monostable multivibrator using transistor experiment

Learning about Electronics

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How to Build a Monostable Multivibrator Circuit with Transistors

Components Needed

• 2 2N4401 NPN transistors
• 2 1KΩ resistors
• 10KΩ resistor
• 510Ω resistor
• 470μF electrolytic capacitor
• 10μF electrolytic capacitor

Monostable Multivibrator Circuit with Transistors

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Monostable MultiVibrator

1.    Study the operation and working principle Monostable Multivibrator.

2.   Study the procedure for conducting the experiment in the lab.

Objectives:

1. To study the operation and observe the wave forms of Monostable Multivibrator.

2. To Design a Monostable multivibrator for the pulse width of 0.3mSec.

1.         CRO   0 to 20 MHz (Dual channel)                                         -           1No.

2.         Function generator  1Hz to 1 MHz                                           -           1No.

3.         Capacitors (0.033µF)                                                               -           2 No.

4.         Capacitor(0.01 µF))                                                                 -           1 No.

5.         Resistors (1 k%u2126, 10k%u2126, 100K%u2126, 47K%u2126)                                              -           2, 2, 1 and 1 Nos .

6.         Transistor (BC 107)                                                                 -           2 No.

7.         Diode(IN4148)                                                                                    -           1 No.

8.         Regulated Power supply 0 – 30 V(dual )                                             -           1 No.

9.         Connecting wires        

10.       Bread board    

Circuit diagram:

     The monostable circuit has one permanently stable and one quasi-stable state. In the monostable configuration, a triggering signal is required to induce a transition from the stable state to the quasi- stable state. The circuit remains in its quasi-stable for a time equal to RC time constant of the circuit. It returns from the quasi-stable state to its stable state without any external triggering pulse. It is also called as one-shot, a single cycle, a single step circuit or a univibrator.

     Assume  initially transistor  Q 2 is  in saturation  as it  gets base bias  from V CC   through R. coupling  from Q 2   collector  to Q 1   base ensures that Q1   is  in cutoff.  If an appropriate negative trigger pulse applied at collector of Q 1 (V C1 ) induces a transition in Q 2 , then Q2  goes to cutoff. The output at Q 2 goes high. This high output when coupled to Q 1    base, turns it  ON.  The Q 1     collector   voltage  falls  by I C1 R C1    and Q2   base  voltage   falls  by   the  same  amount,  as  voltage   across a  capacitor   ‘C’   cannot  change instantaneously.

       The moment,  a negative  trigger  is applied at V C1 ,  Q 2 goes to cutoff and Q 1   starts  conducting.  There is a path for capacitor C to charge from V CC through R and the conducting transistor Q 1 . The polarity should be such that Q 2 base potential rises.  The  moment, it  exceeds  Q 2 base cut-in  voltage,  it  turns ON  Q 2  which due  to coupling  through  R1   from collector  of  Q2 to base of  Q1,  turns Q1   OFF.  Now we are back to the original state i.e.  Q 2 is ON and Q1 is OFF.  Whenever trigger the circuit into the other state, it cannot stay there permanently and it returns back after a time period decided by   R and C.

                        Pulse width is given as T = 0.69RCsec.F

Design Procedure:

To design a monostable multivibrator for the Pulse width of 0.3 mSec.

Let I Cmax = 15mA, V CC = 15V, V BB = 15V, R 1 = 10K%u2126.

                                    T =0.69RC

Choose C = 10nf(0.01µF)          T = 0.69 RC

                                    0.3 x 10 -3 Sec = 0.69 x R x 10 x 10 -9

                                    R = 43.47 Kohms ≈ 47Kohms

                           R C =      (V CC - V CESAT) / I CMAX       =     (15 − 0.3) / 15 X 10 -3

                                                                                                =      1 Kohms

Minimum requirement of  | V B1 | ≤  0.1

For more margin, given  V B1 = -1.185

Substitute the values , R 1 =10kohms    we will get  R 2 = 100Kohms

1. Make the connections as per the circuit diagram.

2. Select the triggering pulse such that the frequency is less than 1/T

3. Apply  the triggering input to the circuit and to the CRO’s channel and  Connect the CRO channel-2 to

    the collector and base of the Transistor Q1&Q2.

4. Adjust the triggering pulse frequency to get stable pulse on the CRO and now measure the pulse width

    and verify with the theoretical value.

5. Obtain waveforms at different points like V B1 , V B2 , V C1 & V C2 and plot the graph.

Expected Waveforms:

Theoretical calculations:        T ON = 0.69 RC

R= 47K%u2126  and  C = 10nF or 0.01µF

Note: Normally Monostable Multivibrator generates single pulse output whenever a trigger is given. To observe this output storage oscilloscope is required.

 Result:     Monostable Multivibrator is designed; the waveforms are observed and verified the results theoretically.

Viva Questions:

•   What is a multivibrator?

          Ans: a multivibrator is a circuit which can operate at a number of frequencies .

• What are applications of Monostable Multivibrator?

           Ans: it is used as  gating circuit  and as a delay circuit.

• The monostable multivibrator is also called __, ___, __, ___ or ___.

            Ans: one shot, a single step circuit, un multivibrator , gating circuit  and delay circuit .

•  A Monostable Multivibrator generates __ wave

           Ans; pulse waveform

• Why is the time period T also called Delay time?

Ans: in Quasi-stable state Q 1  is ON and Q 2 is OFF. The interval during which quasi- stable state of the multi persists i,e Q 2  is dependent upon the rate at which the capacitor C discharges. This duration of Quasi-stable state is termed as delay time.

• Justify, Why Monostable Multivibrator is called one-shot circuit?

         Ans: Because it produces only one pulse.

• What is a quasi state?

           Ans; A quasi-stable state means a temporarily stable state . the circuit remains in the quasi-stable State only for a specified time and afterwards it comes back to other state .

•   In monostable multivibrator, the coupling elements are __.

             Ans: resistor and capacitor

•   What is the formula for the pulse width of a Monostable multivibrator? To get a pulse width of 2 mSec., get the values of R and C.

        Ans: T=0.69RC     R=8K and C=200 µF

•  ___ triggering is used in monostable multivibrator.

Ans; unsymmetrical triggering

Design Projects

        1. Design a collector coupled monostable multivibrator using 2-BC107 transistor with 5ms quasi stable state duration V CC =10V , h­ fe(min) =30 I C(sat) =5mA.

        2. Verify the output of monostable multivibrator by using different triggering methods.

Outcomes: After finishing this experiment students are able to design Monostable Multivibrator and able to explain its operation.

• Updated Sep 29, 2019
• Views 12,681

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Monostable Multivibrator using Transistors

• Electronics , Basic Electronics

Multivibrator is a non linear oscillator or function generator which can generate square, rectangular and pulse waves.

Monostable Multivibrator or One-Shot Multivibrator  has only one stable state. By default monostable multivibrator will be in its stable state, but when triggered it will switch to unstable state (quasi-stable state) for a time period determined by the RC time constant in the circuit.

