Monostable 74122 (retriggerable) timing An RC network for a 74122 retriggerable one-shot uses REXT = 100 kΩ and CEXT = 0.005 µF. What is the output pulse width (approximate)?

Introduction / Context:
The TTL 74122 retriggerable one-shot generates a pulse whose width depends on an external resistor and capacitor. While exact constants vary slightly by logic family and conditions, datasheets provide a proportionality factor so designers can estimate the output pulse width from REXT and CEXT values.

Given Data / Assumptions:

• Device: 74122 retriggerable monostable multivibrator.
• REXT = 100 kΩ.
• CEXT = 0.005 µF = 5 nF.
• We use the typical timing relation tW ≈ K * REXT * CEXT with family-typical constant K.

Concept / Approach:
For the 74122 family, a commonly used approximation is tW ≈ 0.14 * REXT * CEXT under nominal conditions. This constant captures the internal threshold ratios and discharge currents used by the monostable timing network. Calculate RC first, then apply the constant to estimate tW.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check by consulting device timing graphs or using a bench setup: measured tW should fall near 70 µs, with tolerance due to component spread and supply/temperature variations.

Why Other Options Are Wrong:

• 16 µs / 32 µs: Too short compared to the RC and constant typical for 74122.
• 160 µs: Exceeds the estimated value; would require a larger constant or component values.

Common Pitfalls:
Using the constant from a different device (for example, 74121) or mis-converting microfarads and ohms, leading to 10× errors.

Final Answer:
70 µs