555 monostable timing — compute the output pulse width For a one-shot (monostable) 555 timer with R1 = 3.3 kΩ and C1 = 0.02 µF (20 nF), determine the expected output pulse width using the standard formula t = 1.1 * R * C.
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A7.3 µs
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B73 µs
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C7.3 ms
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D73 ms
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E0.73 µs
Answer
Correct Answer: 73 µs
Explanation
Introduction / Context:The 555 timer configured as a monostable generates a single output pulse of width t when triggered. The pulse width is set by the external timing resistor and capacitor according to the widely used relation t = 1.1 * R * C (for the basic one-shot connection). Correct unit handling is vital when choosing component values or interpreting lab measurements.
Given Data / Assumptions:
- R1 = 3.3 kΩ = 3300 Ω.
- C1 = 0.02 µF = 20 nF = 20 × 10^-9 F.
- Formula: t = 1.1 * R * C.
Concept / Approach:Substitute the given component values into t = 1.1 * R * C and convert units carefully. Convert kilo-ohms to ohms and microfarads to farads before multiplying. The resulting time will be in seconds; convert to microseconds (µs) for a convenient expression.
Step-by-Step Solution:
Convert R: 3.3 kΩ = 3300 Ω.Convert C: 0.02 µF = 20 nF = 20 × 10^-9 F.Compute RC: 3300 * 20 × 10^-9 = 66 × 10^-6 s = 66 µs.Apply 1.1 factor: t = 1.1 * 66 µs = 72.6 µs ≈ 73 µs.Verification / Alternative check:Cross-check with a calculator or scope: expected pulse near 73 µs. Small deviations may occur due to component tolerances (e.g., ±5% resistors, ±10% capacitors) and the timer’s internal thresholds, but the nominal value matches 73 µs.
Why Other Options Are Wrong:
- 7.3 µs and 0.73 µs: off by factors of 10 or 100 due to unit mistakes.
- 7.3 ms and 73 ms: millisecond values are three orders of magnitude too large for the given RC.
Common Pitfalls:
- Forgetting the 1.1 multiplier or mixing microfarads and nanofarads.
Final Answer:73 µs