Question

Which of the following is a common application of a 555 timer in monostable mode?

• A. To generate a constant output signal
• B. To generate a fixed pulse with output signal
• C. To oscillate continuously
• D. To provide a frequency response

Hint:

In monostable mode, the 555 timer functions as a "one-shot" pulse generator. It produces a single output pulse of a fixed width determined by \(T = 1.1 R C\) whenever it is triggered.

Answer 1

The Correct Option is B

Concept: The 555 timer is a highly versatile integrated circuit used to implement various timing and multivibrator circuits. It operates primarily in two modes:
• Astable Multivibrator Mode: The circuit has no stable states and oscillates continuously between high and low levels, generating a continuous square wave stream without an external trigger.
• Monostable Multivibrator Mode: The circuit has exactly one stable state (normally low). It remains in this stable state until an external negative-going trigger pulse is applied to the trigger input pin (Pin 2).

Step 1: Analyzing the operational physics of the Monostable mode.
When an external input trigger pulse is received, the internal circuitry flips the output state to HIGH. At the same time, an internal discharge transistor turns off, allowing an external timing capacitor (\(C\)) to begin charging from the supply voltage through a timing resistor (\(R\)). The voltage across the capacitor rises exponentially according to the time constant \(\tau = RC\).

Step 2: Determining the pulse width output.
Once the capacitor voltage reaches two-thirds of the supply voltage (\(\frac{2}{3}V_{CC}\)), an internal comparator resets the circuit, flipping the output back to its stable LOW state and discharging the capacitor instantly. The time duration (\(T\)) that the output remains HIGH is fixed and determined by the component values: \[ T = 1.1 \cdot R \cdot C \] Because this mode produces a single output pulse of a predetermined duration each time it receives a trigger, it is used for applications like pulse width generation, time delays, and switch debouncing. This corresponds perfectly to Option (B).