What does the Eduqas switching and diodes module cover?

It covers diodes (forward and reverse bias, the forward voltage drop, the LED, half-wave rectification, and protection diodes including the flyback diode), the bipolar transistor as a switch (cut-off and saturation, current gain, and the base-resistor calculation), the MOSFET as a voltage-controlled switch (gate threshold, negligible gate current, and when to prefer it over a bipolar transistor), comparators (comparing a sensor voltage with a reference set by a divider, and the digital output), and latching with positive feedback (holding an output on, the reset, and the snap or Schmitt action that avoids chatter).

Which equations recur most across this module?

The current-gain relationship $I_C = h_{FE} I_B$, the base resistor $R_B = \frac{V_\text{drive} - 0.7}{I_B}$, and Ohm's law applied to a diode's series resistor after subtracting the forward voltage. The comparator and latch topics are explained qualitatively, but the threshold reference is found with the potential-divider equation from the previous module.

What is the most common mistake in this module?

Forgetting voltage drops and getting orientation or input order wrong. Common slips are dropping the whole drive voltage across the base resistor instead of $V_\text{drive} - 0.7$, forgetting a diode's forward drop before applying Ohm's law, calling a MOSFET current-controlled when it is voltage-controlled, mixing up the comparator's non-inverting and inverting inputs, and omitting the flyback diode across an inductive load.

How do these components fit into a sensing system?

They form the process and output blocks. A sensor in a potential divider produces a changing voltage; a comparator compares it with an adjustable reference to make a clean on-or-off decision; a transistor or MOSFET switch turns that decision into a real action by driving a lamp, buzzer, relay or motor; a latch can hold the output on until reset; and protection diodes guard the switch from reverse polarity and back-EMF. This input-process-output structure is exactly what the non-exam assessment rewards.

How should I revise switching and diodes?

Make the current-gain and base-resistor calculations automatic, always subtracting the $0.7\ \text{V}$ base-emitter drop. Learn the diode rules (forward conducts at about $0.7\ \text{V}$, reverse blocks) and the four diode uses (LED, rectification, reverse-polarity protection, flyback). Be able to choose between a bipolar transistor and a MOSFET from the drive available and the load current. Practise predicting a comparator output from the two input voltages, and explain latching and the snap action in words. Then build full sensing systems end to end.

EnglandElectronics

Eduqas GCSE Electronics: switching and diodes (diodes, transistor and MOSFET switches, comparators, latches)

A deep-dive Eduqas GCSE Electronics guide to the switching and diodes module within Component 1. Covers diodes (forward and reverse bias, rectification and protection), the bipolar transistor as a switch with current gain and the base resistor, the MOSFET as a voltage-controlled switch, comparators that switch at a reference, and latching with positive feedback and the snap (Schmitt) action.

Generated by Claude Opus 4.813 min readC490 Component 1Updated 2026-06-02

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

Jump to a section

What this module actually demands
Diodes and the transistor switch
MOSFETs, comparators and latches
How this module is examined
Check your knowledge

What this module actually demands

Switching and diodes is where a sensing system gains the power to act. The previous module produced a voltage that changes with light or temperature; this module turns that voltage into a clean decision and then into a real output. It covers the diode and its four uses, the bipolar transistor and MOSFET as switches, the comparator that decides when to switch, and the feedback that lets a circuit latch on or snap cleanly. The examiners reward the base-resistor calculation in base units, correct diode orientation and voltage drops, a sound choice between bipolar and MOSFET, and clear input-process-output reasoning.

This guide walks through the topics in order and sets out the exam patterns Eduqas repeats. Each topic has a matching dot-point page with practice; this overview ties them together.

Diodes and the transistor switch

Diodes and their applications establish the one-way rule (forward conducts above about $0.7\ \text{V}$ , reverse blocks), the LED with its current-limiting resistor, half-wave rectification, and protection diodes (reverse-polarity and the flyback diode for inductive loads). Transistor switching circuits use the bipolar transistor between cut-off and saturation, with $I_C = h_{FE} I_B$ and the base resistor $R_B = \frac{V_\text{drive} - 0.7}{I_B}$ sized so a small input reliably turns on a load.

MOSFETs, comparators and latches

MOSFET switching and driving loads introduces the voltage-controlled switch: a gate voltage above threshold turns it on, it draws negligible steady gate current, and it suits switching large currents from a weak source. Using comparators compares a sensor voltage with a divider-set reference and gives a digital output that drives the switch, with a variable resistor setting the threshold. Latching and feedback switches use positive feedback to hold an output on (needing a reset) and to give a snap (Schmitt) action with two thresholds that avoids chatter.

How this module is examined

A typical Eduqas profile for this content:

Calculations. The minimum base current from $I_C = h_{FE} I_B$ , the base resistor from $\frac{V_\text{drive} - 0.7}{I_B}$ , and the diode series resistor after subtracting the forward drop.
Component questions. Forward and reverse bias, the diode's four uses, the MOSFET gate threshold and negligible gate current, and choosing between a bipolar transistor and a MOSFET.
System questions. Predicting a comparator output from the two input voltages, setting and adjusting the threshold, and describing a complete sensing-switching system.
Explanation. Why a flyback diode is needed, why positive feedback gives a snap action and avoids chatter, and why a latch needs a reset.

Eduqas switching and diodes covers the diode (forward conducts at about $0.7\ \text{V}$ , reverse blocks; LED, rectification, protection and flyback diodes), the bipolar transistor switch (cut-off and saturation, $I_C = h_{FE} I_B$ , base resistor $R_B = \frac{V_\text{drive} - 0.7}{I_B}$ ), the MOSFET voltage-controlled switch (gate above threshold, negligible gate current), the comparator (compares a sensor voltage with a divider-set reference, digital output), and latching with positive feedback (holds the output on until reset, and the snap or Schmitt action with hysteresis that avoids chatter). Subtract the $0.7\ \text{V}$ base-emitter drop, keep diode orientation right, and think in input-process-output blocks.

Check your knowledge

A mix of recall and calculation questions covering the module. Attempt them under timed conditions, then check against the solutions.

State the approximate forward voltage of a conducting silicon diode and what a diode does in reverse bias. (2 marks)
A transistor ( $h_{FE} = 100$ ) must switch a $60\ \text{mA}$ load. Find the minimum base current. (2 marks)
The transistor above is driven from $5.0\ \text{V}$ at a base current of $1.0\ \text{mA}$ . Find the base resistor (base-emitter voltage $0.7\ \text{V}$ ). (2 marks)
State what kind of quantity controls a MOSFET and the condition for it to turn on. (2 marks)
A comparator has $2.5\ \text{V}$ on its inverting input and $3.1\ \text{V}$ on its non-inverting input. State the output. (1 mark)
State why a latched alarm needs a reset switch. (1 mark)

Sources & how we know this

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