B3.4 — Electronic systems application and selection (HL)

Key concepts

B3.4 applies electronic systems theory (A3.4) to real design work — turning circuit ideas into functioning, integrated electronic products. The designer must specify components, verify the circuit works, integrate it with the physical product, and document the design so it can be manufactured or repaired.

Designing functional circuits

A circuit design begins with function:

1. What must the circuit do? (turn on a light when motion detected, read a sensor, drive a motor)
2. What are the inputs and outputs?
3. What processing is needed between them?

From function, the designer sketches a block diagram (input → process → output), then expands each block into a schematic with components and connections.

Component specification

Each component must be specified with:

• Electrical rating — voltage, current, power (resistor wattage matters!)
• Package — through-hole vs surface-mount, size
• Tolerance — permissible variation (e.g. ±5% resistor)
• Cost — per unit, especially at volume
• Availability — standard values, not obscure parts
• Datasheet verified — the designer must read and understand the datasheet

A circuit that works on paper can fail because a selected component is out of stock, out of voltage range, or wrong package.

System testing and fault finding

Electronic prototypes rarely work first time. Systematic testing:

1. Visual inspection — check connections, polarity, missing components
2. Power check — verify supply voltage at appropriate points
3. Signal tracing — use a multimeter or oscilloscope to follow signal through stages
4. Isolation — test sub-circuits in isolation to locate faults
5. Document faults — note what worked, what failed, what was changed

Good designers develop a fault-finding workflow. Random component replacement is expensive and slow; systematic testing finds faults in minutes.

Integrating electronics into products

A bare circuit board is not a product. Integration requires:

• Mechanical mounting — standoffs, clips, screws
• Housing — plastic or metal enclosure with openings for connectors, indicators, and user interface
• User interface — buttons, LEDs, screens properly positioned and labelled
• Connectivity — charging ports, data connectors, antennas positioned to work through housing
• Thermal management — heat-generating components need ventilation or sinking
• EMC (electromagnetic compatibility) — circuit doesn't interfere with or get disturbed by other electronics

Documenting electronic design

A designer leaves behind:

• Schematic diagram — the logical circuit (KiCad, EasyEDA output)
• PCB layout — physical arrangement of components and tracks
• Bill of materials (BOM) — complete list of components with suppliers
• Firmware source code — if microcontroller-based, the program running on it
• Test procedure — how to verify each unit works

Without documentation, the design cannot be manufactured, maintained, or revised.

Safety and regulation

Electronic products face regulations:

• Low-voltage directive — products operating between 50-1000V AC must meet safety standards
• EMC regulations — products must not emit excessive electromagnetic interference
• RoHS — restriction of hazardous substances (lead-free solder, limits on cadmium, mercury)
• WEEE — waste electrical and electronic equipment collection/recycling
• Battery regulations — for rechargeable and primary cells

Student IA projects typically work at low voltage (3–12V DC) where safety risks are limited, but professional designers must design to regulatory standards from day one.

Case studies

Raspberry Pi — a fully documented single-board computer. Schematic, PCB layout, BOM, and firmware are all publicly available. Enabled an enormous education and maker movement precisely because the documentation is complete and accessible.

Dyson vacuum electronics — custom microcontroller-driven brushless motor controllers tuned for specific airflow and efficiency targets. Integration challenges include heat dissipation in a compact housing, EMC compliance, and reliability over tens of thousands of starts.

Arduino ecosystem — the archetype of accessible electronic prototyping. An Arduino Uno on a breadboard can prototype dozens of product concepts in hours. Students and professionals use it identically for early validation.

Tesla vehicle control unit — multi-layer PCB with hundreds of components performing safety-critical functions. Redundant systems, automotive-grade components, formal verification. The far end of electronic design rigour.

Glossary

• Schematic — a logical diagram of a circuit using standard symbols.
• PCB — Printed Circuit Board; physical implementation of a schematic.
• BOM — Bill of Materials; complete list of components with suppliers.
• Datasheet — manufacturer document specifying a component's parameters.
• Fault finding — systematic process of locating circuit faults.
• Firmware — software running on a microcontroller.
• EMC — Electromagnetic Compatibility; how a circuit emits and resists interference.
• RoHS — Restriction of Hazardous Substances (EU regulation).
• Through-hole — component package inserted through PCB holes and soldered on the underside.
• Surface-mount (SMD) — component package soldered directly to PCB pads without holes.

Check your understanding

1. Describe the steps a designer takes to specify a component for a circuit.

Identify the electrical requirements (voltage, current, power, frequency). Check standard values available from suppliers. Read the datasheet to verify the component meets requirements under expected conditions (temperature, load, environment). Select package type based on PCB process and assembly method. Verify availability and cost at required volume. Specify exact part number in the BOM. Designers never specify "a resistor" — they specify "220Ω ±5% 0.25W through-hole carbon film resistor".

2. Give a systematic fault-finding procedure for a non-functioning student prototype circuit.

1) Visual inspection — look for wrong polarity, missing components, solder bridges, reversed connectors. 2) Power check — verify supply voltage at input and key nodes. 3) Signal tracing — start from the input, move step by step through each stage, verifying expected voltage or signal at each point. 4) Isolate sub-circuits — disconnect and test subsystems independently. 5) Substitute suspect components with known-good ones. Document each step so the failure is understood rather than just worked around.

3. Explain why documentation of an electronic design is essential, and list three documents a designer produces.

Without documentation, a circuit cannot be manufactured by anyone other than its designer, cannot be maintained or repaired, and cannot be revised safely. Documentation is how design knowledge survives the designer. Three essential documents: schematic diagram (logical circuit), PCB layout (physical arrangement), bill of materials (complete parts list with suppliers). Firmware source code and a test procedure are equally important for products with embedded code.

4. List three integration challenges when placing an electronic circuit inside a product housing.

Thermal management: power-dissipating components generate heat that must escape without damaging surrounding components or users. Mechanical fit: board must be supported against vibration and shock without distorting. User interface: buttons, LEDs, ports, and antennas must align with openings in the housing, with correct spacing and labelling. EMC: the housing may screen or interfere with wireless signals (Wi-Fi, Bluetooth), and internal components may generate interference that affects external equipment.