How does a capacitor charge through a resistor?

The capacitor voltage rises from zero towards the supply along a curve that is steep at first and then flattens, because the charging current falls as the capacitor voltage rises. The time depends on the product of resistance and capacitance, so a larger resistor or capacitor charges more slowly, creating a time delay.

What does a 555 monostable do?

A monostable has one stable state, with its output normally low. When triggered, the output goes high for a fixed time and then returns low on its own. The pulse duration is T = 1.1RC, where R and C are the timing resistor and capacitor, with R in ohms and C in farads.

What does a 555 astable do?

An astable has no stable state and switches continuously between high and low, producing a square wave. Its frequency is 1.44 divided by (R1 plus 2R2) times C, and the period is one over the frequency. It is a free-running clock used for flashing, beeping and driving counters.

What is the mark-space ratio?

The mark is the time the output is high and the space is the time it is low; the mark-space ratio is the ratio of these times. In a 555 astable the capacitor charges through R1 and R2 but discharges through only R2, so the mark is longer than the space; making R1 small compared with R2 makes them more nearly equal.

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WJEC GCSE Electronics: timing circuits overview

Q: How are timing circuits examined?

It sits in Component 2 (Application of Electronics), a 1 hour 30 minute written paper worth 40% of the GCSE. Expect sketching RC charging and discharging curves, monostable pulse-duration calculations with T = 1.1RC, astable frequency and period calculations, and mark-space ratio reasoning.

An overview of the timing circuits content in Component 2 of WJEC Eduqas GCSE Electronics, covering RC charging and discharging and time delays, the 555 monostable (single pulse, T = 1.1RC) and the 555 astable (continuous square wave, frequency and mark-space ratio).

Generated by Claude Opus 4.86 min readWJEC Eduqas GCSE Electronics, Component 2Updated 2026-06-15

Reviewed by: AI editorial process; not yet individually human-reviewed

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What the topic covers
How this content is examined
How to study it
For the official specification

The timing circuits content of WJEC Eduqas GCSE Electronics is part of Component 2 (Application of Electronics). It covers the RC network that creates time delays and the two key 555 timer modes: the monostable (a single pulse) and the astable (a continuous square wave). This page maps the content and links to a focused answer page for each part.

What the topic covers

RC charging and discharging: How a capacitor charges and discharges through a resistor, the shape of the voltage-time curves, and how the RC product sets a time delay. See RC charging and discharging.
555 monostable timer: A single output pulse of fixed length when triggered, with the pulse duration $T = 1.1RC$ . See 555 monostable timer.
555 astable timer: A continuous square-wave output, with the frequency $f = \dfrac{1.44}{(R_1 + 2R_2)C}$ and the mark-space ratio. See 555 astable timer.

How this content is examined

This content sits in Component 2 (Application of Electronics), a written paper of 1 hour 30 minutes worth 40% of the GCSE. Expect sketching RC charging and discharging curves, monostable pulse-duration calculations, astable frequency and period calculations, and mark-space ratio reasoning.

How to study it

Know the RC curve. Voltage rises (or falls) along a flattening curve; the current falls as the capacitor charges.
Monostable: one pulse. Triggered, output high for $T = 1.1RC$ , then returns low.
Astable: continuous. A square wave at $f = \dfrac{1.44}{(R_1 + 2R_2)C}$ ; remember to double $R_2$ .
Mark and space. Mark is high time, space is low time; the 555 mark is longer than its space.
Watch the units. Convert $\text{k}\Omega$ and $\mu\text{F}$ before every timing calculation.

For the official specification

WJEC Eduqas publishes the full GCSE Electronics specification, past papers and mark schemes at wjec.co.uk. Always revise from the current specification and the board's own past papers, because question style and the printed equation and symbol lists are board-specific.

Sources & how we know this

WJEC Eduqas GCSE Electronics specification (from 2017) — WJEC Eduqas (2017)