555 timer in astable (free-running) mode — timing network contents In a standard 555 astable configuration, which external components form the timing network that sets the output frequency and duty cycle?

Introduction / Context:
The ubiquitous 555 timer can operate in astable mode to generate a continuous square or rectangular waveform. The oscillation frequency and duty cycle depend on a simple timing network connected around the internal comparators and discharge transistor. Recognizing the exact component set used in the astable circuit is a foundational skill for electronics students and technicians.

Given Data / Assumptions:

• Mode: 555 astable (free-running oscillator).
• We are considering the classic single-capacitor, two-resistor timing network.
• No exotic variants (such as CMOS 555 tweaks) are assumed.

Concept / Approach:

In the standard astable configuration, two resistors (often named RA and RB) and one timing capacitor (C) determine the charge and discharge intervals between thresholds of approximately 1/3 Vcc and 2/3 Vcc. The discharge transistor periodically grounds the node between RB and C to create the low interval. Frequency and duty cycle are derived from RA, RB, and C with well-known formulas.

Step-by-Step Solution:

Verification / Alternative check:

Re-derive frequency: f ≈ 1 / (0.693 * (RA + 2RB) * C). Duty cycle ≈ (RA + RB) / (RA + 2RB). Both expressions explicitly reference two resistors and one capacitor.

Why Other Options Are Wrong:

Three resistors: unnecessary for the basic astable.

One resistor, two capacitors: not the standard 555 astable; some duty-cycle-symmetry tricks exist but are not the canonical circuit.

No external R/C: impossible; the 555 requires an external timing network to oscillate.

Common Pitfalls:

Mixing astable with monostable component counts; forgetting that duty cycle can be made 50% using a diode across RB (still the same basic parts plus a diode). Always remember: 2 resistors + 1 capacitor define the timing.

Final Answer:

two external resistors and an external capacitor are used