Monostable 74121 (nonretriggerable) timing A 74121 one-shot uses REXT = 49 kΩ and CEXT = 0.2 µF. Approximately what is the output pulse width tW?

Introduction / Context:The nonretriggerable 74121 monostable produces a fixed-width pulse determined by an external resistor and capacitor. Designers use a simple proportional formula with a device-specific constant to estimate pulse duration from component values.

Given Data / Assumptions:

• Device: 74121 (nonretriggerable) monostable.
• REXT = 49 kΩ.
• CEXT = 0.2 µF.
• Approximate timing relation: tW ≈ 0.7 * REXT * CEXT (typical for 74121).

Concept / Approach:First compute the RC time constant, then multiply by the typical coefficient. Ensure unit conversions are correct (kΩ to Ω, µF to F) to avoid order-of-magnitude mistakes.

Step-by-Step Solution:

Verification / Alternative check:A bench check with nominal parts will produce a pulse near this value, noting tolerance on R and C (often ±5% to ±20%) and supply/temperature effects; the actual width will vary accordingly.

Why Other Options Are Wrong:

• 6.9 µs: Off by a factor of 1000 due to unit error (µF vs nF).
• 69 ms / 690 ms: Too large; would require larger C or R.

Common Pitfalls:Mixing microfarads and nanofarads or forgetting that kΩ * µF yields milliseconds when scaled by a sub-unity constant.

Final Answer:6.9 ms