555 monostable design — find C1 for a required pulse width A 555 timer operates as a monostable (one-shot) with R1 = 220 kΩ. Determine the value of C1 needed to obtain a pulse width of 4 ms using t = 1.1 * R * C.

Introduction / Context:
The 555 timer configured as a monostable multivibrator generates a single pulse whose width is set by an external resistor and capacitor. Selecting the correct timing capacitor for a desired pulse width prevents redesigns and repeated prototype iterations.

Given Data / Assumptions:

• Pulsed one-shot configuration.
• R1 = 220 kΩ = 220,000 Ω.
• Desired pulse width t = 4 ms = 0.004 s.
• Formula: t = 1.1 * R * C.

Concept / Approach:
Rearrange the standard formula to solve for C: C = t / (1.1 * R). Convert all quantities to SI base units to avoid scale errors, then convert the result back to microfarads for readability.

Step-by-Step Solution:

Verification / Alternative check:
Back-calculate: t = 1.1 * 220 kΩ * 0.017 µF ≈ 1.1 * 220,000 * 17 × 10^-9 ≈ 0.004114 s ≈ 4.11 ms, within typical component tolerance ranges for R and C. Fine tuning can be done with precision parts if needed.

Why Other Options Are Wrong:

• 17 pF / 170 pF: three to five orders of magnitude too small.
• 1,700 µF: absurdly large for a 4 ms one-shot with 220 kΩ.
• 0.17 µF: would produce ~41 ms, not 4 ms.

Common Pitfalls:

• Mistaking milli, micro, and nano prefixes; always convert to base units first.

Final Answer:
0.017 µF