A3.4 — Introduction to electronic systems (HL)

Key concepts

Electronic systems move, process, and respond to information using electric current. Almost every modern product contains electronics — phones, appliances, cars, toys, medical devices. Understanding basic electronic architecture is essential for any designer working beyond purely mechanical products.

The input–process–output model

Every electronic system can be described as three subsystems:

• Input — sensors that detect physical quantities (light, temperature, pressure, motion) and convert them to electrical signals. Examples: LDR, thermistor, switch, microphone.
• Process — circuits that manipulate the signal (amplify, filter, compare, calculate). Examples: op-amp, logic gates, microcontroller.
• Output — actuators or indicators that convert electrical signals back to physical effects. Examples: LED, motor, buzzer, speaker.

Analysing a product as input–process–output makes complex systems understandable and modular.

Analogue and digital systems

Every electronic system is either analogue, digital, or a mixture of the two.

An analogue system uses a signal that varies continuously — it can take any value within its range. A classic analogue signal is a sine wave: a smooth oscillation described by its amplitude and its frequency. A moving-coil speaker, a potentiometer volume control and a thermocouple are all analogue: the quantity varies smoothly and the circuit responds smoothly.

A digital system uses discrete values — in practice two, represented as 0 and 1 (binary). A digital signal is a square wave that is either high or low, with nothing meaningful in between. Logic gates, microcontrollers and memory are digital.

The practical difference: analogue signals carry infinite detail but degrade with noise and distance, while digital signals carry only the detail you sampled but can be copied and transmitted without loss. Most real products are mixed — a sensor produces an analogue voltage, an analogue-to-digital converter (ADC) turns it into numbers, a microcontroller processes it, and a digital-to-analogue converter drives an analogue output.

Describing analogue systems — SI units. Analogue behaviour is described using the International System of Units:

SI multipliers scale these across the enormous range electronics spans, from picofarad capacitors to gigahertz processors:

Write values with the multiplier rather than long strings of zeros: 4.7 kΩ, 100 μF, 2.4 GHz.

Feedback: open- and closed-loop systems

The input–process–output model above is only half the story. Many systems add a feedback path that measures the output and reports it back to the process.

An open-loop system acts without checking the result. A toaster runs its heating element for the time you dial in; it does not measure how brown the toast is. A basic desk fan runs at the speed you select regardless of room temperature. Open-loop systems are simple, cheap and predictable — and wrong whenever conditions change.

A closed-loop system measures the output and adjusts. A room thermostat compares measured temperature to the target and switches the heating accordingly. Cruise control measures road speed and varies throttle to hold it on a hill. A 3D printer measures hot-end temperature and modulates the heater to hold 210 °C.

The purpose of feedback in a closed loop is to hold the output at the desired value despite disturbances — a draught, a hill, a change in load. That is the entire reason to accept the extra sensor, extra cost and extra complexity. Ask of any product: does it measure what it is doing? If not, it is open-loop, and it will drift whenever the world changes around it.

Designing electronics responsibly

Electronics are now in almost every product, which makes the designer's choices consequential at scale. Three demands sit on any electronic product: it must be safe, energy-efficient, and built to use minimal energy in operation.

• Safe — isolation of mains voltages, protection against overheating and short circuit, secure battery containment (lithium cells fail dangerously), and materials that do not release harmful substances in use or in a fire.
• Energy-efficient — the efficiency of the conversion itself. A switch-mode power supply wastes far less as heat than a linear one; an LED delivers far more light per watt than a filament.
• Minimal energy — how much the product draws in normal life, including when idle. Standby draw is the classic failure: a device that spends 23 hours a day waiting, consuming power the user never notices.

Beyond use, electronics create the fastest-growing waste stream in the world. Design decisions determine whether a product becomes e-waste early: whether the battery can be replaced, whether the case can be opened without destroying it, whether components are separable for recovery, and whether firmware support outlasts the hardware. A device that still works but can no longer be updated or repaired is scrapped just as surely as a broken one.

Basic components

• Resistor (R) — limits current. Measured in ohms (Ω). Colour bands indicate value.
• Capacitor (C) — stores charge. Smooths signals, times circuits. Measured in farads (F, usually µF or pF).
• Diode — allows current in one direction only. Used in rectifiers, protection circuits, LEDs.
• Transistor — amplifies or switches current. The foundation of all modern electronics.
• LED — light-emitting diode; visual output with low power.
• IC (integrated circuit) — many components on a single chip (op-amp, timer, microcontroller).

Ohm's Law and power

V = I × R (voltage = current × resistance). Fundamental relationship.

P = V × I (power = voltage × current, measured in watts).

These two equations let you calculate any unknown given two knowns. Essential for choosing resistors, predicting battery life, and sizing components.

