Monostable MultiVibrator

Pre-Lab:

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.

Apparatus:

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:

 

Theory:

     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.

Operation:

     Assume  initially transistor  Q₂ is  in saturation  as it  gets base bias  from V_CC  through R. coupling  from Q₂  collector  to Q₁   base ensures that Q1   is  in cutoff.  If an appropriate negative trigger pulse applied at collector of Q₁ (V_C1) induces a transition in Q₂, then Q2  goes to cutoff. The output at Q₂ goes high. This high output when coupled to Q₁   base, turns it  ON.  The Q₁     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,  Q₂ goes to cutoff and Q₁  starts  conducting.  There is a path for capacitor C to charge from V_CC through R and the conducting transistor Q₁. The polarity should be such that Q₂ base potential rises.  The  moment, it  exceeds  Q₂ base cut-in  voltage,  it  turns ON  Q₂  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.  Q₂ 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, R₁ = 10K%u2126.

                                    T =0.69RC

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

                                    0.3 x 10^-3Sec = 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

                        V_B1 =   + 

Substitute the values , R₁=10kohms    we will get  R₂= 100Kohms

Procedure:

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:

1.   What is a multivibrator?

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

2. What are applications of Monostable Multivibrator?

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

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

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

4.  A Monostable Multivibrator generates __ wave

           Ans; pulse waveform

5. Why is the time period T also called Delay time?

Ans: in Quasi-stable state Q₁  is ON and Q₂ is OFF. The interval during which quasi- stable state of the multi persists i,e Q₂  is dependent upon the rate at which the capacitor C discharges. This duration of Quasi-stable state is termed as delay time.

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

         Ans: Because it produces only one pulse.

7. 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 .

8.   In monostable multivibrator, the coupling elements are __.

             Ans: resistor and capacitor

9.   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

10.  ___ 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.