Monostable Multivibrator using Transistors – Circuit Diagram

In the above circuit diagram we can find two transistors which are wired as switches. Please read the article Transistor as a Switch to know more about it. When a transistor is ON, it works in saturation region and its collector – emitter acts as a short circuit. When a transistor is OFF, it works in cut off region and its collector – emitter acts as an open circuit. So in the above circuit, when a transistor is in OFF state, its collector will have voltage equal to supply voltage Vcc and when the a transistor is in ON state, its collector will be grounded.

Function of resistor R is to limit collector current of both transistors Q1 and Q1. Resistors R1 & R2 will provide base current for transistors Q2 & Q1 respectively during ON condition. Capacitor C3 and Resistor R3 is designed as a differentiator circuit to provide sharp trigger pulses to the base of the transistor Q1. The diode D allows only positive pulses to the base. Capacitor C2 is optional, which is called as Speed Up capacitor. It is used for speedy bypassing of signal transitions (LOW to HIGH and HIGH to LOW) at the collector of Q2 to the base of Q1.

When the circuit is switched ON, transistor Q1 will be OFF and Q2 will be ON, which is its stable state. Transistor Q2 is ON since the base of the transistor is connected to Vcc via R1. Transistor Q1 will be OFF because base of the transistor is at 0v since transistor Q2 is ON.

• When the circuit is switched ON, transistor Q1 will be OFF and Q2 will be ON.
• Capacitor C1 gets charged during this state.
• When a positive trigger is applied to the base of transistor Q1 it turns ON, which turns OFF the transistor Q2 due the the negative voltage from the capacitor C1.
• Capacitor C1 starts discharging during this state.
• Transistor Q1 remains in ON state due the positive voltage from the collector of transistor Q2 which is in OFF state.
• Transistor Q2 remains in OFF state until the capacitor C1 discharges compleatly.
• When the capacitor C1 discharged completly, transistor Q2 turns ON, which turns transistor Q1 OFF.

R – Collector Resistor

R c  should be calculated depending upon the collector current requirement.

• Rc  = ( Vcc – Vce (sat) ) / Ic

R1 – Base Resistor

R1 should be chosen such that it will provide enough collector current during saturation to the transistor Q2.

• Min. base current required, Ib min = Ic / β, where β is the hFE of the transistor
• Safe base current, Ib = 3 Ib min = 3Ic / β
• R1 = ( Vcc – Vbe ) / Ib

R2 – Base Resistor Q1

R2 should be chosen such that it should provide enough saturation collector current to the transistor Q1.

• R2 = (( Vcc – Vbe ) / Ib ) – R

T – Pulse Time Period

• T = 0.693R1C1

From this we can find the value of capacitor C.

Speed-Up Capacitor

Speed up capacitor is designed by considering a Compensated Attenuator composed of Base Emitter resistance of Q1, Resistor R2, Base Emitter capacitance of Q1 and seed up capacitor C2.

Base Emitter resistance of a transistor, Rπ = V T / Ib, where V T is the thermal voltage which is approximately equal to 25mV at room temperature and Ib is the base current.

Base Emitter capacitance or Input capacitance will be specified in the datasheet of the transistor. For example, the Input capacitance of BC547 transistor is 9pF.

• RπCeb = C2R2

Note 1: It may be very difficult to get the proper matching speed up capacitor but you can still use a near available one to improve the performance. Note 2: Actually the speed up capacitor should be sized to remove the charge stored at the base of the transistor during saturation period. Unfortunately the value of Rπ and Ceb are not constant. It varies depending up on many factors. So the optimum capacitor value can be find out only through a series of experiments.

R3, C3 – Differentiator

The R3 C3 differentiator should be designed depending up on the frequency of the trigger pulse.

• R3C3 << 0.0016T’ , where T’ is the lowest time period of the signal that should be differentiated.

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Ligo George

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Excellent article. Really helpful.

What would be the max input frequency range and the min. output Pulse Width for this curcuit?

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• 555 Lab - Monostable Multivibrator (One-shot)

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555 Timer Circuit Projects

• 555 Lab - Introduction to the 555 Timer IC
• 555 Lab - Schmitt Trigger
• 555 Lab - Oscillator With Hysteresis
• 555 Lab - Red LED Flasher
• 555 Lab - Blue LED Flasher With Voltage Doubler
• 555 Lab - LED Flasher with Inductive Flyback
• 555 Lab - Advanced Red LED Flasher

In this hands-on electronics experiment, you will build a monostable multivibrator using a 555 timer IC and learn about RC time constant charging and how a one-shot timer operates.

Project overview.

The monostable multivibrator , or one-shot, is one of the most basic 555 circuits . This circuit (Figure 1) is part of the typical 555 datasheets , complete with the math needed to design to specification, and is one of the reasons a 555 is referred to as a timer.

Figure 1. Schematic diagram of the 555 one-shot timer with LED output indicators.

As illustrated in Figure 2, at the falling edge of the trigger, the output goes high. It will stay high for the full time period, T.

Figure 2. Trigger and output waveforms for a one-shot timer (monostable multivibrator).

Parts and materials.

• One 9 V battery
• Battery clip 
• Mini-hook clips 
• A watch with a second hand/display or a stopwatch
• A wire, 11/2” to 2” (3.8 mm to 5 mm) long, folded in half (shown as red wire in the illustration)
• U1 - 555 timer IC
• D1 - Red light-emitting diode (LED)
• D2 - Green LED
• R1, R2 - 1 kΩ 1/4 W resistors
• R t - 27 kΩ 1/4W resistor
• R t - 270 kΩ 1/4W resistor
• C1, C2 - 0.1 µF capacitor
• C t - 10 µF capacitor
• C t - 100 µF capacitor

Learning Objectives

• Learn how a monostable multivibrator works
• Learn a practical application for an RC time constant
• How to use the 555 timer as a monostable multivibrator

Instructions

Step 1:  Build the circuit illustrated in the schematic diagram of Figure 1 and the breadboard implementation of Figure 3.

Figure 3. Breadboard implementation of the 555 one-shot timer with LED output indicators.

Step 2:  Verify that the green LED lights are when the 555 output is high (i.e., switched to V CC ) and the red LED lights when the 555 output is low (switched to ground).

This particular monostable multivibrator (also known as a one-shot timer) is not a re-triggerable type. This means that once triggered, it will ignore further inputs during a timing cycle, with one exception, which will be discussed below.

Step 3:  The timer starts when the input goes low or switches to the ground level and the output goes high. You can prove this by connecting the red wire shown in Figure 1 between the ground and point B, disconnecting it, and reconnecting it.

Step 4:  It is an illegal condition for the input to stay low for this design past timeout. For this reason, R3 and C1 were added to create a signal conditioner, which will allow edge-only triggering and prevent illegal input. You can prove this by connecting the red wire between the ground and point A. The timer will start when the wire is inserted into the protoboard between these two points and ignore further contacts.

Step 5:  If you force the timer input to stay low past timeout, the output will stay high even though the timer has finished. As soon as this ground is removed, the timer will go low.

Step 6:  R t and C t were selected for 3 seconds timing duration. You can verify this with a watch, 3 seconds is long enough that we slow humans can actually measure it. The resistor and capacitor are probably 5% and 20% tolerance, respectively, so the calculated times you measure can vary as much as 25%, though it will usually be much closer.

