# 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)

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. 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

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%

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. 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%

Explanation: Frequency=1.45/(RA+RB)C .
Where RA=100Ω+50Ω=150Ω,
RB=100Ω+20Ω=120Ω.
=>∴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

Explanation: The equation of duty cycle, D = R2/(R1 + R2). If R1 is made equal to R2 then 50% duty cycle is achieved.
Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!

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

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. a) 1450Hz
b) 1333Hz
c) 1871Hz
d) 1700Hz

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

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.

Sanfoundry Global Education & Learning Series – Linear Integrated Circuits. 