Introduction to Electronic Systems | A3.4

Students must be able toDescribe an electronic system in terms of input, process, output and feedback.

Every electronic system can be described using the Input–Process–Output (IPO) model, often extended with a feedback loop:

• Input. A sensor or transducer converts a physical quantity (light, temperature, sound, force, position) into an electrical signal the system can process. Examples: microphone, thermistor, photodiode, push-button switch.
• Process. A control unit (microcontroller, logic circuit, or op-amp) receives the input signal and applies programmed logic or circuitry to decide on a response. This may involve amplification, comparison, timing, or computation.
• Output. An actuator or output transducer converts the processed electrical signal back into a physical effect. Examples: LED (light), loudspeaker (sound), motor (movement), heater (heat).
• Feedback. A signal from the output is returned to the input or process stage to modify behaviour. Negative feedback reduces the difference between desired and actual output, creating stable, self-regulating systems. Example: a thermostat monitors room temperature (feedback) and turns a heater on or off (output) to maintain a set point (input).

Example (electric kettle): Temperature sensor (input) → microcontroller compares measured temperature to 100 °C target (process) → heating element on or off (output) → temperature reading returned to process (feedback).

The IPO Loop

A 1985 NES and the phone in your pocket, different, but the same?

Read spotlight →

Students must be able toIdentify electronic products that are safe, energy-efficient and utilise minimal energy.

Electronics are embedded in almost every product we use: smartphones, medical devices, household appliances, vehicles, infrastructure. This ubiquity creates both opportunity and responsibility for designers.

Responsible electronic design considers:

• Safety. Devices must not harm users or bystanders. Electrical insulation, thermal protection, overvoltage protection (fuses, circuit breakers), and regulatory compliance (CE marking in Europe, UL in the USA) are all safety measures. Medical devices face the most stringent safety standards.
• Energy efficiency. Electronic products consume energy throughout their life. Sleep modes, LED lighting, variable-speed drives, and power factor correction all reduce consumption. The EU Ecodesign Directive requires manufacturers to demonstrate energy efficiency before products can be sold.
• Minimal standby power. Many devices consume power even when not in active use (standby loss, "phantom load"). Efficient designs reduce this to near zero. EU regulations cap standby power at 0.5 W for most devices.
• End-of-life. Electronic waste (e-waste) is the fastest-growing waste stream globally. Designers should minimise toxic materials (lead, mercury, cadmium, now restricted by RoHS), use recyclable materials, and design for disassembly so components can be recovered.
• Data and privacy. Connected electronic products collect data. Designers have responsibilities around secure data storage, minimal data collection, and transparency with users.

Students must be able toDistinguish between analogue and digital systems.

Analogue systems use signals that vary continuously over a range of values. An analogue signal can take any value between a minimum and maximum, just like the physical world it represents. A microphone output, a temperature sensor voltage, and the position of a potentiometer are all analogue signals.

Digital systems represent and process information using only two discrete states: HIGH (logic 1, typically 3.3 V or 5 V) and LOW (logic 0, 0 V). All data (numbers, text, images, audio, video) is encoded as sequences of binary digits (bits). The binary system uses base 2 (only digits 0 and 1).

Most modern systems are mixed signal: an analogue front-end (sensor, amplifier) converts real-world signals, then an Analogue-to-Digital Converter (ADC) converts them to digital form for processing. Digital-to-Analogue Converters (DAC) convert digital results back to analogue for output (e.g., speakers, actuators).

Students must be able toDescribe analogue systems in terms of voltage, current, resistance, frequency and power using SI units: ampere (A), second (s), hertz (Hz), watt (W), volt (V), ohm (Ω). Use SI multipliers: p, n, μ, m, k, M, G, T.

Analogue signals are characterised by quantities that vary continuously. The most common waveform is the sine wave: it models the output of AC generators, audio signals, and many natural phenomena. Key analogue quantities and their SI units:

SI multipliers: Engineers routinely use prefixes to avoid writing many zeros.

AC mains electricity in Australia is 230 V RMS at 50 Hz. A sine wave at 50 Hz completes one full cycle every 20 ms (period T = 1/50 = 0.02 s).

