Representing Electronic Systems
Why We Use Different Representations
Electronic systems can be represented in three different ways: system block diagrams, circuit schematics, and PCB layouts. Each method has a different purpose and level of detail. A block diagram gives an overview of what a system does; a schematic shows exactly which components are used and how they connect; a PCB layout shows where components are physically placed on a board. The OCR specification requires you to interpret — and partially complete — all three types, for systems with up to four blocks.
System Block Diagrams and the Systems Approach
The systems approach breaks any electronic system into three fundamental stages: input, process, and output. In a block diagram, each stage is drawn as a labelled rectangle, and arrows show the direction of signal flow between blocks. A simple torch system might show: battery (input) → switch (process) → LED (output). Block diagrams are quick to draw and easy to understand, making them ideal for planning and communicating a design before any components are chosen.
A key distinction the specification requires is between open loop systems and closed loop systems. In an open loop system, information flows only in one direction from input to output; there is no feedback. A microwave oven set to run for two minutes regardless of food temperature is open loop — the output (heated food) does not influence the input. In a closed loop system, a feedback loop carries information from the output back to the input. A central heating thermostat is closed loop: the room temperature (output) is fed back and compared with the set temperature (input) to switch the boiler on or off.
Open loop: input → process → output (no feedback). Closed loop: input → process → output, plus a feedback arrow from output back to input. Signal arrows in block diagrams must always show the direction of signal flow. The OCR specification tests you on interpreting and completing block diagrams with up to four blocks.
Circuit Schematics
A circuit schematic (also called a circuit diagram) uses standardised symbols to represent components and shows how they are electrically connected. Every component has a specific symbol — a resistor is shown as a rectangle (BS standard) or a zigzag (US standard); an LED as a diode symbol with two outward arrows; a battery as long and short alternating lines. Schematics do not show the physical size or location of components — they focus purely on the electrical connections. When interpreting a schematic, trace each connection to identify the circuit topology (series or parallel) and the function of each component.
PCB Layouts
A printed circuit board (PCB) layout shows the actual physical arrangement of components and the copper tracks that connect them on the board. Unlike a schematic, a PCB layout is drawn to scale and shows where each component is soldered. PCB design software produces both the schematic and the corresponding PCB layout; the layout is then used to manufacture the physical board. When reading a PCB layout, components are identified by reference designators (R1 for first resistor, C1 for first capacitor, etc.), and copper tracks appear as solid lines or polygons connecting pads.
Block diagram: shows system function and signal flow, no component detail. Circuit schematic: shows component types and electrical connections using standard symbols. PCB layout: shows physical size, position, and copper track routing of every component on the board. Each representation serves a different stage of the engineering design process.
When an OCR exam question asks you to complete a block diagram, follow these steps: (1) Identify which blocks are already drawn and what stage they represent (input, process, output, or feedback). (2) Identify what is missing — a block label, a signal arrow, or a feedback path. (3) For each empty box, ask: is it converting an environmental signal into an electrical signal? If so, it is an input/sensor. Is it controlling or switching another component? That is a process block. Is it converting electrical energy into light, sound or movement? That is an output. (4) Draw signal arrows between every block to show the direction of signal flow — arrows on the feedback path must point from output back towards input. Always use a ruler and label every block clearly.
A freezer temperature alarm is a classic closed loop example for the exam. Input: NTC thermistor senses temperature. Process: microcontroller compares measured temperature with a threshold. Output: buzzer sounds if temperature is too high. Feedback: the microcontroller continuously reads the thermistor output, so the system self-regulates — it checks and re-checks the temperature rather than acting once and stopping. This feedback loop makes the system closed loop. In the block diagram, draw the feedback arrow from the output stage back to the process block, with an arrow pointing towards the process block.