The systems approach: input, process and output subsystems, block diagrams, signal flow and feedback.

The systems approach designs an electronic product as connected subsystems (blocks) rather than starting from individual components. Each block performs a function and is defined by what it does (its inputs and outputs), so the designer can develop, test and replace blocks independently. The three core blocks are input (senses a quantity and turns it into an electrical signal), process (makes a decision or modifies the signal) and output (turns the signal into a useful effect).

For an automatic security light:

• Input: a light sensor (LDR in a potential divider) detecting darkness, plus possibly a PIR movement sensor.
• Process: a comparator or logic/timer that decides "dark AND movement" and holds the lamp on for a set time.
• Output: a transistor/relay driving the lamp.

A correct block diagram is: Input (LDR / PIR) -> Process (comparator / timer) -> Output (transistor + lamp), drawn left to right with arrows showing signal flow.

Markers reward the subsystem idea (blocks defined by function, developed and tested separately) and a correctly labelled, correctly ordered block diagram for the example.

Two advantages:

1. Manages complexity / clarity: a block diagram shows the overall function and signal flow simply, without the clutter of every component, so the designer (and client) can understand and plan the whole system at a glance.
2. Modular design and testing: each block can be designed, built and tested independently, and a faulty or unsatisfactory block can be modified or swapped without redesigning the whole circuit, which supports the iterative process.

Other acceptable points: it makes faults easier to locate (test block by block); and it lets standard sub-circuits be reused. Markers want two distinct, correctly explained advantages.