How is an electronic product built from input, process and output sub-systems, and where do programmable components fit?
Electronic systems as input-process-output sub-systems, common sensors and components, and programmable components (microcontrollers) that make products smarter, more functional and reprogrammable.
A focused answer to WJEC A-Level Design and Technology Unit 3 electronic systems and programmable components, covering the input-process-output systems model, common input sensors and output devices, and how programmable microcontrollers make products more functional, flexible and reprogrammable.
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What this dot point is asking
WJEC wants you to use the systems approach (input - process - output) to describe an electronic product, name common sensors and output devices, and explain why programmable components are used. The exam reliably asks you to map a product onto the systems model and to justify a microcontroller over fixed electronics. You need the three-block model, examples of each block, and the advantages of programmability.
The answer
The systems approach
This block thinking is how designers reason about electronic products; you should be able to draw and label a systems block diagram for a given product.
Input sub-system: sensors
Process sub-system
The process block decides what to do with the inputs. Traditionally this was fixed logic (logic gates, timers, counters wired together). Increasingly it is a programmable microcontroller, a single chip whose behaviour is set by a stored program, which can read inputs, make decisions, count and time, and drive outputs.
Output sub-system
Programmable components
A programmable component, usually a microcontroller, stores a program that sets the product's behaviour. The big advantages are flexibility (reprogram rather than rewire), fewer parts (one chip replaces many), smaller and cheaper products, more complex functions, and easy updates and testing. This is why so many modern products are built around a microcontroller.
Examples in context
Example 1. A washing machine. Inputs (door switch, water level and temperature sensors) feed a microcontroller that runs the chosen wash program, switching outputs (motor, heater, valves, pump) in sequence through relays. One programmable chip coordinates the whole cycle and offers many programs from the same hardware.
Example 2. A reversing sensor on a car. An ultrasonic input measures the distance to an obstacle, a microcontroller decides when it is too close, and a buzzer output beeps faster as the gap closes, a clear input-process-output system where the microcontroller sets the warning behaviour in software.
Try this
Q1. State which sub-system (input, process or output) each of these belongs to: LDR, buzzer, microcontroller. [3 marks]
- Cue. LDR - input; buzzer - output; microcontroller - process.
Q2. Give two reasons a designer might choose a programmable microcontroller instead of fixed wired logic. [2 marks]
- Cue. It can be reprogrammed in software rather than rewired, and it replaces many separate components, making the product smaller, cheaper and able to perform more complex functions.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20195 marksUsing the systems approach, describe the input, process and output for an automatic security light that switches on in the dark when movement is detected.Show worked answer →
The systems approach breaks the product into three blocks, and a good answer maps each clearly.
Input: the sensors that detect conditions. Here a light-dependent resistor (LDR) detects darkness and a passive infrared (PIR) sensor detects movement. These provide the signals the system acts on.
Process: the part that makes the decision. A programmable microcontroller (or logic) reads the LDR and PIR and decides to switch the light on only when it is both dark and movement is present, and can include a timer to keep the light on for a set period.
Output: the device that produces the result. A relay or transistor switches the lamp (the output) on, and the lamp provides the light.
Markers reward correctly placing the LDR and PIR as inputs, the decision-making (and timer) as the process, and the lamp via a switching device as the output, ideally noting the AND condition (dark AND movement).
WJEC 20216 marksExplain the advantages of using a programmable microcontroller in a product compared with fixed wired electronics.Show worked answer →
A strong answer gives several clear advantages with reasons.
Flexibility and reprogramming: the behaviour is set in software, so the same hardware can be changed or updated by reprogramming rather than rewiring, and one board can serve many products or be improved later.
Fewer components and smaller size: a microcontroller replaces many separate logic chips, timers and counters, so the circuit is simpler, smaller, cheaper to assemble and more reliable with fewer connections to fail.
More complex functions: it can handle timing, counting, decisions, sensing several inputs and driving several outputs, and respond intelligently in ways that would be very hard to wire with fixed components.
Easier development and testing: behaviour can be simulated and debugged in software before manufacture. Markers reward at least three genuine advantages (reprogrammability, fewer parts and smaller size, more complex functionality, easier updates) each with a reason.
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Sources & how we know this
- WJEC AS/A Level Design and Technology specification — WJEC (2017)