How do we represent an electronic system as input, process and output?
Universal systems diagrams: representing an electronic or control system as input, process and output sub-systems using block diagrams.
An SQA National 5 Engineering Science answer on the universal systems approach, covering input, process and output sub-systems, drawing and interpreting block diagrams, the meaning of feedback, and identifying real components within each block.
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What this key area is asking
The SQA wants you to model any electronic or control system using the universal input, process, output approach and to draw and interpret block diagrams that show how signals flow through a system.
The universal systems approach
Thinking in sub-systems lets engineers design and understand a complex system without worrying about every detail at once. You can describe what each block does (its function) before deciding exactly which components go inside it.
The three sub-systems
- Input sub-system
- A sensor (transducer) that detects a physical condition - light, temperature, movement, a switch being pressed - and converts it into an electrical signal. Examples: a switch, a light-dependent resistor (LDR), a thermistor.
- Process sub-system
- The decision-making or controlling part. It takes the input signal, processes it, and controls the output. Examples: a logic gate or combination of gates, an operational amplifier acting as a comparator, or a microcontroller running a program.
- Output sub-system
- The device that performs the useful action. It converts the electrical signal back into a physical effect - light, sound, movement. Examples: a lamp, an LED, a buzzer, a motor.
Block diagrams
A block diagram represents each sub-system as a labelled box and joins them with arrows that show the direction of signal flow. Block diagrams are read left to right, with the input on the left and the output on the right. They let you communicate a design quickly without drawing every component.
Feedback
Some systems use feedback: information about the output is fed back to the input or process so the system can adjust itself and stay under control. A thermostat is the classic example - it senses the room temperature (which the heater output affects) and switches the heater off once the target is reached, then back on when it cools. Feedback keeps a system at a desired state automatically.
Try this
Q1. State the order of the three sub-systems in a universal systems block diagram. [1 mark]
- Cue. Input, then process, then output.
Q2. Give one example of an output device. [1 mark]
- Cue. A lamp, LED, buzzer or motor.
Q3. Explain what feedback does in a control system. [2 marks]
- Cue. It sends information about the output back to the input or process so the system can adjust itself and stay at a desired state, as in a thermostat.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksAn automatic security light switches on when it gets dark and someone moves nearby. Draw a universal systems block diagram for the light, labelling the input, process and output sub-systems.Show worked answer →
Markers want three labelled blocks connected by arrows in the correct order.
Input: the sensors - a light sensor (LDR) to detect darkness and a movement sensor - feeding signals into the system.
Process: the decision-making sub-system (logic or a microcontroller) that switches the output on only when it is dark and movement is detected.
Output: the lamp that turns on.
The blocks must be joined left to right by arrows showing the signal flow: input arrow into process, process arrow into output. Markers reward three correctly named and ordered blocks with arrows.
SQA N5 style2 marksState what is meant by the process sub-system in a universal systems diagram, and give one example of a component that performs it.Show worked answer →
Markers want a definition plus a valid example.
The process sub-system takes the signals from the input, makes a decision or carries out an operation on them, and controls the output. It is the decision-making or controlling part of the system.
An example component is a logic gate, an operational amplifier acting as a comparator, or a microcontroller.
Markers reward describing the process as the part that acts on the input to control the output, plus one valid processing component.
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