How does a transistor act as a switch?

As a switch a transistor is either fully on (saturated) or fully off (cut-off). An npn bipolar transistor is current-controlled: a small base current through a base resistor switches a large collector current. An n-channel enhancement MOSFET is voltage-controlled: a gate voltage above the threshold switches the drain current, and it draws almost no gate current.

What is the difference between a bipolar transistor and a MOSFET?

A bipolar transistor (base, collector, emitter) is current-controlled and needs a continuous base current through a base resistor. A MOSFET (gate, drain, source) is voltage-controlled, draws almost no gate current and has a very high input resistance, so it is easy to drive directly from a logic output.

How does a voltage comparator work?

A comparator compares two input voltages and drives its output high or low depending on which is larger. One input is usually a sensor voltage from a potential divider and the other a reference voltage from a second divider; the output switches sharply at the threshold where the sensor voltage equals the reference.

Why is an interface circuit needed?

A comparator or logic output can only supply a small current, so it cannot drive a lamp, motor or relay directly. A transistor or MOSFET driver, or a relay, switches the large load current. A relay also gives electrical isolation and can switch a high-voltage or AC load, with a protection diode across its coil.

How are switching circuits examined?

It sits in Component 1 (Discovering Electronics), a 1 hour 30 minute written paper worth 40% of the GCSE. Expect explanations of transistor and MOSFET switching, comparing the two devices, comparator threshold behaviour, setting a reference with a divider, and choosing an interface (transistor or relay) using a data sheet.

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WJEC GCSE Electronics: switching circuits overview

An overview of the switching circuits content in Component 1 of WJEC Eduqas GCSE Electronics, covering the npn transistor and n-channel MOSFET as switches, the voltage comparator as an IC switching circuit, setting a threshold with a potential divider, and interfacing an output with a transistor or relay.

Generated by Claude Opus 4.86 min readWJEC Eduqas GCSE Electronics, Component 1Updated 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
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For the official specification

The switching circuits content of WJEC Eduqas GCSE Electronics is part of Component 1 (Discovering Electronics). It covers the devices that turn a load on and off under control of a small signal: the npn transistor, the n-channel MOSFET and the voltage comparator, plus how to interface the output to a real load. This page maps the content and links to a focused answer page for each part.

What the topic covers

Transistor and MOSFET switching. The npn bipolar transistor (current-controlled, base resistor) and the n-channel enhancement MOSFET (voltage-controlled, high input resistance) as switches, saturation and cut-off, and the differences between them. See Transistor and MOSFET switching.

Comparators and interfacing. The voltage comparator as an IC switching circuit, setting the reference and threshold with a potential divider, and interfacing the output with a transistor driver or relay (with isolation and a protection diode). See Comparators and interfacing.

How this content is examined

This content sits in Component 1 (Discovering Electronics), a written paper of 1 hour 30 minutes worth 40% of the GCSE. Expect explanations of how transistors and MOSFETs switch, comparisons between them, comparator threshold reasoning, setting a reference voltage with a divider, and choosing an interface circuit using a data sheet.

How to study it

Two states only. A switching transistor is fully on (saturated) or fully off (cut-off).
Current versus voltage control. Bipolar transistor needs base current (and a base resistor); MOSFET needs only a gate voltage.
Learn the terminal names. Base/collector/emitter for bipolar; gate/drain/source for MOSFET.
Comparator switches at a threshold. Output high when the non-inverting input exceeds the inverting input; the reference divider sets the threshold.
Interface every real load. Use a transistor, MOSFET or relay; remember relay isolation and the coil protection diode.

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)