Monostable MultiVibrator
PreLab:
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 quasistable 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 quasistable for a time equal to RC time constant of the circuit. It returns from the quasistable state to its stable state without any external triggering pulse. It is also called as oneshot, a single cycle, a single step circuit or a univibrator.
Operation:
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 cutin 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
V_{B1} = +
Substitute the values , R_{1}=10kohms we will get R_{2}= 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 channel2 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 Quasistable 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 Quasistable state is termed as delay time.
 Justify, Why Monostable Multivibrator is called oneshot circuit?
Ans: Because it produces only one pulse.
 What is a quasi state?
Ans; A quasistable state means a temporarily stable state . the circuit remains in the quasistable 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 2BC107 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.

UpdatedSep 29, 2019

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