Step 7:  Try changing R t from a 270 kΩ to a 27 kΩ resistor and C t from a 10 µF to a 100 µF capacitor. Since the RC time constant is the same, there should be no difference in how it operates.

Step 8:  Next, try replacing R t with the 270 kΩ resistor. Since the RC time constant is now 10 times greater, you should get close to 30 seconds.

Another nice feature of the 555 is its immunity from the power supply voltage. If you were to swap the 9V battery with a 6V or 12 battery, you should get identical results, though the LED light intensity will change.

C2 isn’t actually necessary. The 555 IC has this option in case the timer is being used in an environment where the power supply line is noisy.

You can remove it and not notice a difference. The 555 itself is a source of noise since there is a very brief period of time that the transistors on both sides of the output are both conducting, creating a power surge (measured in nanoseconds) from the power supply.

Theory of Operation

Figure 4 is the starting and ending point for this circuit, where the output is low and it is waiting for a trigger to start a timing cycle. Looking at the functional schematic shown in Figure 4, you can see that pin 7 (d ischarge ) is connected to a transistor going to the ground. 

Figure 4.  The stable state of the 555 timer one-shot waiting for a trigger input.

This transistor is simply a switch. When the flip-flop output Q   is high, as indicated by the red lines in the figure, the transistor is on and connects the capacitor C t to the ground. Figure 5 shows what happens when the 555 receives a trigger, starting the sequence. 

Figure 5. Triggering the 555 timer one-shot by grounding pin 2 (trigger).

Pin 2 ( trigger ) is forced low, which causes comparator C2 output to go high. This resets the flip-flop output Q low, which turns off the transistor connected to pin 7 and the capacitor C t .

The capacitor C t now begins charging through resistor R t as illustrated in Figure 6. 

Figure 6.  Capacitor voltage during a 555 one-shot timing sequence.

The red line represents the power supply voltage, V CC , and the green line represents the ground (0 volts). The capacitor C t charges up from the ground towards V CC following an RC exponential charging curve. Note: it will not reach V CC due to the operation of the circuit. 

Figure 7 shows the state of the circuit while the capacitor is charging. 

Figure 7. State of the 555 timer one-shot while the capacitor is charging.

During this time, the circuit is in a stable configuration, and the output is high.

Figure 8 shows the circuit in the middle of switching off when it hits timeout. 

Figure 8. When the capacitor voltage reaches 2/3 V CC and sets the flip-flop.

Capacitor C t has charged to 2/3 V CC . This forces comparator C1 to switch high and set the flip-flop. The flip-flop output Q   goes high, which will turn on the discharge transistor at pin 7. This also resets the output low.

At this point, the pin 7 transistor is on, keeping the capacitor C t discharged. The circuit is now in the state illustrated in Figure 9, which is identical to the starting state of Figure 4. 

Figure 9.  Return to the stable state of the 555 timer one-shot.

Related content.

Learn more about the fundamentals behind this project in the resources below.

• Electric Fields and Capacitance
• Capacitors and Calculus
• Voltage and Current Calculations
• Solving for an Unknown Time
• Monostable Multivibrators

Calculator:

• 555 Timer Monostable Circuit Calculator
• RC Time Constant Calculator

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• Oscillator Circuits Worksheet
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Monostable Multivibrator Using 555 Timer

• May 21, 2024
• By Ravi Teja

A monostable multivibrator using a 555 timer is a circuit that generates a single output pulse of a defined duration in response to an input trigger. This configuration of the 555 timer is also known as a one-shot multivibrator because it returns to its stable state after a single pulse. The duration of the output pulse is determined by an RC (resistor-capacitor) network connected to the timer. These guides cover everything from the basic principles, circuit diagrams, and practical assembly tips, to applications of the monostable multivibrator using the 555 timer.

Circuit and Operation

The following figure is the schematic of IC 555 as a Monostable Multivibrator. This is the basic mode of operation of the IC 555. It requires only two extra components to make it work as a monostable multivibrator: a resistor and a capacitor.

As the name specifies, a monostable multivibrator has only one stable state. When a trigger input is applied, a pulse is produced at the output and returns back to the stable state after a time interval. The duration of time for which the pulse is high will depend on the timing circuit that comprises of a resistor (R) and a capacitor (C).

The details of the connection are as follows. The pins 1 and 8 are connected to ground and supply (VCC) respectively. Output is taken at pin 3. To avoid accidental reset of the circuit, pin 4 is connected to the VCC. Pin 5, which is the control voltage input, should be grounded when not in use. To filter the noise, it is connected to the ground via a small capacitor of capacitance 0.01µF.

The monostable mode is also called “one-shot” pulse generator. The sequence of events starts when a negative going trigger pulse is applied to the trigger comparator. When this trigger comparator senses the short negative going trigger pulse to be just below the reference voltage (1/3 VCC), the device triggers and the output goes HIGH.

The discharge transistor is turned OFF and the capacitor C that is externally connected to its collector will start charging to the max value through the resistor R. The HIGH output pulse ends when the charge on the capacitor reaches 2/3 VCC. The internal connection of the IC 555 in monostable mode along with the RC timing circuit is shown below.

The detailed operation can be explained as follows. Initially, the flip-flop is RESET. This will allow the discharge transistor to go to saturation. The capacitor C, which is connected to the open collector (drain in case of CMOS) of the transistor, is provided with a discharge path. Hence the capacitor discharges completely and the voltage across it is 0. The output at pin 3 is low (0).

When a negative going trigger pulse input is applied to the trigger comparator (comparator 2), it is compared with a reference voltage of 1/3 VCC. The output remains low until the trigger input is greater than the reference voltage. The moment trigger voltage goes below 1/3 VCC, the output of comparator goes high and this will SET the flip-flop. Hence the output at pin 3 will become high.

At the same time, the discharge transistor is turned OFF and the capacitor C will begin to charge and the voltage across it rises exponentially. This is nothing but the threshold voltage at pin 6. This is given to the comparator 1 along with a reference voltage of 2/3 VCC. The output at pin 3 will remain HIGH until the voltage across the capacitor reaches 2/3 VCC.

The instance at which the threshold voltage (which is nothing but the voltage across the capacitor) becomes more than the reference voltage, the output of the comparator 1 goes high. This will RESET the flip-flop and hence the output at pin 3 will fall to low (logic 0) i.e. the output returns to its stable state. As the output is low, the discharge transistor is driven to saturation and the capacitor will completely discharge.

Hence it can be noted that the output at pin 3 is low at start, when the trigger becomes less than 1/3 VCC the output at pin 3 goes high and when the threshold voltage is greater than 2/3 VCC the output becomes low until the occurrence of next trigger pulse. A rectangular pulse is produced at the output. The time for which the output stays high or the width of the rectangular pulse is controlled by the timing circuit i.e. the charging time of the capacitor which depends on the time constant RC.

Pulse Width Derivation

We know that the voltage across the capacitor C rises exponentially. Hence the equation for the capacitor voltage VC can be written as

VC = VCC (1 – e -t/RC )

When the capacitor voltage is 2/3 VCC, then

2/3 VCC = VCC (1 – e -t/RC )

2/3 = 1 – e -t/RC

e -t/RC = 1/3

– t/RC = ln (1/3)

– t/RC = -1.098

t = 1.098 RC

∴ t ≈ 1.1 RC

The pulse width of the output rectangular pulse is W = 1.1 RC.

