Describing Systems: Block Diagrams, Schematics and Flowcharts
Thinking in Systems
Every engineered product — from a car braking system to a factory conveyor — is a system. Engineers break complicated products down into smaller, manageable pieces so they can design, build and test each part separately. The three main tools for doing this are block diagrams, schematic drawings and flowcharts. Understanding when to use each one is essential.
The input–process–output (IPO) model is the foundation of all system design. Every system takes something in, does something to it, and produces a result. Identifying these three stages is the first step when analysing or designing any engineering system.
The Input–Process–Output Model
The input–process–output model (IPO) lets engineers describe any system with three boxes. In an electric kettle: the input is the mains electricity and cold water; the process is the heating element converting electrical energy to heat; the output is boiling water. In a car engine: the input is fuel and air; the process is combustion; the output is rotary motion at the crankshaft.
A system also has a system boundary — an imaginary line that separates what belongs to the system from the wider environment. Drawing the boundary forces the engineer to decide exactly what is and is not part of their design.
System Block Diagrams
A system block diagram uses rectangular blocks joined by arrows to show how sub-systems connect. Each block is labelled with what it does (e.g. “temperature sensor”, “microcontroller”, “motor driver”). The arrows show the direction of signal or energy flow.
Block diagrams are NOT circuit diagrams and NOT flowcharts. A circuit diagram shows how components are wired using standard symbols (resistor, capacitor, LED). A flowchart shows the sequence of steps a process or program follows. A circuit diagram offered in place of a block diagram is unlikely to earn the marks for the block-diagram question, because it does not show the system stages the question asks for.
A system block diagram uses labelled rectangles and arrows only — no component symbols (resistor zigzag, capacitor plates, logic gate shapes). If the diagram looks like a circuit, it is wrong. Blocks describe WHAT each sub-system does, not HOW it is built.
Open-Loop and Closed-Loop Control
In an open-loop system the output has no effect on the process. A toaster set to a timer is open-loop: it heats for a fixed time regardless of whether the toast is burnt or pale. In a closed-loop system a sensor monitors the output and feeds information back to the controller. An oven thermostat is closed-loop: it measures actual temperature and switches the heating element on or off to match the set point.
Closed-loop systems are more accurate but more complex to design. Most industrial control systems — from a car cruise-control to a furnace temperature controller — are closed-loop because small disturbances (draughts, varying load) would cause the output to drift without feedback correction.
Large systems are broken into sub-systems. A car has sub-systems for engine management, braking, steering, lighting, infotainment and climate control. Each sub-system can be designed and tested independently. On a block diagram, sub-systems are shown as blocks that can themselves be expanded into more detailed block diagrams — this is called hierarchical decomposition.
The system boundary decides what counts as an input and what counts as an output. For a car engine management sub-system, “throttle position” is an input (from outside the sub-system, controlled by the driver), whereas “fuel injector pulse width” is an output (leaving the sub-system to control the fuel delivery).
Schematic Drawings
A schematic drawing (or schematic) uses standard symbols to show how components connect, not how they are physically arranged on a PCB or in a machine. Electrical schematics use British Standard (BS 3939) symbols: zigzag for a resistor, two parallel lines for a capacitor, a circle with an arrow for a motor. Hydraulic and pneumatic schematics use ISO 1219 symbols: rectangles for cylinders, diamonds for filters, arrows on lines for flow direction.
Schematics are the standard language of engineering workshops. If you hand a schematic to a technician, they can identify every component and trace every connection regardless of what the physical product looks like. This is why drawing interpretation is a key skill examined in the written paper.
Flowcharts show the sequence of steps in a process or control program. There are four standard symbols you must know:
- Terminator (oval) — marks the START or END of the process. Every flowchart must begin and end with a terminator.
- Process box (rectangle) — shows an action to be carried out, e.g. “Switch motor ON” or “Heat element for 30 s”.
- Input/Output parallelogram — shows data entering or leaving the system, e.g. “Read sensor value” or “Display temperature”.
- Decision diamond — asks a YES/NO question, e.g. “Is temperature > 80°C?” with two arrow paths leaving it.
Feedback loops in flowcharts must loop back to a decision diamond or an earlier process box — never to the start terminator. A common error is drawing the feedback arrow going all the way back to START, creating an infinite loop with no way to exit.
Choosing the Right Representation
You need to choose the correct type of diagram for the question. Use a block diagram when asked to show how sub-systems relate and where signals flow. Use a schematic when asked to show component connections using standard symbols. Use a flowchart when asked to show the sequence of steps or decisions in a process or control program. Mixing these up is one of the most common reasons students lose marks on systems questions.