Students must be able toDescribe digital systems in terms of using discrete values such as binary digits and on and off signals. Define logic gates.

Digital systems store, process, and transmit all information as binary (base-2) numbers: sequences of 0s and 1s. Each binary digit is a bit; 8 bits = 1 byte. Advantages of digital over analogue: noise immunity, perfect copying, easy storage, encryption, and compression.

Logic gates are the fundamental building blocks of digital circuits. They take one or more binary inputs and produce a single binary output according to a defined Boolean function:

Logic gates combine to form adders, comparators, flip-flops (memory cells), counters, and all the complex functions of a microprocessor. NAND and NOR are universal gates: any logic function can be built using only one type.

Students must be able toExplain the purpose of passive electronic components, including fixed and variable resistors, capacitors, switches, relays; and active components such as diodes and transistors.

Passive components do not require an external power supply to function and cannot amplify signals; they can only attenuate, store, or redirect energy:

• Fixed resistors. Limit current flow according to Ohm's Law (V = IR). Used for biasing, current limiting (protecting LEDs), and voltage division. Measured in ohms (Ω). Colour codes indicate value.
• Variable resistors (potentiometers / rheostats). Resistance adjustable by turning a dial or sliding a wiper. Used for volume controls, dimmer switches, position sensors.
• Capacitors. Store electrical charge (energy) in an electric field. Used for filtering, smoothing power supplies, timing circuits, and coupling/decoupling. Measured in farads (F), typically μF or pF. Electrolytic capacitors are polarised (must be connected the right way round).
• Switches. Mechanically open or close a circuit. Types: SPST (single-pole single-throw: on/off), SPDT (changeover), DPDT, push-to-make, push-to-break, reed switches (activated by a magnet).
• Relays. Electrically operated switches. A small current through an electromagnet coil controls a separate, larger-current switching contact. Used to allow a low-voltage microcontroller to switch mains-voltage circuits safely.

Active components require an external energy source and can amplify or switch signals:

• Diodes. Allow current to flow in one direction only (anode to cathode). Used for rectification (AC to DC), voltage clamping, and protection. LED (Light Emitting Diode) emits light when forward biased.
• Transistors (BJT and MOSFET). Three-terminal devices that act as electronically controlled switches or amplifiers. A small base/gate signal controls a much larger collector/drain current. Fundamental to all digital and analogue circuits: a modern processor contains billions of MOSFET transistors.

Interactive Tool

Macropad Builder

Plan the switches, wiring and components for a real macropad, then generate downloadable code to build it with an ESP32.

Students must be able toIdentify appropriate input devices for a given electronic system, including light, sound, temperature, motion, and touch sensors.

Input devices (sensors and transducers) convert a physical change in the environment into an electrical signal. Choosing the right sensor is a key design decision. Common sensor types:

When selecting a sensor, designers consider: measurand (what physical property is detected), range, sensitivity, response time, linearity, power requirements, size, and cost.

Students must be able toDescribe the role of processing devices in an electronic system, including logic ICs, microcontrollers, and single-board computers.

The process stage in an electronic system receives input signals, applies logic or computation, and generates output signals. Processing devices range in complexity:

• Logic ICs. Integrated circuits containing fixed logic gates (AND, OR, NOT, etc.) in a single chip package. Examples: 74HC series (CMOS logic). Used for simple combinational logic: no programming required, but no flexibility.
• Microcontrollers (MCUs). A complete computer on a single chip: CPU, RAM, flash memory (for program storage), and programmable input/output pins. Program written in C, MicroPython, or a block-based language. Examples: Arduino (ATmega328), Raspberry Pi Pico (RP2040), PIC, STM32. Widely used in consumer electronics, automotive systems, and industrial control.
• Single-board computers (SBCs). Full Linux computers on one board. Examples: Raspberry Pi 4, BeagleBone. Have operating systems, USB, HDMI, ethernet; more powerful than MCUs but consume more power and require more time to boot.
• FPGAs (Field-Programmable Gate Arrays). Chips that can be configured to implement any digital circuit. Used in high-speed signal processing, prototyping, and aerospace/defence applications.