The waveforms of the monostable operation are shown below.

Back to top

Applications of Monostable Multivibrator

Frequency divider.

When the IC 555 is used as a monostable multivibrator, a positive going rectangular pulse is available at the output when a negative going pulse of short duration is applied at the trigger input. By adjusting the time interval t of the charging or timing circuit the device can be made to work as a Frequency Divider circuit.

If the timing interval t is made slightly larger than the time period of the input pulse (trigger pulse), the device can act as a Divide – by – two circuit. The timing interval can be controlled by appropriately choosing the values of the resistor R and the capacitor C in the timing circuit. The waveforms of the input and output signals corresponding to the divide–by–two circuit are shown below.

The circuit will trigger for the first negative pulse of the trigger input. As a result, the output will go to high state. The output will remain high for the time interval t. During this interval, even if a second negative going trigger pulse is applied, the output will not be affected and continues to remain high as the timing interval is greater than the time period of the trigger pulse. On the third negative going trigger pulse, the circuit is retriggered.

So the circuit will trigger on every alternate negative going trigger pulse i.e. there is one output pulse for every two input pulses and hence it is a divide–by–two circuit. By adjusting the timing interval, a monostable circuit can be made to produce integral fractions of the input frequency.

Pulse Width Modulation

The monostable mode of operation of the IC 555 can be turned into a Pulse Width Modulator by applying a modulating signal as the control voltage at the pin 5. The circuit for a Pulse Width Modulator using monostable multivibrator is shown below.

The control signal will modulate the threshold voltage and as a result, the output pulse width is modulated. As the control voltage varies, the threshold voltage; which is the input to the comparator 1, also varies. As a result, the time for charging the capacitor to the threshold voltage level will vary, resulting in a pulse width modulated wave at the output. The waveforms of the input, output and the modulating signal are shown below.

Due to the application of the control signal, the upper threshold voltage level for the capacitor will be different. The new upper threshold level UTL is given by

UTL = 2/3 VCC + VMOD

Where VMOD is the voltage of the modulating signal.

Because of the new threshold level, the pulse width of the output is given by

W = -RC ln (1 – UTL/VCC)

The time period of the output is same as the input.

Linear Ramp Generator

The monostable multivibrator will act as a Linear Ramp Generator with the addition of a constant current source. A current mirror, consisting of a diode and a PNP transistor, is used as a Constant Current Source. This constant current source is positioned in place of the timing resistor. The circuit for a linear ramp generator with IC 555 in monostable mode is shown below.

The current IC from the constant current source will charge the capacitor at a constant rate towards the peak voltage (VCC) resulting in a rising linear ramp. As the voltage across the capacitor reaches 2/3 VCC, the comparator 1 will drive the discharge transistor to saturation. As a result, the capacitor starts discharging. While discharging, as the voltage across the capacitor falls to 1/3 VCC, the comparator 2 will turn off the discharge capacitor.

Hence the capacitor will start charging again. The discharge time of the capacitor is very less when compared to the charging time. As a result, the downward ramp is very steep (almost an immediate discharge). Hence, the time period of the ramp output is practically equal to the charging time of the capacitor. The time period of the ramp output is approximately given by

T = (2/(3 ) Vcc Re (R1+R2)C)/(R1 Vcc – Vbe (R1+R2))

The waveforms of the ramp output and the pulse output of a ramp generator are shown below.

Switching the Relay ON

The monostable multivibrator can be used to drive a relay. The circuit is shown below.

These circuits are called as Time Delay Relays. In this circuit, the relay will stay ON for a certain period of time once activated. This time, for which the relay is ON, can be anywhere between 0 to 20 sec depending on the values of R and C in the timing circuit.

For example, if the relay is to be ON for a period of 10s in order to energize an external device, then values of the resistor and the capacitor can be calculated as follows using the equation t = 1.1 RC.

By assuming the value of the capacitor to be its least possible value i.e. 10µF, the value of the resistor is

10 = 1.1 * R * 10µF

∴ R = 909090.9090 ≈ 909 KΩ.

A potentiometer can be used to adjust the resistance and therefore adjusting the time delay.

Missing Pulse Detector

The circuit of a Missing Pulse Detector is shown below. A PNP transistor is connected across the capacitor and the input trigger pulse train is given to the base terminal of the transistor as well as the pin 2 trigger input of the IC 555.

The train of trigger pulses will continuously reset the timing cycle. Hence the output is always high. If any trigger pulse is missing, the device detects this missing pulse and the output goes low. The detailed working is as follows. When the input is 0, the PNP transistor is turned ON and the voltage across the capacitor is clamped to 0.7 V and the output is HIGH. When the input trigger voltage is high, the transistor is cut-off and the capacitor will start charging.

If the input trigger signal goes low again before the completion of the timing cycle, the voltage across the capacitor falls to 0.7 V before reaching the threshold voltage (2/3 VCC) and the output continues to remain HIGH. If the input trigger signal doesn’t go low before the completion of the timing cycle due to a missing pulse, it allows the capacitor to charge to the threshold voltage and the output will become LOW.

In order to make this circuit work as a Missing Pulse Detector, the time period of the input trigger signal should be slightly lesser than the timing interval. Because of this, the continuous negative going input pulses will not allow the capacitor to charge till the threshold voltage. And the output continues to stay high. In case of change of input frequency or a missing pulse, the capacitor will charge to the threshold voltage and the output falls low. The waveforms of the input pulse, voltage on the capacitor and the output signal are shown below.

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One Response

I can’t figure out how to make a one shot that fires on the input but does not stay on even if the trigger stays on. In other words, if the monostable multivibrator (mvb) is triggered by a switch and the switch is held for 5 seconds the mvb outputs its pulse for however long the RC time is calculated for. I hope I’m making myself clear. These 555 multivibrators will continue the output pulse until the momentary switch is released. So, if you hold the switch down for three (3) seconds and the RC time is 0.5 seconds the 555 will continue the output pulse until the switch is released.

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Table of Contents

Background:, objectives:, directions:, hardware setup:, for further reading:, activity: bjt multivibrators, for adalm2000.

A multivibrator circuit consists generally of two inverting amplifier stages. The two amplifiers are connected in series or cascade, and a feedback path connects from the output of the second amplifier back to the input of the first. Because each stages inverts the signal, the overall feedback around the loop is positive. There are three main types of multivibrators. In the astable multivibrator capacitors are used to couple the two amplifier stages and provide the feedback path. Since the capacitors block any DC signals (sometimes referred to as state) from passing from one stage to the next the astable multivibrator has no stable DC operating point and is thus a free-running oscillator. In the monostable multivibrator the coupling from one of the stages to the other uses one capacitor while the second connection is through a DC path. Thus the monostable multivibrator has one stable DC stage. Hence, monostable or as it is sometimes referred to as a one-shot. The circuit maintains this single stable state except when a triggering pulse is applied. Then the state changes for a predetermined length of time set by the RC time constant of the AC coupled part of the signal path. In the bistable multivibrator both coupling paths are DC coupled and thus the circuit has two different stable states and uses no capacitors. The bistable multivibrator is also called a flip-flop, with either of two DC stable states.

