What does a rising-edge-triggered D-type flip-flop do?

On the rising edge of the clock it copies its D input to its Q output, and between clock edges it holds that value, ignoring changes at D. It is a one-bit memory, used as a latch and for data transfer, and it makes sequential (memory) logic different from combinational logic.

How is a binary counter built from flip-flops?

Feeding a D-type's inverted output back to its D input makes it toggle, dividing the clock frequency by two - a 1-bit counter. Clocking a second toggling flip-flop from the first gives a 2-bit counter that counts 00, 01, 10, 11 (0 to 3). Each stage halves the frequency and doubles the count range.

How are decimal digits displayed?

A decade counter counts 0 to 9 and outputs the count as BCD (binary-coded decimal). A decoder/driver converts the BCD code into the right pattern of segments and supplies the current to light a seven-segment display, so the chain counter to decoder/driver to display shows the digit, advancing one each clock pulse.

What is the 4017 and how do you shorten a count?

The 4017 is a decade counter with ten outputs, only one high at a time, stepping along on each clock pulse, so it acts as a sequencer. To count fewer steps, feed the output one beyond the last wanted step back to the reset input, so the counter resets early and repeats a shorter sequence.

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WJEC GCSE Electronics: sequential systems overview

Q: How are sequential systems examined?

It sits in Component 2 (Application of Electronics), a 1 hour 30 minute written paper worth 40% of the GCSE. Expect D-type timing diagrams, building and reading binary counters, frequency division, decade counter and seven-segment display chains, 4017 sequencing, and resetting for a custom count length.

An overview of the sequential systems content in Component 2 of WJEC Eduqas GCSE Electronics, covering the rising-edge-triggered D-type flip-flop, building 1-bit and 2-bit binary counters, frequency division, BCD and decade counters, seven-segment displays with a decoder/driver, the 4017 sequencer, and resetting for a custom count length.

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 sequential systems content of WJEC Eduqas GCSE Electronics is the memory-based digital part of Component 2 (Application of Electronics). It covers the D-type flip-flop, binary counters, decade counters with displays, and the 4017 sequencer. This page maps the content and links to a focused answer page for each part.

What the topic covers

D-type flip-flops: The rising-edge-triggered D-type that copies D to Q on the clock edge, its use as a latch and for data transfer, and reading timing diagrams. See D-type flip-flops.
Binary counters: Building 1-bit and 2-bit counters from toggling D-types, frequency division by two per stage, and reading the count from a timing diagram. See Binary counters.
Decade counters and displays: BCD and decade counters, the seven-segment display with its decoder/driver, the 4017 sequencer, and resetting for a custom count length. See Decade counters and displays.

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 D-type timing diagrams, building and reading binary counters, frequency division, decade counter and display chains, 4017 sequencing, and resetting for a custom count length.

How to study it

D-type rule. Copies D to Q on the rising clock edge; holds Q between edges.
Toggle and divide. $\overline{Q}$ to D makes it toggle, dividing the clock by two; chain stages for more bits.
Read counts in binary. A 2-bit counter goes 00, 01, 10, 11 (0 to 3) then repeats.
Display chain. Decade counter then decoder/driver then seven-segment display.
Reset early. Feed the output one beyond the last wanted step to reset for a custom count.

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)