How do input, process and output sub-systems combine to build a complete electronic system?
The systems approach to electronics: the input (sensing), process and output sub-systems, the use of system block diagrams, common input sensors, processing units and output devices, and why transducer drivers are needed between a processing sub-system and an output device.
A focused answer to WJEC Eduqas GCSE Electronics on the systems approach, covering the input, process and output sub-systems, system block diagrams, common sensors, processing units, output devices and why a transducer driver is needed.
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What this topic is asking
WJEC Eduqas wants you to think about electronics as a system built from sub-systems. You should be able to split any electronic product into an input (sensing) stage, a process stage and an output stage, draw a system block diagram to show how a signal flows through them, recognise common sensors, processing units and output devices, and explain why a transducer driver is often needed to connect the process stage to the output. This is the foundation for the whole of Component 1.
The systems approach
Thinking in sub-systems is the central idea of the course. You do not need to know what is inside every block; you need to know what goes in, what comes out, and what the block does to the signal. This is why engineers build complex products quickly: they connect tested sub-systems together.
System block diagrams
When you draw a block diagram, give each box a sensible name that matches the task (for example "light sensor", "comparator", "lamp"), not a vague label. The arrows are essential: they show that the input drives the process and the process drives the output. You should be able to draw the three-box chain input then process then output for any everyday electronic product.
Input (sensing) sub-systems
You should recognise sensors for these quantities:
- Light - a light-dependent resistor (LDR), whose resistance falls as light increases.
- Temperature - a thermistor (usually NTC: resistance falls as temperature rises).
- Sound - a microphone.
- Pressure or force - a pressure sensor or force-sensing resistor.
- Moisture - a moisture sensor (two probes whose resistance falls when wet).
- Magnetic field - a reed switch or Hall sensor.
- Position or movement - a variable resistor (potentiometer), switches or a rotation sensor.
Most resistive sensors are used inside a potential divider so that the change in resistance becomes a change in voltage that the process sub-system can read.
Process sub-systems
The process stage is where decisions are made. A comparator switches when the sensor voltage crosses a threshold; logic gates combine several inputs by a rule; a latch remembers that an event has happened; a timing circuit produces a pulse or a stream of pulses. Component 2 develops these blocks in detail.
Output sub-systems and transducer drivers
The output transducer is the opposite of an input transducer: it converts electricity into light, sound, heat or movement. The problem is current. A logic gate or comparator output can only deliver a few milliamps, far too little to turn a motor or a mains relay. The transducer driver solves this: the small process signal switches a transistor or MOSFET, which switches a much larger current from the supply through the output device. This is why almost every motor, solenoid or filament lamp is driven through a transistor rather than directly from a logic chip.
Try this
Q1. Name the three main types of sub-system in an electronic system, in the order the signal passes through them. [3 marks]
- Cue. Input (sensing), then process, then output.
Q2. State one suitable input transducer for sensing light and one suitable output transducer for producing sound. [2 marks]
- Cue. Light input: an LDR. Sound output: a loudspeaker or buzzer.
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.
Eduqas style3 marksA security light switches on a lamp when it gets dark. Draw a system block diagram for the light, naming the input, process and output sub-systems.Show worked answer →
A Component 1 systems question. Draw three boxes in a row with arrows pointing left to right: Input (sensor) then Process then Output (1 mark for the correct order and arrows showing the signal flow). Label the input as a light sensor (LDR in a potential divider), the process as a switching or comparator sub-system, and the output as a lamp (1 mark for sensible, named sub-systems that match the task). State that the input sub-system turns the change in light into a voltage, the process decides when to act, and the output sub-system produces the visible effect (1 mark for the role of each stage). Markers reward the correct left-to-right order, named sub-systems and the signal flow.
Eduqas style2 marksExplain why a transducer driver is needed between a logic processing sub-system and a motor.Show worked answer →
A Component 1 Explain question on driving outputs. The logic output can only supply a very small current at a low voltage, which is not enough to run a motor (1 mark). A transducer driver, such as a transistor or MOSFET switch, uses the small logic signal to switch a much larger current from the supply through the motor, so the small signal controls the large load (1 mark). Markers reward the idea that the logic output cannot supply enough current and that the driver provides current gain or switches the load.
Related dot points
- Electric charge and current as the rate of flow of charge, the charge equation, voltage (potential difference) as energy per unit charge, standard circuit symbols, and the test equipment used to measure electrical quantities (multimeter, oscilloscope, logic probe).
A focused answer to WJEC Eduqas GCSE Electronics on charge, current and voltage, covering charge and the current equation, voltage as energy per unit charge, standard circuit symbols, and the test equipment used to measure them.
- The potential divider: how two resistors share a supply voltage, the potential divider equation, and designing and analysing a divider to produce a required output voltage.
A focused answer to WJEC Eduqas GCSE Electronics on potential dividers, covering how two resistors share a supply voltage, the potential divider equation, and designing and analysing a divider for a required output voltage.
- Input sensors as variable resistors: the light-dependent resistor (LDR) and the NTC thermistor, how their resistance varies with light and temperature, and using them in a potential divider so the output voltage responds to the physical quantity.
A focused answer to WJEC Eduqas GCSE Electronics on input sensors, covering how an LDR and an NTC thermistor change resistance with light and temperature, and how a sensor in a potential divider produces a voltage that responds to the physical quantity.
- Logic levels (logic 1 and logic 0 as high and low), the NOT, AND, OR, NAND, NOR and XOR gates, their logic symbols, and constructing and using truth tables to describe combinational logic.
A focused answer to WJEC Eduqas GCSE Electronics on logic gates and truth tables, covering logic levels, the NOT, AND, OR, NAND, NOR and XOR gates and their symbols, and constructing truth tables for combinational logic.
- The npn bipolar transistor and the n-channel enhancement MOSFET used as switches: how a small input controls a larger output current, the meaning of saturation and cut-off, and the differences between the two devices.
A focused answer to WJEC Eduqas GCSE Electronics on transistor and MOSFET switching, covering how an npn transistor and an n-channel enhancement MOSFET act as switches, saturation and cut-off, and the differences between the two devices.
Sources & how we know this
- WJEC Eduqas GCSE Electronics specification (from 2017) — WJEC Eduqas (2017)