The Astable Multivibrator

The objective of this first experiment is to build an astable multivibrator. Two identical resistance-capacitance networks determine the frequency at which oscillation will occur. The amplifying devices (transistors) are connected in a common-emitter configuration, as shown in figure 1.

ADALM2000 Active Learning Module Solder-less breadboard Jumper wires 2 - 470 Ω Resistors 2 - 20 KΩ Resistors 2 - small signal NPN transistors (2N3904) 1 - Red LED 1 - Green LED 2 - 47 uF Capacitors

Construct the circuit as shown in figure 1 on your solder-less breadboard. The green boxes indicate connections to the ADALM2000. Note: there is no input from the ADALM2000 board just the power supply. The first inverting amplifier stage consists of Q 1 with R 1 and the Red LED serving as the output load. The second inverting amplifier stage consists of Q 2 with R 2 and the Green LED serving as the load. C 1 couples the output of the first stage at the collector of Q 1 to the input of the second stage at the base of Q 2 . Similarly, C 2 couples the output of the second stage at the collector of Q 2 back to the input of the first stage at the base of Q 1 .

Figure 1, Astable Multivibrator

Figure 2, Astable Multivibrator Breadboard Circuit

The frequency of oscillation is very slow due to the large values of capacitors C 1 and C 2 . Replace C 1 and C 2 with 0.1uF capacitors. The circuit should oscillate much faster now such that both LEDs seem to be on at the same time. Using the scope channels you should now measure the frequency and period of the output waveforms.

Figure 3, Astable Multivibrator interval at 47uF capacitor

Figure 4, Astable Multivibrator interval at 0.1uF capacitor

1. What are the two most important components in the multivibrator circuit shown in figure 1? 2. What would be the effect of increasing or decreasing the value of only one capacitor? 3. What would be the effect of increasing or decreasing the value of both capacitors?

Add more questions here:

The Monostable Multivibrator

The objective of this second experiment is to build an monostable multivibrator. One resistance-capacitance network determines the duration of the one-shot output. The amplifying devices (transistors) are connected in a common-emitter configuration, as shown in figure 2.

ADALM2000 Active Learning Module Solder-less breadboard Jumper wires 2 - 470 Ω Resistors 1 - 1 KΩ Resistor 1 - 20 KΩ Resistor 1 - 47 KΩ Resistor 1 - small signal diode (1N914) 2 - small signal NPN transistors (2N3904) 1 - Red LED 1 - Green LED 1 - 47 uF Capacitor

Construct the circuit as shown in figure 2 on your solder-less breadboard. The green boxes indicate connections to the ADALM2000. Starting with the circuit from experiment 1, remove one of the 20K? resistors (old R 3 ) and replace capacitor C 1 with a 47K? resistor (new R 3 ). Add diode D 1 and resistor R 5 as shown to the base of Q 2 . Be sure to replace C 2 with the original 47 uF capacitor.

Figure 5, Monostable Multivibrator

Figure 6, Monostable Multivibrator Breadboard Circuit

Turn on the Vp power supply only after you have completely built and checked the circuit. The red LED should be lit and the green LED should be dark. With a length of wire, momentarily touch the trigger input (end of R 5 ) to Vp and immediately let go. The red LED should go out and the green LED come on for about a second and then go back to the stable state with the red on and green off. Try this a few times.

Figure 6, Monostable Multivibrator Behavior on trigger

Add questions here:

The Bistable Multivibrator ( or flip-flop )

The objective of this third experiment is to build an bistable multivibrator. The amplifying devices (transistors) are connected in a common-emitter configuration, as shown in figure 3.

ADALM2000 Active Learning Module Solder-less breadboard Jumper wires 2 - 470 Ω Resistors 2 - 1 KΩ Resistors 2 - 47 KΩ Resistors 2 - small signal NPN transistors (2N3904) 2 - small signal diodes (1N914) 1 - Red LED 1 - Green LED

Construct the circuit as shown in figure 3 on your solder-less breadboard. The green boxes indicate connections to theADALM2000.

Figure 7, Bistable Multivibrator

Figure 8, Bistable Multivibrator Breadboard Circuit

Turn on the Vp power supply only after you have completely built and checked the circuit. Either the red LED should be lit with the green LED dark or the green LED should be lit with the red LED dark. With a length of wire, momentarily touch the either the SET or RESET input (end of R 5 or R 6 ) to Vp and immediately let go. The LEDs should change state or toggle back and forth depending which input is touched to Vp. Try this a few times.

Figure 9, Bistable Multivibrator behavior triggering Set pin

Figure 10, Bistable Multivibrator behavior triggering Reset pin

Connect the SET and RESET inputs to two of the digital I/O pins on the ADALM2000 connector. Configure the pins as push-pull outputs. Used the static digital I/O screen to control the digital pins.

Add Questions here:

D-Type Flip-Flop

The objective of this fourth experiment is to use the bistable or set - reset flip-flop from experiment 3 to build what is known as a D-Type flip-flop.

ADALM2000 Active Learning Module Solder-less breadboard Jumper wires 3 - 1 KΩ resistors 1 - 100 KΩ resistor 2 - 200 KΩ resistors 2 - 47 KΩ resistors 3 - small signal NPN transistors (2N3904) 2 - small signal diodes (1N914) 2 - 39 pF capacitors 2 - 100 pF capacitors

Construct the D type flip-flop circuit as shown in figure 4 on your solder-less breadboard. Note that the polarity of the two diodes is reversed compared to figure 3. Because this experiment will be done at much higher frequencies, the LEDs have been removed and simple 1 K ? load resistors are used.

Switching between the two flip-flop states is achieved by applying the D (data) signal and a single clock pulse which, depending on the state of the D input with respect to the current state will, cause the “ON” transistor to turn “OFF” and the “OFF” transistor to turn “ON” on the negative or falling edge of the clock pulse. The true D signal and complement DB signal ( output of Q 3 , R 7 inverting stage ) are used to bias diodes D 1 and D 2 to steer the clock pulse to the correct base, the equivalent of the SET and RESET inputs in figure 3.

To illustrate how the circuit operates we will assume the circuit is in one of its two stable states with the QB output low ( collector voltage of Q 1 at 0 V ), and the Q output high ( collector voltage of Q 2 high at 5 V ). With the D input low ( DB high ) D 1 has a low voltage on its cathode via R 6 and a high voltage ( V BE of on transistor Q 1 ) on its anode via R 4 , making it forward biased. D 2 has a high voltage ( from DB ) on its cathode via R 5 and a low voltage on its anode via R 3 ( V BE of off transistor Q 2 ), making it reverse biased.

A negative going pulse on the Clock input, coupled through C 1 and C 2 , is steered to the base of Q 1 since D 1 is forward biased, but blocked from the base of Q 2 by reverse biased D 2 . Q 1 is turned off and Q 2 is turned on by the cross coupled connection through the parallel combination of C 3 and R 3 . This happens very quickly because of the positive feedback effect we saw earlier in the simple bistable multivibrator. The circuit is now in the other stable state with the Q output high and the QB output low. The circuit will remain in that state until the D input becomes high and after another negative going clock pulse arrives.

