AQA AQA 2022-style practice: An LED needs a current of 20 mA and drops 2 V across itself. It is connected to a 9 V supply. Calculate the value of the series resistor needed to protect the LED.

A good answer finds the voltage across the resistor, then uses Ohm's law. The supply is 9 V and the LED drops 2 V, so the resistor must drop the rest: 9 - 2 = 7 V. The same current flows through the resistor as the LED, 20 mA = 0.02 A. Using Ohm's law, R = VI = 70.02 = 350. So a series resistor of about 350 (the nearest standard value, 390, would be safe) limits the current and protects the LED. Markers reward the resistor voltage of 7 V, the current in amperes, and R = 350.

EnglandEngineeringSyllabus dot point

What are the building blocks of an electronic system and what do they do?

Input, process and output components in electronic systems (sensors, resistors, transistors, logic, LEDs, motors, buzzers) and how they are combined.

A focused answer to AQA GCSE Engineering on electronic input, process and output components (switches, LDRs, thermistors, resistors, transistors, logic gates, LEDs, motors and buzzers) and how they form a working system.

Generated by Claude Opus 4.89 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
Input components
Process components
Output components
Combining components into a system
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What this dot point is asking

AQA wants you to sort electronic components into input, process and output, say what each does, and combine them into a simple working system that solves a problem. Calculation marks can appear too, especially sizing a series resistor for an LED.

Input components

A potential divider is two resistors in series across the supply; the voltage at the join depends on their ratio, with the larger share of the supply voltage appearing across the larger resistance. Replacing one resistor with an LDR or thermistor makes that voltage change as the light or temperature changes, which is how a sensor produces a useful changing signal. By choosing which way round the sensor and the fixed resistor sit, the designer decides whether the output voltage rises or falls as the conditions change, for example making the signal go high in the dark for a night light or high in the heat for a cooling fan.

Process components

Resistors: limit and set the current in a circuit, using $I = V/R$ . They protect components such as LEDs from too much current.
Transistors: act as electronic switches, turning on a larger current to an output when a small signal arrives at the base.
Logic gates: make decisions from one or more inputs. AND needs both inputs high to give a high output; OR needs at least one high; NOT inverts the input.

Output components

Combining components into a system

A frost alarm uses a thermistor (input) feeding a potential divider, a transistor (process) that switches at a set temperature, and a buzzer (output). Each stage maps onto input, process and output, which is why naming the model first makes the design clear.

Worked example: sizing the series resistor for an indicator LED

A panel LED needs $15 \text{ mA}$ and drops $2 \text{ V}$ . It runs from a $5 \text{ V}$ supply. Find the series resistor.

step Find the voltage across the resistor

The supply is $5 \text{ V}$ and the LED uses $2 \text{ V}$ , so the resistor takes the remaining $5 - 2 = 3 \text{ V}$ .

step Identify the current

In a series circuit the same current flows everywhere, so the resistor carries the LED current, $15 \text{ mA} = 0.015 \text{ A}$ .

step Apply Ohm's law

$R = \dfrac{V}{I} = \dfrac{3}{0.015} = 200 \text{ }\Omega$ .

step Choose a real value and check

$200 \text{ }\Omega$ is a standard value, so use it (or the next higher value to be safe, which slightly lowers the current). The resistor drops the excess voltage and limits the current, protecting the LED, which is exactly why a process-stage resistor is needed with every LED output.

Try this

Q1. Name an input component that senses temperature. [1 mark]

Cue. A thermistor.

Q2. State what a transistor does in a process stage. [1 mark]

Cue. It acts as an electronic switch, turning on a larger current from a small input signal.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksDescribe the input, process and output components you would use in an automatic night light that switches on in the dark.

Show worked answer →

A good answer assigns a sensible component to each part of the system.

The input is a light-dependent resistor (LDR), whose resistance rises as it gets darker; it is usually used in a potential divider so the voltage signal changes with light level.

The process stage compares this signal and switches: a transistor (or a logic gate or microcontroller) turns on when the dark signal is reached.

The output is an LED (or lamp) that lights up. Together the LDR senses the dark, the transistor switches, and the LED provides the light. Markers reward one correct component for each of input, process and output, with a brief reason.

AQA 20224 marksAn LED needs a current of

20 \text{ mA}

and drops

2 \text{ V}

across itself. It is connected to a

9 \text{ V}

supply. Calculate the value of the series resistor needed to protect the LED.

Show worked answer →

A good answer finds the voltage across the resistor, then uses Ohm's law.

The supply is $9 \text{ V}$ and the LED drops $2 \text{ V}$ , so the resistor must drop the rest: $9 - 2 = 7 \text{ V}$ .

The same current flows through the resistor as the LED, $20 \text{ mA} = 0.02 \text{ A}$ . Using Ohm's law, $R = \dfrac{V}{I} = \dfrac{7}{0.02} = 350 \text{ }\Omega$ .

So a series resistor of about $350 \text{ }\Omega$ (the nearest standard value, $390 \text{ }\Omega$ , would be safe) limits the current and protects the LED. Markers reward the resistor voltage of $7 \text{ V}$ , the current in amperes, and $R = 350 \text{ }\Omega$ .

Related dot points

Sources & how we know this

AQA GCSE Engineering (8852) specification — AQA (2017)