Electronic Systems and Sub-Systems
Thinking in Systems
Every electronic product — from a smoke alarm to a traffic light controller — can be broken into three connected stages: an input sub-system, a processing sub-system and an output sub-system. Thinking in these three stages is one of the most powerful tools an electronics engineer has. It lets you design a complex product by solving one small section at a time.
The signal flows in one direction: from input to processing to output. Each stage has a clear role. Once you can sort components and circuits into these three categories, you can read and sketch block diagrams confidently — a skill tested on almost every past paper.
Every electronic system follows the same pattern: Input → Processing → Output. Input sub-systems sense the physical world. Processing sub-systems make decisions. Output sub-systems act on those decisions.
Input Sub-Systems — Sensing the World
Input sub-systems contain transducers that convert a physical quantity into an electrical signal. The specification requires you to know these sensing units:
- Light sensor — a light-dependent resistor (LDR) whose resistance falls as light intensity increases. Used in street-light controllers and security systems.
- Temperature sensor — an NTC thermistor whose resistance falls as temperature rises. Used in fire alarms and thermostat circuits.
- Magnetic field sensor — a reed switch that closes when a magnet is brought close. Used in door alarms and bicycle speed sensors.
- Pressure sensor — a force-sensitive resistor whose resistance changes with applied force. Used in electronic scales and car park barriers.
- Moisture sensor — a pair of conducting tracks whose resistance falls when wet. Used in plant-watering monitors and rain detectors.
- Sound sensor (microphone) — a small electret microphone that converts sound pressure waves into a varying voltage. Used in clap-activated switches and intruder alarms.
- Rotation sensor — an encoder or potentiometer that produces a voltage or pulse train proportional to angular position or speed. Used in robotic arms and motor controllers.
- Pulse generator — an oscillator that produces a regular stream of pulses. It acts as a timing input sub-system, providing a clock signal to processing circuits such as counters and timers. It is never placed where a sensing transducer is needed — its role is timing, not physical measurement.
Processing Sub-Systems — Making Decisions
The processing stage receives electrical signals from the inputs and decides what to do. Common processing sub-systems include:
- Logic gates — digital circuits (AND, OR, NOT, NAND, NOR) that combine binary signals according to a truth table.
- Latch (bistable) — a memory element that holds its output HIGH or LOW until told to change. Useful where the output must stay on after the trigger is removed.
- Time delay — a monostable circuit that produces a single output pulse of a set length when triggered. Used in burglar alarm hold-off timers and courtesy lights.
- Comparator — a circuit that compares two voltages and outputs HIGH when one is larger than the other. Links the analogue sensor voltage to a digital decision.
Processing sub-systems operate on signals — they do not interact directly with the physical world. A comparator, logic gate, latch or timer all sit in the processing stage, never in the input or output stage.
Output Sub-Systems — Acting on Decisions
Output sub-systems convert the processed electrical signal back into something physically useful. The specification lists these output devices:
- Lamp / LED — visual indicators. LEDs require a current-limiting resistor in series.
- Buzzer — an audio output. Some are “active” (buzzes at a fixed frequency when supplied with DC); others are “passive” (require an AC or pulsed signal).
- Solenoid — an electromagnetic coil that produces linear motion when energised. Used to release door locks or trigger valves.
- Motor — converts electrical energy into continuous rotation. Used in fans, conveyor belts and robots.
- Servo — a geared motor with a position feedback circuit, allowing controlled angular movement. Used in robotic arms and model aircraft control surfaces.
- Loudspeaker — a high-power audio output that requires an amplifier stage between the processing circuit and the speaker coil.
Most output devices cannot be driven directly by a logic gate or comparator because they need more current than those circuits can supply. A transducer driver — typically a transistor or MOSFET switch — sits between the processing output and the device to boost current capacity.
A security light turns on when it is dark AND someone is moving nearby. Break this into three stages:
Input: Two sensors are needed — an LDR (detects darkness) and a PIR motion sensor (detects movement). Each is an input sub-system.
Processing: The light should only come on if it is dark AND movement is detected. An AND logic gate combines the two signals: output is HIGH only when both inputs are HIGH.
Output: The lamp (output device). Because the AND gate cannot supply enough current to drive the lamp directly, a transistor switch (transducer driver) is added between the gate output and the lamp.
Block diagram: The LDR sensor and PIR sensor both feed the AND gate. The AND gate feeds the transistor driver, which feeds the lamp.
Three types of error come up again and again in block diagrams:
1. Mixing up stages. Putting a logic gate in the input stage or a thermistor in the output stage. Remember: transducers are always inputs; decision circuits are always processing; actuators are always outputs.
2. Inventing blocks not needed by the specification. Adding a pulse generator where no timing is required (e.g. a simple light sensor to LED circuit needs no clock). Only include a block if the system description justifies it.
3. Omitting the transducer driver. When the output is a motor, relay, solenoid or lamp, a transistor or MOSFET driver is almost always required. Logic gates and comparators cannot supply the current these devices need directly.