Figure 11 D type flip-flop

Figure 12 D type flip-flop breadboard circuit

Turn on the Vp power supply and enable the AWG outputs only after you have completely built and checked the circuit. You should observe a square wave on the Q output which is aligned with the falling edge of the Clock input signal. Change the phase of AWG2 ( D input signal ) while observing this alignment. Does this change as the phase of the D input change? Move the channel 1 scope input to the D input. You should see a similar square wave signal but ahead in time with respect to the Q output. In other words the Q output is delayed until the falling edge of the Clock signal.

Figure 13: Plot of Q and Clock signal

Figure 14: Plot of Q and D signal

What is the purpose of capacitors C 3 and C 4 ?

What if the highest clock frequency at which the circuit continues to function? What limits this maximum frequency?

The capacitor coupling ( AC coupling ) of the clock input relies on the rise and fall time ( dV/dT ) of the input pulse to switch the state of the flip-flop. Use the trapezoidal waveform option for AWG 1 and use the symmetry control to adjust the rise/fall time of the waveform. What is the slowest dV/dT that will change the state of the flip-flop?

Divide by 2 Flip-Flop

The objective of this fifth experiment is to modify the D-type flip-flop from experiment 4 to build a circuit that divides the frequency of an input signal by 2.

ADALM2000 Active Learning Module Solder-less breadboard Jumper wires 2 - 1 KΩ resistors 2 - 200 KΩ resistors 2 - 47 KΩ resistors 2 - small signal NPN transistors (2N3904) 2 - small signal diodes (1N914) 2 - 39 pF capacitors 2 - 100 pF capacitors

Modify the D-type flip-flop from experiment four to construct the divide by 2 circuit as shown in figure 5 on your solder-less breadboard.

Switching between the two states is achieved by applying a single clock pulse which in turn will cause the “ON” transistor to turn “OFF” and the “OFF” transistor to turn “ON” on the negative or falling edge of the clock pulse. The circuit will switch sequentially by applying a pulse to each base in turn and this is achieved from a single input clock pulse using biasing the two diodes to steer the pulse to the correct base based on the current state of the flip-flop.

To illustrate how the circuit operates we will assume the circuit is in one of its two stable states with the collector voltage of Q 1 low (0 V ), and that of Q 2 high (5 V ). D 1 has a low voltage on its cathode via R 6 and a high voltage ( V BE of on transistor Q 1 ) on its anode via R 4 , making it forward biased. D 2 has a high voltage on its cathode via R 5 and a low voltage on its anode via R 3 ( V BE of off transistor Q 2 ), making it reverse biased.

An external negative going pulse, coupled through C 1 and C 2 , is steered to the base of Q 1 since D 1 is forward biased, but blocked from the base of Q 2 by reverse biased D 2 . Q 1 is turned off and Q 2 is turned on by the cross coupled connection through the parallel combination of C 3 and R 3 . This happens very quickly because of the positive feedback effect we saw earlier in the simple bistable multivibrator. The circuit is now in its second stable state and waits for another negative going clock pulse.

Since the collector voltage of Q 2 , the Q output node, changes state for every clock pulse, there is one pulse appearing at the output for every two clock input pulses. It can therefore be used as a divide by two circuit.

Figure 15 Divide by 2 circuit

The AWG1 output and scope channel 1 input should both be connected to the input marked Clock in figure 13. The second input scope channel 2 should be connected to the Q output of the flip-flop in figure 5. The AWG1 should be configured as a square wave with a 5 V amplitude peak-to-peak and 2.5 V offset ( 0 - 5V swing ). Set the frequency to 10 KHz.

Figure 16 Divide by 2 flipflop breadboard circuit

Turn on the Vp power supply and enable AWG1 output only after you have completely built and checked the circuit. You should observe a square wave on the Q output which is one half the frequency of the AWG 1 signal. Move the channel 2 scope input to the QB output. You should see a similar square wave signal but inverted with respect to the Q output.

Figure 15: Plot of Clock and Q output

Figure 16: Plot of Clock and QB output

Reverse the polarity ( direction ) of the two steering diodes, D 1 and D 2 . What is the effect on the relative timing of the Q and QB outputs with respect to the input clock signal? Explain why it has changed.

• Fritzing files: bjt_multivib_bb
• LTspice files: bjt_multivib_ltspice

http://en.wikipedia.org/wiki/Multivibrator

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• Pulse Circuits Tutorial
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• Multivibrator (Overview)
• Astable Multivibrator
• Monostable Multivibrator
• Bistable Multivibrator
• Pulse Circuits Time Base Generators
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Pulse Circuits - Monostable Multivibrator

A monostable multivibrator, as the name implies, has only one stable state . When the transistor conducts, the other remains in non-conducting state. A stable state is such a state where the transistor remains without being altered, unless disturbed by some external trigger pulse. As Monostable works on the same principle, it has another name called as One-shot Multivibrator .

Construction of Monostable Multivibrator

Two transistors Q 1 and Q 2 are connected in feedback to one another. The collector of transistor Q 1 is connected to the base of transistor Q 2 through the capacitor C 1 . The base Q 1 is connected to the collector of Q 2 through the resistor R 2 and capacitor C. Another dc supply voltage –V BB is given to the base of transistor Q 1 through the resistor R 3 . The trigger pulse is given to the base of Q 1 through the capacitor C 2 to change its state. R L1 and R L2 are the load resistors of Q 1 and Q 2 .

One of the transistors, when gets into a stable state, an external trigger pulse is given to change its state. After changing its state, the transistor remains in this quasi-stable state or Meta-stable state for a specific time period, which is determined by the values of RC time constants and gets back to the previous stable state.

The following figure shows the circuit diagram of a Monostable Multivibrator.

Operation of Monostable Multivibrator

Firstly, when the circuit is switched ON, transistor Q 1 will be in OFF state and Q 2 will be in ON state. This is the stable state. As Q 1 is OFF, the collector voltage will be V CC at point A and hence C 1 gets charged. A positive trigger pulse applied at the base of the transistor Q 1 turns the transistor ON. This decreases the collector voltage, which turns OFF the transistor Q 2 . The capacitor C 1 starts discharging at this point of time. As the positive voltage from the collector of transistor Q 2 gets applied to transistor Q 1 , it remains in ON state. This is the quasi-stable state or Meta-stable state.

The transistor Q 2 remains in OFF state, until the capacitor C 1 discharges completely. After this, the transistor Q 2 turns ON with the voltage applied through the capacitor discharge. This turn ON the transistor Q 1 , which is the previous stable state.

Output Waveforms

The output waveforms at the collectors of Q 1 and Q 2 along with the trigger input given at the base of Q 1 are shown in the following figures.

The width of this output pulse depends upon the RC time constant. Hence it depends on the values of R 1 C 1 . The duration of pulse is given by

$$T = 0.69R_1 C_1$$

The trigger input given will be of very short duration, just to initiate the action. This triggers the circuit to change its state from Stable state to Quasi-stable or Meta-stable or Semi-stable state, in which the circuit remains for a short duration. There will be one output pulse for one trigger pulse.

