What is the systems approach, and how do input, process and output describe a control system?
The systems approach: representing electronic and control systems as input, process and output blocks, with feedback, using block (systems) diagrams.
A CCEA GCSE Technology and Design answer on the systems approach: describing an electronic or control system as input, process and output blocks, the idea of feedback, and using block (systems) diagrams to design and analyse a system.
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What this dot point is asking
CCEA wants you to understand the systems approach: describing any electronic or control system as input, process and output blocks, the idea of feedback, and using block (systems) diagrams to design and analyse a system. This way of thinking underpins the whole electronics topic.
The answer
Input, process, output
This lets a complex circuit be understood as three simple blocks, regardless of the detail inside each. A security light is: input (detect movement) -> process (decide to switch on) -> output (lamp on).
Block (systems) diagrams
Block diagrams are quick to draw, easy to read and ideal for planning a system before choosing components. Each block can later be filled with a real subsystem.
Feedback
A thermostat-controlled heater is a closed-loop system: the temperature reached (output) is fed back so the heater switches off at the set point and on again when it cools. A simple timer that just runs for a set time, ignoring the result, is open-loop.
Why the systems approach helps
The systems approach makes design manageable: you decide what input, process and output you need, draw the block diagram, then design or select a subsystem for each block. It also makes fault-finding easier - you can test each block (input, process, output) in turn to find where a system fails.
Worked example: modelling a system with blocks
Examples in context
- Example 1. A burglar alarm
- Input (door switch or sensor) -> process (control circuit that latches on) -> output (siren). A block diagram makes the whole alarm easy to plan and explain.
- Example 2. A thermostat heater
- A closed-loop system: the temperature output is fed back so the heater switches off at the set point, keeping the room steady.
- Example 3. A washing machine
- Many subsystems described as input (buttons, sensors) -> process (controller running the program) -> output (motor, valves, heater), showing the systems approach scaled up.
Being able to draw a labelled block diagram and explain feedback lets you answer both the "draw a block diagram" and "explain feedback" questions.
Try this
Q1. Name the three stages of a system in order. [3 marks]
- Cue. Input, process, output.
Q2. Give one example of an input device and one example of an output device. [2 marks]
- Cue. Input: a sensor or switch. Output: a lamp, buzzer or motor.
Q3. What do the arrows on a block diagram show? [1 mark]
- Cue. The direction in which the signal flows between blocks.
Q4. What is feedback in a control system? [2 marks]
- Cue. Information about the output sent back to the input or process so the system adjusts itself.
Q5. Is a thermostat-controlled heater open-loop or closed-loop, and why? [2 marks]
- Cue. Closed-loop, because the temperature (output) is fed back to switch the heater on and off automatically.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDraw a block diagram for an automatic security light and label the input, process and output.Show worked answer →
Award marks for three labelled blocks in order with arrows showing signal flow:
Input: a sensor (e.g. a PIR movement sensor or an LDR detecting darkness) (1).
Process: a circuit that decides whether to switch on (e.g. logic or a controller) (1).
Output: the lamp that turns on (1).
The blocks are connected left to right with arrows: input then process then output (1).
CCEA style3 marksExplain what is meant by feedback in a control system, using an example.Show worked answer →
Feedback is when information about the output is sent back to the input or process so the system can adjust itself (1).
Example: a thermostat measures the room temperature (output condition) and feeds it back, so the heater switches off when the set temperature is reached and on again when it falls (1). This keeps the output at the desired level automatically (1).
Related dot points
- Electronic components and quantities: conductors and insulators, resistors, capacitors, diodes and LEDs, and using Ohm's law V = I R.
A CCEA GCSE Technology and Design answer on basic electronics: conductors and insulators, resistors, capacitors, diodes and LEDs, current, voltage and resistance, and using Ohm's law V = I R including a current-limiting resistor calculation.
- Input subsystems: switches, the light-dependent resistor, the thermistor, and the voltage divider that turns a sensor's resistance change into a voltage signal.
A CCEA GCSE Technology and Design answer on input subsystems: switches, the light-dependent resistor (LDR) and the thermistor, and how a voltage divider (potential divider) turns a change in a sensor's resistance into a changing voltage signal for the process stage.
- Output devices: lamps, buzzers, motors and relays, and the transistor used as an electronic switch to control a larger current from a small input.
A CCEA GCSE Technology and Design answer on output devices: lamps, buzzers, motors and relays, and how a transistor acts as an electronic switch, turning a small input current into the switching of a much larger output current, with a protection diode for inductive loads.
- Logic gates and digital control: the AND, OR, NOT, NAND and NOR gates with truth tables, combining gates, and flowcharts for program control.
A CCEA GCSE Technology and Design answer on digital control: the AND, OR, NOT, NAND and NOR logic gates with their truth tables, combining gates to make a decision, and using flowcharts (sequence and decision) for program control.
- Microcontrollers (PICs) and timing: programmable control with a microcontroller, and resistor-capacitor timing where the capacitor charges to create a delay.
A CCEA GCSE Technology and Design answer on microcontrollers (PICs) and timing circuits: how a programmable microcontroller controls inputs and outputs, the advantages of programmable control, and how a resistor-capacitor circuit charges to create a time delay.
Sources & how we know this
- CCEA GCSE Technology and Design specification — CCEA (2017)