Question E7A06:
What is a characteristic of a monostable multivibrator?  

Choices:  
A: It switches temporarily to an alternate state for a set time  
B: It produces a continuous square wave  
C: It stores one bit of data  
D: It maintains a constant output voltage, regardless of variations in the input voltage    

Correct Answer is  A: It switches temporarily to an alternate state for a set time  

Monostable multivibrator = “one-shot.” It has one stable state. When triggered, it switches to the other (quasi-stable) state for a predetermined time, then returns to the stable state.

Why A is correct:
- A monostable has exactly one stable state; a trigger causes a single output pulse of fixed duration, then it returns to the stable state. Hence, it “switches temporarily to an alternate state for a set time.”

Why the others are incorrect:
- B: “It produces a continuous square wave” describes an astable multivibrator (free-running oscillator), not monostable.
- C: “It stores one bit of data” describes a bistable multivibrator (flip-flop/latch), not monostable.
- D: “Maintains a constant output voltage” describes a voltage regulator, not any multivibrator.

Useful formulas and rules of thumb:
- Pulse width for RC-timed monostables is set by the RC time constant:
  $$T \approx k\,R\,C$$
  where $k$ depends on the specific circuit.
  - 555 timer in monostable mode: $$T = 1.1\,R\,C$$
  - Many transistor/comparator monostables (threshold at 50%): $$T \approx 0.693\,R\,C$$
- Family summary:
  - Monostable: 1 stable state → one-shot pulse on trigger.
  - Bistable: 2 stable states → stores 1 bit.
  - Astable: 0 stable states → continuous oscillation.