Electronic Systems and the Input-Process-Output Model
How Electronic Systems Work
Electronic systems are at the heart of almost every modern product — from a bedside lamp that dims automatically to a fire alarm that wakes a building. Rather than thinking about individual wires and components, designers use a systems approach to plan how a product will function. This means dividing any electronic product into three linked stages: input, process and output. Understanding this model is essential for answering questions about how electronic products detect, decide and respond.
The Input–Process–Output Model
The input–process–output (IPO) model describes all electronic systems. The three stages always occur in this sequence:
- Input — a sensor or switch detects a change in the environment and converts it into an electrical signal. Examples include a light-dependent resistor detecting low light, or a microswitch being pressed by a moving part.
- Process — a processor (such as a transistor circuit or microcontroller) receives the signal and decides what to do. It may compare the input against a set threshold and trigger a response only when a condition is met.
- Output — the system produces a visible, audible or physical response. Common output devices include LEDs, buzzers, motors and display screens.
Every electronic system has three stages: input (sensor detects a change), process (circuit or microcontroller decides) and output (device responds). No electronic product exists that skips one of these stages.
Input Sensors
Sensors are the ‘senses’ of an electronic system. They convert a physical change in the environment into a changing electrical signal. The most commonly tested sensors are:
- Light-dependent resistor (LDR) — its resistance is high in darkness and falls as light intensity increases. A street lamp controller uses an LDR so the lamp switches on at dusk and off at dawn.
- Thermistor — a thermistor has high resistance at low temperatures and low resistance when warm. A central heating thermostat uses a thermistor to detect when the room drops below the set temperature.
- Microswitch — a mechanical switch with a small lever; a tiny force triggers it. Printers use microswitches to detect when a paper tray is open.
- Moisture sensor — two conductive tracks separated by a gap; when water bridges the gap, current flows, completing the circuit. A plant-watering alert uses this type. Important: simply naming “moisture sensor” is not enough in the exam — you must explain that water completes the circuit between the two tracks, changing the current flowing to the processor.
Sensors produce either analogue or digital signals. An analogue signal varies continuously — the output of a thermistor changes smoothly as temperature rises. A digital signal is either on or off: a microswitch is either pressed (1) or not pressed (0). Most microcontrollers work in the digital domain, so analogue signals from sensors like LDRs are converted using an analogue-to-digital converter (ADC) before the processor can act on them.
Understanding this distinction matters for exam questions asking you to explain how a sensor interfaces with a microcontroller. A thermistor gives a continuously varying voltage; the ADC samples this many times per second and converts each sample into a binary number the processor can compare against a stored threshold value.
Output Devices
The output stage converts the electrical signal back into a physical form the user can see, hear or feel:
- LED (light-emitting diode) — emits light when current flows in the correct direction. Used as indicator lights, display digits and, increasingly, as the primary light source in products such as cycle lamps.
- Buzzer — converts electrical pulses into sound via a vibrating disc. Common in alarms, timers and door entry systems.
- Motor — converts electrical energy into rotary motion. Used to move conveyor belts, open automatic doors and drive robotic arms.
- Relay — an electromagnetic switch allowing a low-current circuit to control a high-current device safely, such as a microcontroller switching a mains-voltage motor.
An LDR has high resistance in darkness and low resistance in bright light — the opposite of what many students expect. A thermistor has high resistance when cold and low resistance when hot. Getting these directions wrong is the most common error in sensor questions.
Block Diagrams and Circuit Diagrams
Designers use two main types of diagram to represent electronic systems:
- Block diagram — shows the function of each stage as a labelled box connected by arrows. It does not show individual components or wiring. A block diagram for a security light might show: [LDR] → [Processor] → [LED array].
- Circuit diagram — shows every component using standard British Standard (BS) symbols and the wiring between them. More detailed than a block diagram and used by engineers building the product.
A basic IPO system runs in one direction: input → process → output. A more advanced system uses feedback to make the output self-correcting. In a closed-loop system, the output is monitored and compared to a desired value; any difference (error) is used to adjust the process. A central heating system is a classic example: the thermistor measures room temperature (feedback), the controller compares it to the target, and the boiler fires or cuts off accordingly.
In contrast, an open-loop system has no feedback: a toaster with only a timer heats for a fixed period regardless of whether the bread is actually toasted. Open-loop systems are simpler and cheaper but less accurate. Closed-loop systems cost more but respond dynamically to changing conditions, making them preferable in safety-critical applications such as industrial robots and medical devices.
Flowcharts
A flowchart maps the logical steps in a control system. Standard shapes are used: ovals for start/stop, rectangles for process steps, diamonds for decisions (with yes/no branches). A flowchart for an automatic night-light would show: Start → Is it dark? (No → loop back; Yes → Turn LED on) → loop back to check again. Flowcharts are particularly useful for describing how a programmable component such as a microcontroller responds to sensor inputs.