff region the collector-emitter voltage \(V_{CE}\) is equal to supply voltage \(V_{CC}\). For it to be in cutoff region means that the base current is almost zero and the collector current is also also very small which can be effectively neglected.

Operation of Transistors Monostable Multivibrator

Circuit diagram of Transistors Monostable Multivibrator.

The following shows the Monostable Multivibrator.circuit diagram implemented using BJT transistors.

Monostable Multivibrator using Transistors circuit diagram

 The Q1 and Q2 transistor used in the circuit diagram are BC107 NPN transistor. The diode D1 used is 1N4817 switching diode. A power supply of +5V has been used. The trigger pulse is applied using a push button which is pulled high using 4.7KOhm resistor. The trigger signal enters the monostable multivibrator circuit via the 100nF capacitor.

Monostable Multivibrator implementation on breadboard

The following picture shows monostable multivibrator implemented on breadboard using BC108 transistors.

Monostable Multivibrator using Transistors on breadboard

Monostable Multivibrator Waveform

The following picture shows trigger signal input to the monostable multivibrator circuit and the output pulse signal on oscilloscope.

monostable multivibrator signal oscilloscope

Video demonstration

Following is video demonstration of how the monostable multivibrator implemented using discrete transistors BC108 and build on breadboard works.

Operation of transistors based monostable multivibrator

The working principle of the transistor based monostable multivibrator is explained below.

Initial Momentary state

When power is applied to the circuit, due to initial rush of current into the circuit, the transistor Q1 goes momentary into saturation and transistor Q2 goes momentary into cutoff. This momentary state causes the capacitor C1 to charge the current flow from supply, through the resistor R1 into C1, then into transistor Q1 collector into emitter and finally ground. So initially the capacitor C1 is charged momentarily.

Stable State

When the momentary action is completed, the transistor Q1 goes into cutoff and the transistor Q2 goes into saturation. That is, transistor Q1 is OFF(non-conducting) and transistor Q2 is ON(conducting). The transistor Q2 goes into saturation because of the current flowing from the supply into R1 and into the base of the transistor Q2. This turns on the transistor Q2, and the output is nearly 0V because all the current from the supply source goes into the collector resistor R5 though the transistor collector into the emitter and then ground. 

The following shows circuit diagram shows current flow direction when the monostable multivibrator circuit is in stable state.

Stable state Monostable Multivibrator Circuit Diagram

When the transistor is saturated we have,


Since, \(V_{CE}=0\)

If we want collector current to be \(I_C=5mA\) then we can calculate the value of the collector resistor \(R_5\) which is,


So in stable state of the multivibrator, the transistor Q2, the output voltage nearly zero and current is 5mA.

Quasi Stable State 

When a negative going pulse is applied to the input, it causes the base of the transistor Q1 is pulled low. Because of the low voltage at the Q1 base, current is drawn from supply into resistor R5 and then into the Q1 base(and not into Q2 collector). So this current flowing now into the Q1 base turns the transistor Q1 ON. Because of this current drawn into the Q1 the transistor Q2 gets turned OFF. Also during the same triggering interval, the current flows from supply into R4 into collector of Q1 and out of the Q1 emitter. The following circuit diagram shows the direction of current flow when the monostable multivibrator is in quasi stable state or unstable state.

Quasi-Stable state Monostable Multivibrator Circuit Diagram

 As soon as, the triggering level goes high, the transistor Q1 again is turned OFF and Q2 is turned ON because of the biasing action. At the same time, the charge stored in the capacitor C1 gets released into the base of the transistor. The resistor R1 and C1 sets the pulse duration of the output signal(Vout) at Q2 collector when the circuit is triggered by external signal(Vin). The pulse duration of signal from monostable multivibrator is given by the following equation.

\(T = 0.69R_1C_1\)

Let the time duration of the triggered output signal from monostable multivibrator be T=200ms. Also suppose we choose a capacitor of \(C_1=10\mu F\). Then we can calculate the resistor R1 as follows,

\(R_1=\frac{T}{0.69C_1}=\frac{200ms}{0.69\times 10\mu F} = 29K\Omega\)

The Q1 collector resistor R4 value is not much importance because we do not constraint on collector current as it is not the output of the monostable multivibrator. But due to symmetry we usually use R4=R5.

The following shows what happens to the current flow when the monostable multivibrator is triggered using external signal.

triggering action on monostable multivibrator

The following shows the pulse trigger signal waveform and output pulse waveform on oscilloscope.
trigger pulse and output pulse waveform

Retriggerable Monostable Multivibrator

Retriggerable monostable multivibrator is a monostable multivibrator that remains in unstable state or quasi stable state by applying input trigger pulses such that the interval between the input trigger pulses is smaller than the output pulse time period. This is due to the use of capacitor which has instantaneous recharging recovery time. When the monostable multivibrator is driven into quasi-stable state and the trigger times out, it takes some time for the capacitor to recharge. This recharging time inhibits the application of subsequent trigger to the monostable multivibrator. But with retriggerable monostable multivibrator, the recharging of the capacitor happens instantaneously and so the monostable multivibrator can re triggered immediately after the first trigger. This retriggerable feature makes it possible to generate long period output pulse from the monostable multivibrator.

Consider the following diagram which illustrates what happens in retriggerable monostable multivibrator.

Retriggerable Monostable Multivibrator

After the first trigger pulse goes low, the monostable multivibrator circuit goes from stable state into the unstable state. Now if the output pulse has period T=0.69RC longer than the time interval between the input trigger pulse then output will not change. The above diagram shows that the second input trigger pulse is applied before the output changes to stable state and hence the monostable multivibrator continues in its unstable state and the output remains high. This is how retriggerable monostable multivibrator operates. 

Example of retriggerable monostable multivibrator are 74HCT4538 IC, 74LS122,74LS123, 4528 IC.

Video demonstration of Monostable Multivibrator using Transistors

The following video shows how the monostable multivibrator using transistors works. It shows how the current flows during the stable state and unstable state of monostable multivibrator. It also shows the input trigger pulse and output pulse waveforms on oscilloscope.

Tutorial Summary and Further work

So in this monostable multivibrator using transistor tutorial we have explained how the monstable multivibrator operates. A monostable multivibrator has two states- one is a stable state and one is a quasi-stable state(or unstable state). We explained the operation of mono-stable multivibrator explaining the initial momemtary phase and then how it operates in the stable state and the unstable state(quasi stable state). Example Calculation of Monostable Multivibrator with Transistors was provided where we calculated the biasing resistor values and the time period of the output triggered pulse. We also provided the current flow diagram for both states in the monostable multivibrator operation.

BJT transistor based Monostable multivibrator circuit was illustrated here. It could also be build using FET transistors and operational amplifier. Monostable multivibrator also comes in IC(Integrated Circuit). Example of Monostable multivibrator IC includes SN74123, 74LS123, 555 timer as monostable multivibrato, CD4098 IC etc.

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