555 Timer MCQ [Free PDF] – Objective Question Answer for 555 Timer Quiz

555 Timer MCQ


1. Monostable multivibrator may be used to generate

  1. sweep voltage
  2. pulses
  3. sinusoidal voltage
  4. sweep current

Answer.2. pulses

A monostable multivibrator may be used to generate pulses whereas an Astable multivibrator may be used to generate a square wave.


2. A 1 μs pulse can be converted into a 1 ms pulse by using

  1. A monostable multivibrator
  2. An astable multivibrator
  3. A bistable multivibrator
  4. A J – K flip-flop

Answer.1. A monostable multivibrator

  • Given that the 1 μs pulse is converted into a 1 ms.
  • This is pulse widening. This can be achieved with the help of the monostable multivibrator.


3. The pulse width out of a one-shot multi-vibrator increase when the

  1. Supply voltage increases
  2. Timing resistor decreases
  3. UTP decreases
  4. Timing capacitance increases

Answer.4. Timing capacitance increases

The pulse width of a one-shot multivibrator depends on the formula,

τ = 1.1 RTCT

Hence, the timing cycle of the monostable multivibrator is determined by the timing capacitor, CT, and the resistor RT until it resets or returns to the original state.

It remains in the original stable state indefinitely until another input pulse or trigger signal is received. Hence, it has only one stable state.


4. Which of the following is not the other name for the Monostable Multi-vibrator circuit?

  1. Univibrator
  2. One-shot
  3. Gating circuit
  4. Square wave generator

Answer.4. Square wave generator

Square wave generator:

  • It is defined as an oscillator that gives the output without any input.
  • This generator is used in digital signal processing and electronic applications.
  • The square wave generator is also known as an Astable multivibrator or free-running.
  • The frequency of the square wave generator is independent of the output voltage.

​So, a square wave generator is an astable multivibrator and not monostable multivibrator.


5. Which among the following multivibrators is unstable in any state?

  1. Astable multivibrator
  2. Bistable multivibrator
  3. Monostable multivibrator
  4. Both astable and bistable multivibrator

Answer.1. Astable multivibrator

  • An Astable multivibrator is such a circuit that it automatically switches between the two states continuously without the application of any external pulse for its operation, i.e. it is unstable in any state.
  • As this produces a continuous square wave output, it is called a free-running multivibrator.


6. Determine the time period of a monostable 555 multivibrator.

A. T = 0.33RC
B. T = 1.1RC
C. T = 3RC
D. T = RC

Answer: B

The time period of a monostable 555 timer is

T =  RC×ln(1/3) = 1.1.RC.


7. How to overcome mistriggering on the positive pulse edges in the monostable circuit?

A. Connect an RC network at the input
B. Connect an integrator at the input
C. Connect a differentiator at the input
D. Connect a diode at the input

Answer: C

To prevent the mistrigger on positive pulse edges, a resister & capacitor combined 10kΩ and 0.001µF at the input to form a differentiator.

The circuit shows the differentiator to be connected between trigger input and the +VCC.


8. A monostable multivibrator has R = 120kΩ and the time delay T = 1000ms, calculate the value of C?

A. 0.9µF
B. 1.32µF
C. 7.5µF
D. 2.49µF

Answer: C

Time delay for a monostable multivibrator

T = 1.1RC

=> C = T/(1.1R)

= 1000ms/(1.1×120kΩ) = 7.57µF.


9. Which among the following can be used to detect the missing heartbeat?

A. Monostable multivibrator
B. Astable multivibrator
C. Schmitt trigger
D. None of the mentioned

Answer: A

A monostable multivibrator can be used as a missing pulse detector by connecting a transistor between trigger inputs. If a pulse misses, the discharge trigger input goes high & the transistor becomes cut-off and the output goes low. So, this type of circuit can be used to detect a missing heartbeat.


10. A 555 timer in monostable application mode can be used for

A. Pulse position modulation
B. Frequency shift keying
C. Speed control and measurement
D. Digital phase detector

Answer: C

In monostable operation mode, if input trigger pulses are generated from a rotating wheel, the circuit will determine the wheel speed whenever it drops below a predetermined value. Therefore, it can be used for speed control and measurement.

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