For most embedded product designs, a microcontroller is the most common choice: it is purpose-built, inexpensive, energy-efficient, and available in a huge range of sizes and capabilities.

Students must be able toDescribe the function of control circuits in everyday products, and explain how they monitor and respond to changing conditions.

A control circuit continuously monitors one or more inputs, applies logic, and switches outputs on or off to maintain a desired condition. Control circuits underpin all automated systems.

Examples of control circuits in everyday products:

• Thermostat (home heating). Temperature sensor monitors room temperature. When temperature falls below the set point, the control circuit closes a relay to activate the boiler. When temperature reaches the set point, the relay opens, turning the boiler off. This is an example of on–off control (bang-bang control).
• Automatic street lights. LDR monitors light level. When ambient light falls below a threshold (dusk), the control circuit switches the lamp on. At dawn, the lamp switches off. The time delay prevents flickering in brief cloud shadow.
• Washing machine. Multiple control circuits manage water temperature, water level (pressure sensor), drum motor speed (tachometer feedback), door lock, and timer. The main microcontroller coordinates all these through a programmed wash cycle.
• Battery management system (BMS). Monitors cell voltage, current, and temperature in a lithium battery pack. Controls charging rate, prevents overcharging or over-discharging, and balances cells. Critical for safety in electric vehicles and portable electronics.

Control circuits may be implemented as hardwired analogue circuits (comparators with hysteresis) or as software running on a microcontroller. Software-based control is more flexible: parameters can be changed by reprogramming rather than replacing components.

Students must be able toIdentify appropriate output devices for a given electronic system, including lights, displays, motors, speakers, and solenoids.

Output devices convert an electrical signal into a physical effect: light, sound, motion, or heat. The output device chosen must match the application's requirements for power, speed, precision, and size.

Power management is critical at the output stage: most microcontrollers can only supply ~40 mA per GPIO pin. A transistor or MOSFET driver, or a relay, is needed to switch higher-current outputs such as motors and solenoids.

Students must be able toExplain the role of negative and positive feedback in electronic systems, and identify how feedback creates self-regulating systems.

Feedback is the process of routing part of the output signal back to the input, where it influences the system's behaviour. Feedback is fundamental to creating stable, accurate, self-correcting systems.

Negative feedback. The feedback signal opposes the change in output, reducing the difference between the desired (set point) and actual output. Negative feedback makes systems stable and predictable.

• Example: A cruise control system measures actual vehicle speed (output), compares it to the set speed (set point), and adjusts the throttle (process) to reduce the error. If the car slows going uphill, the throttle opens; if it accelerates downhill, the throttle closes.
• Example: An op-amp with negative feedback has its output fed back to its inverting input, dramatically improving linearity, bandwidth, and output stability compared to open-loop operation.

Positive feedback. The feedback signal reinforces the change, amplifying the output further in the same direction. Positive feedback leads to instability or latching behaviour, often used deliberately in oscillators and Schmitt triggers.

• Example: Microphone held too close to a speaker: sound is picked up, amplified, played back, and picked up again, producing a loud squeal (runaway positive feedback).
• Example: Schmitt trigger: uses positive feedback to create sharp, clean transitions between HIGH and LOW, eliminating noise-induced jitter on the edges of slowly changing input signals.

Students must be able toDescribe the characteristics of an ideal op-amp and explain the operation of inverting and non-inverting amplifier configurations.

An operational amplifier (op-amp) is a high-gain, DC-coupled voltage amplifier with two inputs, a non-inverting input (+) and an inverting input (−), and a single output. Op-amps are integrated circuits (the μA741 is the classic example; the LM358 and TL071 are widely used modern variants).

Ideal op-amp characteristics: infinite open-loop gain, infinite input impedance (draws no current), zero output impedance, zero offset voltage, infinite bandwidth. Real op-amps approach these ideals.

