Duty-cycle conversion at constant frequency using a 74121 one-shot Convert a 33 kHz waveform from 30% duty to 60% duty while keeping the frequency at 33 kHz. Choose suitable Rint and Cext.

Introduction / Context:Changing duty cycle at a fixed frequency can be achieved by pulse stretching (or re-timing) with a monostable. The 74121 is a non-retriggerable one-shot whose output width is set by its timing network. Here, we need to expand the high time to 60% while keeping the input frequency at 33 kHz.

Given Data / Assumptions:

• Input frequency fin = 33 kHz → period T ≈ 1 / 33,000 ≈ 30.303 µs
• Target duty = 60% → desired high time th ≈ 0.60 * 30.303 µs ≈ 18.18 µs
• 74121 approximate timing: t_w ≈ K * Rint * Cext, where K is a family constant on the order of 0.7 (device- and family-dependent)
• Options provide discrete Rint and Cext choices

Concept / Approach:

Select Rint and Cext such that t_w ≈ desired high time. Using a common approximation t_w ≈ 0.7 * Rint * Cext guides us to an R*C product near th / 0.7.

Step-by-Step Solution:

Verification / Alternative check:

Accounting for component tolerances and the device's K variation, option (b) yields a practical pulse width near the target. Fine-tuning is typically done by slight value adjustments or trimming if required.

Why Other Options Are Wrong:

(a) and (c): RC products in the nanoseconds range cannot realize an ~18 µs pulse.(d): Produces a pulse too long (~33.6 µs), overshooting the 60% target.

Common Pitfalls:

Confusing 74121 timing constant with 555 values; misreading nF vs µF; forgetting frequency–period conversion when converting duty cycles.

Final Answer:

Rint = 2 kΩ, Cext = 0.012 µF