The advantages of Monostable Multivibrator are as follows −

• One trigger pulse is enough.
• Circuit design is simple
• Inexpensive

Disadvantages

The major drawback of using a monostable multivibrator is that the time between the applications of trigger pulse T has to be greater than the RC time constant of the circuit.

Applications

Monostable Multivibrators are used in applications such as television circuits and control system circuits.

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Transistor Multivibrator Circuits [Astable, Bistable, Monostable]

Last Updated on August 28, 2022 by Swagatam 1 Comment

A transistor or BJT multivibrator is a two-stage switching circuit that is configured in a cross-coupled manner. This allows each of the active BJT stages to be regeneratively cross-coupled with the other BJT stage, so that one switches ON when the other switches OFF, and vice versa.

This cross-coupling may be set up to switch either a stable or semi-stable manner. Whenever a stable cross-coupling is required, the transistor switch continues in either an ON or OFF condition until an external signal input forces it to change.

Once the circuit is cross-coupled in a semistable mode, the transistor originally latches into an ON or OFF position. However after some delay, as specified by the RC time constant of the cross-coupling, the transistor immediately gets "unlocked" again.

Types of Transistor Multivibrator

Figures 1 to figure 4 illustrate circuits for the four most basic types of transistor multivibrator circuits.

• Bistable : A bistable transistorized multivibrator can be used to generate a fixed ON or OFF output states in response to the pressing of a push button.
• Astable: In an astable multivibrator the transistors switch ON/OFF alternately and this cycle continues indefinitely as long as the circuit is powered. The switching frequency is determined by the RC components of the circuit.
• Monostable: In a transistorized monostable circuit , the output can be switched ON momentarily for a desired period of time by pressing an associated trigger button. The time delay is decided by the RC timing component of the circuit.

A triggered bistable (two stable state) multivibrator example is shown in Fig. 1, which can be triggered manually.

Each transistor's base-bias is derived through the collector of the second transistor, causing the first transistor to switch off when the second transistor turns on, and vice versa.

By momentarily shutting off Q1 with S1, the output may be pushed low; the circuit will remain in this condition until Q2 is switched off by S2.

The output locks into the high state at that point, and the process could be repeated for so long as the circuit remains energized.

A monostable (single stable state) multivibrator or one-shot pulse generator circuit is shown in Figure 2. 

The output of this configuration is normally low, however if Q2 is momentarily switched off with S1, it turns high for a certain duration of time (defined by the values of C1 and R2).

Astable Multivibrator

An astable (without stable states) multivibrator or free-running square-wave generator is shown in Figure 2.

The values of R3 and C1, as well as R2 and C2, define the square wave's on and off times.

A Schmitt trigger, often known as a sine-to-square waveform converter, is shown in Figure 3.

Transistor Q2 quickly flips from ON to OFF, or vice versa, when the base of transistor Q1 rises above or falls below the predefined trigger voltage thresholds.

The following paragraphs explains the above 3 basic transistor multivibrator circuits in more details:

Monostable Transistor Multivibrator

The triggered pulse generator in Fig. 4 indicates an example of a monostable multivibrator circuit.

Normally, R2 drives transistor Q2 into saturation, resulting in a low output (derived from transistor Q2's collector).

Under this circumstance, transistor Q1, which gets its base-bias via transistor Q2's collector and resistor R4, is switched off, and its collector is now at 100% supply voltage.

When switch S1 is briefly closed, a START signal is applied to Q2. Q2 turns off, forcing the output high and turning on Q1 via R4. The switching off of S1 causes regenerative switching activity.

When the regenerative reaction is implemented, the charge on C1 drives the base of transistor Q2 negative.

Through R2, C1 begins to discharge. Its charge finally drops to the point where Q2 switches on again, causing the triggering of yet another regenerative reaction.

Next, the output pulse stops, and both transistors restore to their original states, completing the circuit's activity.

Therefore, when an input trigger signal is applied by briefly shutting switch S1, a positive-going pulse is created at the output of the monostable multivibrator circuit. The values of R2 and C1 define the pulse period. 

The following equation provides the relationship:

Pulse Period = -0.7 x R2 x C1

The pulse time is measured in microseconds, C is measured in microfarads, and R is measured in kilohms. Simply activating a momentary switch or supplying an input command signals can both trigger the circuit in Fig. 2.

A negative pulse applied to the base of Q2 or a positive pulse applied to the base of Q1 can be used as the trigger signal.

A realistic design for a mechanically triggered monostable multivibrator is shown in Figure 5-a. It may be initiated by delivering a positive pulse to Q1's base via R2 using the momentary switch S1.

The waveforms of the circuit are shown in Figure 5-b. While operating, the base-to-emitter junction of Q2 is reverse-biased by a peak voltage which is equal to the supply voltage level, as shown in Fig. 5.

This implies that to avoid damage to the transistor, the maximum supply voltage must be restricted to around 9 volts. 

If silicon diode D1 is connected in series with Q2's base, a supply voltage larger than the reverse base-emitter breakdown value of Q2 can be reliably supplied, as illustrated in Fig. 5 . 

In the Fig. 5 circuit, the magnitude of timing resistor R3 has to be significantly bigger compared to R1, but lower than the product of R5 and the hFE of Q1.

The pulse time in Fig. 5 is 50 milliseconds divided by the value of capacitor C1 in microfarads. With the value of Cl as indicated in the diagram, pulse timing output will be 5 seconds.

Transistor Multivibrator with Delay Periods

When Q2 in Fig. 5 is replaced by a Darlington transistor combination, the circuit may give very long timing intervals. As demonstrated in Fig. 6, this replacement leads to a very relatively high hFE and allows for the use of large R3 values.

With the values of the resistors and capacitors illustrated, the Fig. 6 circuit may be supplied through any DC source with an output between +6 and +15 volts to produce a pulse output length of roughly 100 seconds.

Please remember that the length of the input trigger signal is critical for a manually triggered monostable circuit like those shown in Figures 5 and 6.

When a positive-going pulse is supplied to the base of Q1 in Fig. 5 or Q3 in Fig. 6, the circuit is activated.

The period will finish regeneratively as soon as this pulse is withdrawn before the monostable multi-vibrator ends its usual timing period, as earlier mentioned.

The timing cycle will finish non-regeneratively in case the trigger signal has not been eliminated within the period the monostable achieves its normal timing period.

The output pulse would have a longer duration and fall time as a result of this compared to if the trigger signal was terminated sooner.

How to Trigger Using Waveforms

Figures 7 and 8 demonstrate two different approaches of activating the monostable pulse generator using input waveforms. A square-wave input signal with a brief rise time triggers the circuit for each scenario.

The differentiation circuit, which consists of C1 and R1, differentiates this waveform to generate a short activation pulse.

The differentiated input signal is rectified using diode D1 in the Fig. 7 circuit, resulting in a positive trigger pulse at transistor Q1 base, in case an external trigger signal is introduced.

However, the differentiated signal is supplied to the gate of transistor Q1 in the Fig. 8 circuit.

