# Multivibrators and 555 Timer MCQ Questions & Answers

Multivibrators and 555 Timer MCQs : This section focuses on the "Multivibrators and 555 Timer". These Multiple Choice Questions (MCQs) should be practiced to improve the Multivibrators and 555 Timer skills required for various interviews (campus interview, walk-in interview, company interview), placement, entrance exam and other competitive examinations.

Question 1

When a capacitor charges:

A. the voltage across the plates rises exponentially
B. the circuit current falls exponentially
C. the capacitor charges to the source voltage in 5×RC seconds
D. all of the above

Question 2

An astable 555 timer has the following number of stable states:

A. 0
B. 1
C. 2
D. 3

Question 3

A monostable 555 timer has the following number of stable states:

A. 0
B. 1
C. 2
D. 3

Question 4

What does the discharge transistor do in the 555 timer circuit?

A. charge the external capacitor to stop the timing
B. charge the external capacitor to start the timing over again
C. discharge the external capacitor to stop the timing
D. discharge the external capacitor to start the timing over again

Question 5

If Vcc of a 555 timer circuit is set to +10 V what is the level of output voltage from the circuit?

A. VOH = 10 V, VOL = 0 V
B. VOH = 10 V, VOL = 0.1 V
C. VOH = 8.5 V, VOL = 0 V
D. VOH = 8.5 V, VOL = 0.1 V

Question 6

With most monostable multivibrators, what is the Q output when no input trigger has occurred?

A. LOW
B. +5 V
C. SET
D. HIGH

Question 7

Design a circuit using a 74121 to convert a 33-kHz, 30% duty cycle waveform to a 33-kHz, 60% duty cycle waveform.

A. Rint = 2 k, Cext = 0.012 nF
B. Rint = 2 k, Cext = 0.012 F
C. Rint = 4 k, Cext = 0.012 nF
D. Rint = 4 k, Cext = 0.012 F

Question 8

A retriggerable one shot has a pulse of 10 ms. 3 ms after being triggered, another trigger pulse is applied. The resulting output pulse will be ________ ms.

A. 3
B. 7
C. 10
D. 13

Question 9

Triggering a retriggerable one shot during pulse generation will:

A. time out the original pulse
B. extend the pulse to this trigger width
C. have no effect
D. double the original pulse width

Question 10

A crystal demonstrates the ________ effect when a mechanical force across the crystal causes a small voltage to be generated.

A. photoelectric
B. Co-pitts
C. piezoelectric
D. flywheel

Question 11

Pulse stretching, time-delay, and pulse generation are all easily accomplished with which type of multivibrator circuit?

A. astable
B. monostable
C. multistable
D. bistable

Question 12

Is there any limit to the number of times that a 74123 can be retriggered?

A. yes
B. no

Question 13

Does the 74123 one shot have an internal timing resistor?

A. yes
B. no

Question 14

An astable multivibrator requires:

A. balanced time constants
B. a pair of matched transistors
C. no input signal
D. dual J-K flip-flops

Question 15

To obtain a 50% duty cycle in an astable 555 timer circuit:

A. tLO = tHI
B. RA = RB and short RB with a diode during the capacitor charging cycle
C. capacitor voltage must rise above 1/3 VCC
D. tLO = tHI, RA = RB, and short RB with a diode during the capacitor charging cycle

Question 16

What is the difference between a retriggerable one shot and a nonretriggerable one shot?

A. The nonretriggerable can only be triggered once.
B. The retriggerable can be triggered many times.
C. The output pulse can be stretched with a nonretriggerable.
D. The output pulse can be stretched with a retriggerable.

Question 17

In a typical IC monostable multivibrator circuit, at the falling edge of the trigger input, the output switches HIGH for a period of time determined by the ________.

A. value of the RC timing components
B. amplitude of the input trigger
C. frequency of the input trigger
D. magnitude of the dc supply voltage

Question 18

What is another name for a bistable multivibrator?

A. an on-off switch
B. an oscillator
C. a flip-flop

Question 19

The internal circuitry of the 555 timer consists of ________, an R-S flip-flop, a transistor switch, an output buffer amplifier, and a voltage divider.

A. a comparator
B. a voltage amplifier
C. two comparators
D. a peak detector

Question 20

If the resistor in the Schmitt trigger astable multivibrator is a variable resistor, what part of the output voltage waveform will change when the resistance is changed?

A. the shape of the waveform
B. the amplitude of the waveform
C. the period of the waveform
D. none of the above

Question 21

The output of the astable circuit ________.

A. constantly switches between two states
B. is LOW until a trigger is received
C. is HIGH until a trigger is received
D. floats until triggered

Question 22

What is the function of the comparators in the 555 timer circuit?

A. to compare the output voltages to the internal voltage divider
B. to compare the input voltages to the internal voltage divider
C. to compare the output voltages to the external voltage divider
D. to compare the input voltages to the external voltage divider

Question 23

The pulse width out of a one-shot multivibrator increases when the:

A. supply voltage increases
B. timing resistor decreases
C. UTP decreases
D. timing capacitance increases

Question 24

The ________ is defined as the time the output is active divided by the total period of the output signal.

A. on time
B. off time
C. duty cycle
D. active ratio

Question 25

Which of the following is NOT a characteristic of a retriggerable monostable multivibrator?

A. It is a dual multivibrator.
B. It has an active-HIGH reset, which terminates all timing functions.
C. It has no internal timing resistor.
D. none of the above

Question 26

What is the difference between an astable multivibrator and a monostable multivibrator?

A. The astable is free running.
B. The astable needs to be clocked.
C. The monostable is free running.
D. none of the above

Question 27

The monostable multivibrator circuit is not an oscillator because ________.

A. its output switches between two states
B. it requires a trigger to obtain an output signal
C. it requires a sine wave input signal
D. the circuit does not require a dc power supply

Question 28

Which mode of operation is being used when a 555 timer chip has two external resistors and an external capacitor?

A. monostable
B. pulse stretching
C. Schmitt triggering
D. astable