Logic gates and digital systems

Digital systems use only two voltage levels: HIGH (1) and LOW (0). Logic gates combine these:

• AND — output HIGH only if all inputs HIGH
• OR — output HIGH if any input HIGH
• NOT (inverter) — output opposite of input
• NAND, NOR, XOR — combinations of the above

Complex digital behaviour is built from networks of gates. A microprocessor contains billions of them.

Microcontrollers

A microcontroller is a programmable IC containing a processor, memory, and input/output pins. Designers write code (in C, MicroPython, or visual languages) that reads sensors, makes decisions, and drives outputs. Arduino and Raspberry Pi Pico are popular prototyping platforms. A single microcontroller can replace hundreds of discrete components.

Sensors and actuators

Sensors convert physical quantities into electrical signals: - LDR (light), thermistor (temperature), ultrasonic (distance), accelerometer (motion), hall-effect (magnetic field), force-sensing resistor (pressure).

Actuators convert electrical signals into physical action: - DC motor (rotation), servo (precise angle), solenoid (linear pull), piezo buzzer (sound), LED (light), stepper motor (incremental rotation).

Circuit design and simulation

Modern designers draft circuits in software before soldering:

• Schematic capture — draw the circuit diagram (KiCad, EasyEDA, Fritzing).
• Simulation — run the circuit virtually to check behaviour (LTspice, Tinkercad Circuits).
• PCB layout — convert schematic to a printable circuit board.

Simulation catches errors before hardware is built, saving time and components.

Case studies

Smart thermostat (Nest, Ecobee) — sensors (temperature, occupancy, humidity) feed a microcontroller that processes data against learned schedules and user preferences, outputting signals to HVAC relays. Input-process-output at product scale.

Arduino-based prototypes — have democratised electronic product prototyping. A $25 board, a handful of sensors, and a breadboard let designers test electronic ideas in an evening.

Electric toothbrush — timer IC drives a small DC motor through a H-bridge transistor circuit, with a pressure sensor input that reduces intensity when the user presses too hard. LED indicators signal battery and operating mode.

Automatic hand sanitiser dispenser — infrared distance sensor (input), comparator or microcontroller (process), small pump and LED (output). A textbook IPO example.

Glossary

• Analogue — a signal that varies continuously and can take any value in its range.
• Digital — a signal restricted to discrete values, in practice binary 0 and 1.
• Sine wave — the characteristic smooth oscillation of an analogue signal.
• Open-loop system — a system that acts without measuring its output.
• Closed-loop system — a system that measures its output and adjusts to hold a target.
• Feedback — the path returning output information to the process, enabling closed-loop control.
• Input — component or subsystem that converts physical quantity to electrical signal.
• Process — circuit or code that manipulates signals.
• Output — component that converts electrical signal to physical action.
• Resistor — passive component limiting current.
• Capacitor — component storing electrical charge.
• Transistor — semiconductor that amplifies or switches current.
• Diode — component allowing current flow in one direction.
• LED — light-emitting diode.
• IC — integrated circuit; many components on one chip.
• Microcontroller — programmable IC with processor, memory, and I/O pins.
• Sensor — component converting a physical quantity into an electrical signal.
• Actuator — component converting an electrical signal into physical action.
• Logic gate — digital circuit performing a Boolean operation (AND, OR, NOT).
• Ohm's Law — V = I × R.

Check your understanding

1. Describe the input–process–output model and apply it to an automatic bathroom light.

Every electronic system can be divided into an input (detecting a physical quantity), process (making decisions or transformations), and output (producing a physical effect). For an automatic bathroom light: input is a passive infrared (PIR) motion sensor; process is a timer circuit or microcontroller that switches on for a set period after movement is detected; output is an LED or filament lamp providing illumination. This modular view makes complex products designable in stages.

2. Using Ohm's Law, calculate the current through a 220 Ω resistor connected to a 5 V supply.

I = V / R = 5 / 220 = 0.0227 A = 22.7 mA. This is typical of an LED-limiting resistor, keeping the current safely below the LED's maximum rating (~20 mA for standard LEDs).

3. Explain the function of a microcontroller and why it often replaces discrete component circuits in modern products.

A microcontroller is a small programmable IC containing a processor, memory, and I/O pins. It reads signals from connected sensors, makes decisions in software, and drives outputs — replacing what would otherwise be a large assembly of logic gates, timers, and comparators. The advantage is flexibility: a single microcontroller can be re-programmed for different products or updated later, whereas discrete circuits are fixed at manufacture. Cost, size, and power consumption have all dropped sufficiently that microcontrollers now appear in children's toys.

4. Identify and describe three sensors and three actuators you might use in a student-designed "smart plant pot" that waters itself.

Sensors: soil moisture sensor (detects dryness), light sensor/LDR (detects sunlight levels), temperature sensor (detects ambient conditions). Actuators: small peristaltic pump (delivers water), LED indicator (status light), piezo buzzer (alert when water reservoir is empty). A microcontroller reads the sensors, compares to thresholds, and drives the actuators accordingly.