A comparator has 2.0\ V on the inverting input and 2.6\ V on the non-inverting input. State the output. [1 mark]

WJEC Eduqas Eduqas 2019-style practice: A comparator has a reference voltage of 2.5\ V on its inverting input. A light sensor produces 1.8\ V in the dark, rising to 3.6\ V in bright light, connected to the non-inverting input. State the comparator output (high or low) in the dark and in bright light, and explain your answer.

In the dark (up to 2 marks): the non-inverting input (1.8\ V) is below the inverting reference (2.5\ V), so the output is **low**. A comparator drives its output low when the non-inverting input is below the inverting input. In bright light (up to 2 marks): the non-inverting input (3.6\ V) is above the reference (2.5\ V), so the output switches **high**. Markers reward comparing each sensor voltage with the 2.5\ V reference and the correct high/low output, with the rule that the output is high when the non-inverting input exceeds the inverting input.

EnglandElectronicsSyllabus dot point

How does a comparator switch its output when a sensor voltage crosses a reference level?

Comparators: comparing two voltages, the reference set by a potential divider, the digital output, and using a comparator to make a sensing system switch at a threshold.

An Eduqas GCSE Electronics answer on comparators: how a comparator compares two input voltages and switches its output high or low, setting the reference with a potential divider, the digital nature of the output, and combining a sensor divider with a comparator to switch a circuit at a chosen threshold.

Generated by Claude Opus 4.813 min answerUpdated 2026-06-02

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

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What this dot point is asking

Eduqas wants you to use a comparator: it compares two voltages and gives a digital (high or low) output depending on which is larger, with one input set as a reference by a potential divider. You must combine a sensor divider with a comparator to make a system switch at a chosen threshold. The comparator is the processing block that turns a slowly changing sensor voltage into a clean on/off decision.

The answer

Comparing two voltages

The reference voltage

The digital output

Building a threshold-switching system

Setting the threshold for a dark-detecting light

An LDR-and-resistor divider gives $1.5\ \text{V}$ in bright light, rising to $4.0\ \text{V}$ in darkness, fed to the non-inverting input. We want a lamp to switch on when the output voltage reaches $3.0\ \text{V}$ (getting dark). Set the reference and check the action.

step 1: Set the reference voltage

Put $3.0\ \text{V}$ on the inverting input using a reference potential divider (for a $6.0\ \text{V}$ supply, equal resistors give $3.0\ \text{V}$ ).

step 2: Work out the output in the light

In bright light the sensor gives $1.5\ \text{V}$ , which is below the $3.0\ \text{V}$ reference, so $V_+ < V_-$ and the output is low: the lamp is off.

step 3: Work out the output in the dark

As it darkens the sensor voltage rises; once it passes $3.0\ \text{V}$ , $V_+ > V_-$ and the output switches high, turning on the transistor switch and the lamp. A variable reference resistor lets you tune how dark it must be.

Examples in context

The comparator is the standard processing block of GCSE sensing systems: a light-operated switch, a frost alarm, a temperature-controlled fan and a liquid-level alarm all use a sensor divider, a comparator with an adjustable reference, and a transistor output stage. The same component appears as an op-amp used open-loop, linking this topic to operational amplifiers, and the input-process-output structure is the backbone of the non-exam assessment system design.

Try this

Q1. State the condition for a comparator output to be high. [1 mark]

Cue. The non-inverting ( $+$ ) input voltage is above the inverting ( $-$ ) input voltage.

Q2. A comparator has $2.0\ \text{V}$ on the inverting input and $2.6\ \text{V}$ on the non-inverting input. State the output. [1 mark]

Cue. High ( $V_+ > V_-$ ).

Q3. State how the switching threshold of a sensing comparator is usually set and made adjustable. [2 marks]

Cue. By a potential divider giving the reference voltage, with a variable resistor (potentiometer) to adjust it.

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 20194 marksA comparator has a reference voltage of

2.5\ \text{V}

on its inverting input. A light sensor produces

1.8\ \text{V}

in the dark, rising to

3.6\ \text{V}

in bright light, connected to the non-inverting input. State the comparator output (high or low) in the dark and in bright light, and explain your answer.

Show worked answer →

In the dark (up to 2 marks): the non-inverting input ( $1.8\ \text{V}$ ) is below the inverting reference ( $2.5\ \text{V}$ ), so the output is low. A comparator drives its output low when the non-inverting input is below the inverting input.

In bright light (up to 2 marks): the non-inverting input ( $3.6\ \text{V}$ ) is above the reference ( $2.5\ \text{V}$ ), so the output switches high.

Markers reward comparing each sensor voltage with the $2.5\ \text{V}$ reference and the correct high/low output, with the rule that the output is high when the non-inverting input exceeds the inverting input.

Eduqas 20225 marksDescribe how to build a circuit that switches on a heater when the temperature falls below a set value, naming the subsystems and explaining how the threshold is set.

Show worked answer →

Subsystems (up to 3 marks): a sensing subsystem (an NTC thermistor and a fixed resistor in a potential divider) produces a voltage that changes with temperature; a comparator compares this with a reference voltage set by a second potential divider; the comparator output drives a processing/output stage (a transistor switch) that turns on the heater.

Threshold and behaviour (up to 2 marks): the reference divider sets the temperature at which switching occurs. Arrange the thermistor so the sensor voltage rises as the temperature falls; when it crosses above the reference the comparator output goes high and switches on the heater. A variable resistor in the reference (or sensor) divider lets the threshold be adjusted.

Markers reward the input-process-output structure, the comparator comparing the sensor voltage with a divider-set reference, and a correct explanation of how crossing the threshold switches the heater.

Related dot points

Sources & how we know this

WJEC Eduqas GCSE (9-1) Electronics specification (C490) — WJEC Eduqas (2017)