How is a binary count turned into a readable decimal digit on a seven-segment display?
Seven-segment displays and decoders: the seven-segment display, the decoder that drives it from a binary count, and common-anode and common-cathode types with current-limiting resistors.
An Eduqas GCSE Electronics answer on seven-segment displays and decoders: how a seven-segment display forms digits, the decoder that converts a binary count into the segment pattern, common-anode and common-cathode types, and the current-limiting resistor each segment needs.
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What this dot point is asking
Eduqas wants you to describe the seven-segment display, the decoder that drives it from a binary count, and the common-anode and common-cathode types, including the current-limiting resistor each segment needs. This is the standard output stage that turns a digital count into a number a person can read.
The answer
The seven-segment display
The decoder
Common anode and common cathode
Current-limiting resistors
Examples in context
The seven-segment display is the standard numeric output of the course: digital clocks, counters, timers, thermometers and meters all present their value on one or more seven-segment digits driven by a decoder. It reuses the LED current-limiting-resistor calculation from the resistive-components module for each segment, and the decoder is a combinational circuit of the kind designed in the combinational-logic module. Driven by a counter, it completes the chain from clock to count to readable number that the non-exam assessment often requires.
Try this
Q1. State how many segments a seven-segment display has (excluding the decimal point). [1 mark]
- Cue. Seven (labelled to ).
Q2. State the purpose of a decoder between a counter and a seven-segment display. [2 marks]
- Cue. It converts the counter's binary code into the pattern of segments to light for the matching decimal digit.
Q3. State why each segment of the display needs a series resistor. [1 mark]
- Cue. Each segment is an LED, so it needs a current-limiting resistor to set a safe current.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20204 marksExplain the purpose of a decoder between a counter and a seven-segment display, and state how many segments such a display has.Show worked answer →
Purpose (up to 3 marks): a counter outputs a number in binary (or binary-coded decimal), which a seven-segment display cannot use directly. A decoder converts that binary code into the pattern of segments that must be lit to show the matching decimal digit; for each input code it drives the correct combination of segment outputs.
Segments (1 mark): a seven-segment display has seven segments (labelled to ), often with an eighth for the decimal point.
Markers reward the binary-to-segment-pattern conversion role of the decoder and the seven segments.
Eduqas 20224 marksExplain the difference between a common-anode and a common-cathode seven-segment display, and state why each segment needs a series resistor.Show worked answer →
Common anode versus common cathode (up to 2 marks): in a common-anode display all the segment LEDs share their anodes (connected to the positive supply), so a segment lights when its cathode is driven low. In a common-cathode display all the cathodes are joined (to ), so a segment lights when its anode is driven high. The decoder must be the matching type.
Series resistor (up to 2 marks): each segment is an LED, which needs a series current-limiting resistor to set a safe current, otherwise it would draw too much current and burn out.
Markers reward the shared-anode versus shared-cathode distinction (with the matching active-low or active-high drive) and the current-limiting resistor on each segment.
Related dot points
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An Eduqas GCSE Electronics answer on counters: how chained flip-flops count clock pulses in binary, how each stage divides the frequency by two, the modulus (number of states) of a counter, and using counters to divide frequency and count events.
- Driving LEDs: the need for a current-limiting series resistor, the forward voltage drop of an LED, and calculating the resistor value for a chosen current.
An Eduqas GCSE Electronics answer on driving LEDs: why an LED needs a series current-limiting resistor, the LED forward voltage drop, and calculating the resistor value from the supply voltage, forward voltage and forward current.
- Flip-flops and latches: storing one bit, the difference between sequential and combinational logic, the D-type flip-flop, and edge triggering by a clock.
An Eduqas GCSE Electronics answer on flip-flops and latches: how a flip-flop stores a single bit, the difference between sequential and combinational logic, the D-type flip-flop and how it captures its input on a clock edge, and using flip-flops as memory and to divide frequency.
- Designing combinational logic: building a circuit from a truth table or word description, combining gates, and the universal NAND and NOR gates.
An Eduqas GCSE Electronics answer on designing combinational logic: turning a word description or truth table into a Boolean expression and a gate circuit, combining gates into a system, and using the universal NAND and NOR gates to build any function from one gate type.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Electronics specification (C490) — WJEC Eduqas (2017)