Linear Integrated Circuit Multiple Choice Questions 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)
Answer: A
Clarification: 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
Answer: B
Clarification: 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%
Answer: D
Clarification: 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%
Answer: B
Clarification: 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. R12
B. R1 > R2
C. R1 = R2
D. R1 ≥ R2
Answer: C
Clarification: The equation of duty cycle, D = R2/(R1 + R2). If R1 is made equal to R2 then 50% duty cycle is achieved.
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
Answer: b
Clarification: 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
Answer: C
Clarification: 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
Answer: A
Clarification: 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.