The trigger signal has now become independent of Q2 due to this alteration in the design. To optimize the structure of the output waveform, "speed-up" capacitor C3 is hooked up in parallel with feedback resistor R5 in Fig. 8.

With the indicated values of resistors and capacitors in Figs. 7 and 8, both circuits produce an output pulse duration of roughly 110 microseconds.

With appropriate values for the capacitor C2 and resistor R4, this duration may be adjusted through a fraction of a microsecond to many seconds.

When regulated by a Schmitt trigger or equivalent sinewave-to-squarewave converter circuit, the circuits in Figures 7 and 8 could be activated through a sine wave or other non-rectangular waves.

Transistor Bistable Multivibrator circuits 

Figure 9 shows a realistic design for the mechanically triggered bistable multivibrator depicted in Figure 1 and explained before.

This circuit is also characterized as a R/S (reset/set) flipflop and is a rudimentary digital memory, similar to a SPDT switch.

By briefly locking switch S2, the output could be set to the high state. (Put another way, apply a negative pulse to the base of Q2.)

The circuit subsequently "remembers" this condition until S1 is momentarily closed, resetting it to the low state (which can be also done by applying a negative pulse to the base of Q1).

This new state is then "remembered" by the circuit until S2 resets it. For so long as power is supplied, the cycle keep repeating forever.

A couple of  guiding diodes (diodes D1 and D2) and accompanying parts could be added to the circuit in Fig. 9 to give a divide-by-two or counting function, as illustrated in Fig. 10.

When a negative-going trigger pulse is supplied to the Fig. 10 circuit, it switches state.

The circuit will create a squarewave output signal at 1/2 the input frequency in case for example the input pulses are generated from a squarewave pulse.

The circuit creates a couple of 180° out of phase output signals, designated Q1 and Q2. The arrival of CMOS IC equivalents of bistable counter circuits has essentially eliminated the requirement for discrete transistorized assembly of such multivibrator designs.

Transistor Multivibrators with Schmitt triggers

The Schmitt trigger circuit is the last member of the multi-vibrator family to be explored.

It's a voltage-sensitive switching circuit which switches its output state whenever the input signal rises beyond and falls below certain higher and lower threshold values.

The Schmitt trigger transforms sine waves to square waves, as seen in Figure 11.

The Schmitt trigger circuit is an emitter-coupled configuration, which consists of a cross-coupling between the base and collector of transistor Q1, allowing for a regenerative switching.

By shunting R4, capacitor C2 accelerates the switching speed. A DC voltage is superimposed on the sine-wave input signal. (The voltage delivered to the base of Q1 is adjusted by trimmer potentiometer R8 and resistors R1 and R2).

A sinewave input signal having an amplitude of a minimum of 0.5 volts rms is required for an effective Schmitt trigger. R8 should be modified to maximise the uniformity of the squarewave output signal amplitude with the input signal amplitude.

At frequencies approximately to a few hundred kilohertz, the Schmitt trigger works well as a sinewave-to-squarewave converter.

The circuit can generate squarewave output signals with rise times of just a few microseconds.

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About Swagatam

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April 29, 2023

Nice am gonna try one

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Astable, Monostable and Bistable Multivibrator

A digital electronic circuit known as a multivibrator produces digital signals like pulses, square waves, and other types of signals. It often consists of passive elements like resistors and capacitors as well as digital logic gates.

What is a Multivibrator?

A multivibrator is a switching circuit that is a two-stage resistance-coupled amplifier with positive feedback from the output of one amplifier to the input of the other. The name multivibrator is derived from the fact that the square wave generated consists of a large number of sinusoidal of different frequencies. Multivibrators are crucial parts of digital circuits and computer architecture, to sum up. They are utilized for data storage, synchronization between various components, clock signal generation, timing control, and data transfer time. The three basic types of multivibrators are astable, monostable, and bistable, each with distinct properties and uses.

• A multivibrator circuit oscillates between a “HIGH” state and a “LOW” state producing a continuous output.
• In a multivibrator, the two transistors are connected in feedback so that one transistor controls the state of the other. 
• Multivibrators are used are widely used to implement two-state devices like Relaxation Oscillators, Timers, and flip-flops. 

The multivibrators are classified into three categories-

• Astable Multivibrator.
• Monostable Multivibrator.
• Bistable Multivibrator.

Astable Multivibrator

An astable multivibrator, also called a free-running multivibrator, is a circuit that continuously produces square waves or pulses without the use of an external trigger. The term “astable” refers to the absence of a stable state in this particular type of multivibrator. 

• The circuit is built to alternate between two stable states, resulting in a steady oscillation. 
• By changing the values of the resistors and capacitors in the circuit, the frequency and duty cycle of the output waveform can be altered.
• In digital circuits, the astable multivibrator is frequently employed as a clock source. The timing of data transfers between various components can be synchronized using the frequency of the output waveform.

The output of an astable multivibrator does not have any stable state and it changes its state from high to low and low to high repeatedly.

• It is also known as a free-running multivibrator.
• It has no stable state, hence the name astable.
• It produces a continuous series of pulses with a predetermined frequency and duty cycle.
• It requires two identical transistors two capacitors, and a few resistors.
• It is commonly used in oscillator circuits, pulse generators, and clock circuits.

Applications

• Used in square wave frequency generator.
• Used as a timing oscillator in the computer system.
• Used in flashing lights.

Monostable Multivibrator

A monostable multivibrator, also called a one-shot multivibrator, is a circuit that responds to an external trigger by producing a single pulse with a set duration. A pulse from outside causes this sort of multivibrator to flip from its stable state to an unstable one. 

• The circuit returns to its stable condition after a certain amount of time and generates a single output pulse. 
• By altering the values of the resistors and capacitors in the circuit, the output pulse’s duration can be changed.
• In digital circuits, the monostable multivibrator is frequently used for pulse shaping, debouncing, and time delay functions. 
• Other circuits can use it as a trigger as well.

The output of a monostable multivibrator has only one stable state.

• Also known as a one-shot multivibrator.
• It has only one stable state.
• It produces a single output pulse of a predetermined width when triggered by an input signal.
• It requires two transistors, two capacitors, and a few resistors.
• It is commonly used in timing circuits, delay circuits, and pulse width modulation circuits.
•  Used for regenerating weak signals.
• Used in the pulse generator.
• Used in memory.

Bistable Multivibrator

A flip-flop, or bistable multivibrator, is a circuit with two stable states that can alternately exist indefinitely. 

• A signal from outside causes it to change from one stable condition to another. 
• The circuit will stay in its stable state until another trigger signal enters it. 
• A bistable multivibrator typically produces a square wave with two separate voltage levels as its output waveform.

Digital circuits frequently use bistable multivibrators for memory storage, data transport, and synchronization. They can also be utilized in shift registers and counters.

The output of a bistable multivibrator has two stable states. We have to apply external inputs in order to change the existing state of the output.

• Also known as a flip-flop multivibrator.
• It has two stable states and can remain in either state indefinitely without any input signal.
• It is commonly used in digital circuits as a memory element, latch, or flip-flop.
• It is also used in applications where a simple on/off switch is required.
• Used for changing the supply to two circuits.
• Used in digital operation in computers.

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