Linear Integrated Circuit Questions and Answers – 555 Timer as an Astable Multivibrator

This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “555 Timer as an Astable Multivibrator”.

1. Free running frequency of Astable multivibrator?
a) f=1.45/(RA+2RB)C
b) f=1.45(RA+2RB)C
c) f=1.45C/(RA+2RB)
d) f=1.45 RA/( RA+RB)
View Answer

Answer: a
Explanation: The frequency of the Astable multivibrator is T=0.69(RA+2RB)C.
Therefore, f = 1/T =1.45/(RA+2RB)C.

2. Find the charging and discharging time of 0.5µF capacitor.
Find the charging and discharging time of 0.5µF capacitor
a) Charging time=2ms; Discharging time=5ms
b) Charging time=5ms; Discharging time=2ms
c) Charging time=3ms; Discharging time=5ms
d) Charging time=5ms; Discharging time=3ms
View Answer

Answer: b
Explanation: The time required to charge the capacitor is tHigh=0.69(RA+RB)C =0.69(10kΩ+5kΩ)x0.5µF =5ms.
The time required to discharge the capacitor is tLow=0.69xRC =0.69x5kΩx0.5µF=2ms.

3. Astable multivibrator operating at 150Hz has a discharge time of 2.5m. Find the duty cycle of the circuit.
a) 50%
b) 75%
c) 95.99%
d) 37.5%
View Answer

Answer: d
Explanation: Given f=150Hz.Therefore,T=1/f =1/150 =6.67ms.
∴ Duty cycle, D%=(tLow/T) x 100% = (2.5ms/6.67ms)x100% = 37.5%.

4. Determine the frequency and duty cycle of a rectangular wave generator.
Determine the frequency and duty cycle of a rectangular wave generator
a) Frequency=63.7kHz; Duty cycle=50%
b) Frequency=53.7kHz; Duty cycle=55%
c) Frequency=43.7kHz; Duty cycle=50%
d) Frequency=60kHz; Duty cycle=55%
View Answer

Answer: b
Explanation: Frequency=1.45/(RA+RB)C .
Where RA=100Ω+50Ω=150Ω,
=>∴f=1.45/((150+120)x0.1µF) = 53703Hz = 53.7kHz.
Duty cycle, D% = [RB/(RA+RB)] x 100% = 120Ω/(150Ω +120Ω) x 100% = 0.55×100% = 55%.

5. How to achieve 50% duty cycle in adjustable rectangular wave generator? (Assume R1 –> Resistor connected between supply and discharge and R2 –> Resistor connected between discharge and trigger input.)
a) R1 < R2
b) R1 > R2
c) R1 = R2
d) R1 ≥ R2
View Answer

Answer: c
Explanation: The equation of duty cycle, D = R2/(R1 + R2). If R1 is made equal to R2 then 50% duty cycle is achieved.
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6. How to obtain symmetrical waveform in Astable multivibrator?
a) Use clocked RS flip-flop
b) Use clocked JK flip-flop
c) Use clocked D-flip-flop
d) Use clocked T-flip-flop
View Answer

Answer: b
Explanation: Symmetrical square wave can be obtained by adding a clocked JK flip-flop to the output of Astable multivibrator. The clocked flip-flop acts as a binary divider to the times output and produces 50% duty cycle without any restriction on the choice of resistors.

7. Determine the output frequency of the circuit.
Determine the output frequency of the circuit
a) 1450Hz
b) 1333Hz
c) 1871Hz
d) 1700Hz
View Answer

Answer: c
Explanation: The output frequency of the frequency shift keying generator is
f=1.45/[(RA||RC)+2(RB)]xC = 1.45/[(2.3kΩ||2.3kΩ) + (2×3.3kΩ)] x 0.1µF = 1.45/{[(2.3×2.3)/(2.3+2.3)] + 6.6kΩ}x0.1µF = 1.45/(7.75×10-4) = 1870.9 ≅ 1871Hz.

8. How does a monostable multivibrator used as frequency divider?
a) Using square wave generator
b) Using triangular wave generator
c) Using sawtooth wave generator
d) Using sine wave generator
View Answer

Answer: a
Explanation: Monostable multivibrator can be used as a frequency divider when a continuously triggered monostable circuit is triggered using a square wave generator. Provided the timing interval is adjusted to be longer than the period of triggering square wave input signal.

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