Common configurations:

• Comparator (open loop). No feedback. Output swings to positive or negative rail depending on which input is higher. V+ > V− → output HIGH; V− > V+ → output LOW. Used to compare a sensor voltage to a reference (e.g., trigger an alarm when temperature exceeds a threshold).
• Inverting amplifier (negative feedback). Input resistor R₁ at the inverting input; feedback resistor R₂ from output to inverting input. Voltage gain A = −R₂/R₁ (negative = inverted). Example: R₁ = 1 kΩ, R₂ = 10 kΩ → gain = −10.
• Non-inverting amplifier (negative feedback). Input at non-inverting (+) terminal; feedback resistors R₁ and R₂ set gain. Voltage gain A = 1 + R₂/R₁ (always positive, output in phase with input). Example: R₁ = 1 kΩ, R₂ = 9 kΩ → gain = +10.
• Voltage follower (unity gain buffer). Output connected directly to inverting input. Gain = 1; used to buffer a high-impedance source without drawing current from it.

Students must be able toDescribe what an embedded system is and explain how embedded systems communicate with each other using standard protocols.

An embedded system is a dedicated computer system designed to perform a specific function within a larger product or system. Unlike a general-purpose computer, an embedded system runs a fixed program and is not intended to be reprogrammed by the end user.

Key characteristics: dedicated function, real-time response, constrained resources (limited RAM and flash memory), low power consumption, high reliability, long service life.

Examples: Engine control unit (ECU) in a car, anti-lock brake system, insulin pump controller, smart thermostat, industrial PLC (Programmable Logic Controller), washing machine control board.

Communication between embedded systems. Multiple embedded systems within a product or across products use standard serial communication protocols:

• I²C (Inter-Integrated Circuit). Two-wire bus (SDA data, SCL clock). Supports multiple devices on one bus using addresses. Typical speed: 100 kHz or 400 kHz. Used for sensors, displays, EEPROMs in short-range connections.
• SPI (Serial Peripheral Interface). Four-wire bus (MOSI, MISO, SCK, CS). Higher speed than I²C, full-duplex. Used for SD cards, ADCs, and fast displays.
• UART (Universal Asynchronous Receiver/Transmitter). Two-wire (TX, RX). Simple, no clock line; devices must agree on baud rate. Used for GPS modules, Bluetooth serial links, debugging.
• CAN bus (Controller Area Network). Robust two-wire differential bus used in automotive and industrial applications. Supports up to 1 Mbit/s; resistant to electrical noise; each node can communicate with all others.

Further Reading

Computer Parts: A History

An interactive and (I promise) interesting look into the parts that make up personal computers, and how we got to where we are with them.

Students must be able toDraw and interpret simple electronic circuit diagrams using standard IEC symbols, and distinguish between series and parallel circuits.

A circuit diagram (schematic) is a standardised graphical representation of an electronic circuit, using universally recognised symbols defined by IEC 60617. Circuit diagrams allow engineers worldwide to communicate circuit designs unambiguously.

Key IEC schematic symbols (know these for examination):

• Battery / DC supply: long and short parallel lines; + terminal at long line
• Resistor: rectangle (IEC) or zigzag (ANSI)
• Capacitor: two parallel lines (electrolytic adds a + sign)
• Switch (SPST): line with a gap and pivoting arm
• LED: diode triangle pointing to bar, with arrows indicating emitted light
• Transistor (NPN BJT): three-terminal symbol with arrow on emitter pointing outward
• Op-amp: triangle with two inputs on left (+ and −) and output on right
• Ground: downward-pointing lines (horizontal bars decreasing in size)

Series circuits: Components connected end-to-end in a single path. Same current flows through all components. Total resistance = R₁ + R₂ + R₃. If one component fails (open), the entire circuit stops working. Example: Old-style Christmas lights in series: one blown bulb stops all.

Parallel circuits: Components connected across the same two nodes, providing multiple current paths. Same voltage across all branches. Total resistance is less than any individual resistor (1/R_total = 1/R₁ + 1/R₂ + ...). If one branch fails (open), others continue. Example: Household mains wiring: all appliances share the same 230 V, and switching one off does not affect others.