System synthesis: the systems approach, block diagrams, building a system from input, process and output sub-systems, and interfacing between blocks.

What this dot point is asking

System synthesis is the foundation of the whole WJEC Electronics course. The specification asks you to think of every product as a chain of sub-systems and to build, draw and analyse systems using block diagrams before you ever reach for a soldering iron. Because both written components are synoptic, examiners constantly drop a half-finished block diagram in front of you and ask which sub-system fills the gap, so fluency with the systems approach pays off on almost every paper.

The answer

The systems approach

Thinking in blocks is powerful because it lets you separate concerns. You can specify that a block must, say, "output 5 V when the light level falls below a set point" without yet deciding whether that block is a comparator, a logic gate or a microcontroller. The internal design becomes a later, smaller problem.

The three-block model

Arrows between the blocks show the direction of signal flow. A more elaborate product simply has more blocks, and processing blocks can themselves be broken down further. A burglar alarm, for example, has several input sensors feeding a logic processing stage that drives both a siren and an indicator output.

Interfacing between blocks

The point where one block's output meets the next block's input is an interface, and a system only works if the blocks are compatible there. A comparator output that can supply only a few milliamps cannot directly drive a motor that needs an amp, so an interfacing block (a transistor switch or a relay driver) sits between them. Voltage levels, current capability and signal type (analogue or digital) must all match across an interface.

Why systems thinking matters in the exam

Because questions are synoptic, you are rewarded for naming a sensible device in each block and for justifying the interface. A common task is to add a missing stage, redraw a chain with an extra processing block, or explain why a system fails because two blocks are incompatible at an interface.

Examples in context

Example 1. A digital thermometer

A thermistor divider (input) feeds an analogue-to-digital converter and microcontroller (process) that drives a seven-segment display (output). Each block has a clean interface: the divider hands the ADC a voltage in range, and the microcontroller hands the display the right logic levels. Designing it as three blocks means the display can be changed without touching the sensor.

Example 2. A public-address system

A microphone (input) feeds a mixer and amplifier (process) that drives a loudspeaker (output). The interface that matters here is power: the small signal from the microphone must be raised by the amplifier before it can move a speaker cone, so the processing block exists precisely to bridge that interface.

Example 3. A reversing sensor on a car

An ultrasonic transducer (input) feeds a timing and processing circuit (process) that drives a buzzer whose pulse rate rises as an obstacle nears (output). The systems view makes the design tractable: the engineer specifies each block's job and worries about the internal electronics only block by block.

Try this

Q1. Draw a three-block diagram for an automatic night-light that switches an LED on when it gets dark. Name a device for each block. [3 marks]

• Cue. Input: LDR potential divider. Process: comparator. Output: transistor switch driving an LED. Arrows left to right.

Q2. Explain why an interfacing block is often needed between a logic processing stage and a motor. [2 marks]

• Cue. Logic outputs supply only a small current at a fixed voltage; a motor needs far more current, so a transistor or relay driver (the interface) is required to avoid overloading the logic and to switch the larger load.