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How is an electronic system structured, and what do input and process subsystems do?

Electronic systems as input, process and output, input sensors (switches, LDR, thermistor) and the potential divider, process subsystems (transistors, comparators, logic gates, timers) and how a system senses a condition and decides a response.

A focused answer to Eduqas A-Level Product Design on electronic input and process systems: the input, process, output model, input sensors such as switches, LDRs and thermistors, the potential divider, and process subsystems including transistors, comparators, logic gates and timers, with a worked potential divider calculation.

Generated by Claude Opus 4.812 min answerUpdated 2026-06-02

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

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Quick answer

An electronic system is modelled as input, process and output. The input subsystem senses a condition with sensors: switches (push, toggle, reed, microswitch), a light-dependent resistor (LDR) (resistance falls as light rises), a thermistor (resistance changes with temperature), and others. A sensor is usually wired in a potential divider so a changing resistance produces a changing voltage: $V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$ . The process subsystem decides a response: a transistor switches on above a threshold, a comparator (op-amp) compares the sensor voltage to a reference, logic gates (AND, OR, NOT) combine conditions, and timers (the 555) create delays. The process output then drives the output stage. The whole system senses, decides and acts.

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What this dot point is asking
The input, process, output model
Input sensors and switches
The potential divider
Process subsystems: deciding a response

What this dot point is asking

Eduqas wants you to model an electronic system as input, process and output, know the common input sensors and the potential divider, and the process subsystems (transistors, comparators, logic gates, timers) that decide a response. This is how products sense their surroundings and react, combining technical knowledge with the applied maths of the potential divider, so it is examined as the system model, a calculation and applied design reasoning.

The input, process, output model

Input sensors and switches

The potential divider

Key fact

A potential divider is two resistors in series across a supply, which shares the supply voltage in proportion to the resistances. The voltage across the lower resistor is $V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$ . By making one resistor a sensor (an LDR or thermistor), the output voltage changes as the sensed quantity changes, turning a changing resistance into a changing voltage the process stage can use. For example, with an LDR and a fixed resistor, the output voltage rises or falls as it gets light or dark, depending on which resistor the output is taken across. A variable resistor in the divider lets the user set the threshold (the sensitivity), which is why night lights and thermostats have an adjustment. The potential divider is the most important and most-calculated circuit in this topic.

Process subsystems: deciding a response

The process subsystem decides the response from the input signal. A transistor acts as an electronic switch: when its base voltage rises above about $0.6$ V it turns on, switching a larger current to the output (it can also amplify). A comparator (an op-amp) compares the sensor voltage to a reference voltage and switches its output sharply when one crosses the other, giving a clean threshold (better than a single transistor for a precise trigger). Logic gates (AND, OR, NOT, NAND, NOR) combine digital inputs by rules, so an output depends on a combination of conditions (an alarm that triggers if a door AND a window are open). Timers, such as the 555, create delays and pulses (a light that stays on for a set time). Choosing the right process component depends on whether the decision is a threshold, a combination of conditions or a timed delay.

Designing the input and process of a frost alarm

Choose the input sensor

A frost alarm must sense low temperature, so use a thermistor (an NTC type whose resistance rises as it gets colder) as the input sensor.

Put the sensor in a potential divider

Wire the thermistor with a fixed (or variable) resistor in a potential divider across the supply, so the output voltage changes with temperature. Using $V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$ , the output voltage crosses a threshold as it gets cold.

Choose the process component

Feed the divider output into a comparator, with a reference voltage set by a variable resistor to choose the alarm temperature. When the temperature drops to the set point, the divider voltage crosses the reference and the comparator switches its output cleanly.

State the system logic

Conclude that the system senses temperature (thermistor and potential divider), decides at a set threshold (comparator with adjustable reference), and would then act through an output stage (buzzer). State the potential divider relationship and the threshold logic, because those are the marked ideas.

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 marksA potential divider uses a fixed 10 kilohm resistor and an LDR in series across a 9 V supply. In darkness the LDR resistance is 90 kilohms. Calculate the voltage across the fixed resistor in darkness, showing your working.

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A Component 1 potential divider calculation. Marks for the formula, the substitution and the answer.

The potential divider shares the supply voltage in proportion to resistance. The voltage across the fixed resistor is $V_{out} = V_{in} \times \frac{R_{fixed}}{R_{fixed} + R_{LDR}} = 9 \times \frac{10}{10 + 90} = 9 \times \frac{10}{100} = 0.9$ volts.

Award marks for the correct formula, substitution and answer (0.9 V). In darkness the LDR resistance is high, so most of the voltage is dropped across the LDR and little across the fixed resistor. A common dropped mark is mixing up which resistor is in the numerator: the output is taken across the resistor named in the formula.

Eduqas 20216 marksDiscuss how an automatic night light could be built as an input, process and output system. Name a suitable input sensor and process component, and explain how the system decides to switch the light on.

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A Component 1 extended question marked by levels of response. Reward the system model, named components and the decision logic.

Input: a light-dependent resistor (LDR) in a potential divider senses the light level, producing a voltage that changes as it gets dark (the LDR resistance rises, changing the divider output). Process: a transistor or a comparator (op-amp) compares this voltage to a reference set by a variable resistor; when the light falls below the set level, the process stage switches. Output: the switched signal turns on a transistor or relay that powers an LED or lamp.

A top answer structures the answer as input (LDR potential divider), process (transistor or comparator threshold) and output (lamp driver), explains that the system senses darkness and decides to switch when the voltage crosses the threshold, and concludes with the benefit of the variable resistor for setting sensitivity.

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Sources & how we know this

Eduqas A Level Design and Technology specification (Product Design) — Eduqas (2017)