Key Idea: An electronic system is input → process → output, plus a feedback loop that carries a measurement of the output back. Input and output boxes hold devices, never quantities. Analogue is continuous, digital is discrete. Converting loses whatever happened between samples and rounds each reading — once, and then never again. Six quantities (V, A, Ω, W, Hz, s), two relationships (V = I × R, P = V × I), and multipliers p n µ m k M G T, each step a thousand. Passive components limit, store or divert. Active ones control the flow — and a transistor lets a small signal control a large current. A process is built either by signal conditioning (analogue) or program control (digital). A microcontroller is the second one on a chip. Open loop does what it was told; closed loop does what it was asked.
Paper 1 — multiple choice
- Name a component from its symbol or its behaviour
- Classify a signal or a system as analogue or digital
- Apply Ohm's law, the power equation or a prefix
- Complete a truth-table row or pick the right gate
Paper 2 — analysing a product
- Describe a product as an electronic system
- Explain the purpose of a component in a given circuit
- Justify an input or output device for a user and a place
- Evaluate what adding electronics costs a product
Carried into the design project
- Criterion D — a circuit claim must be built and measured
- Every sensor choice needs a user and a place behind it
- Standby, batteries and repairability belong in Criterion E
Almost every long question in this topic starts by putting a product into the systems model. The marks are lost in the same place every time — in the boxes, not in the physics.
| Box | What belongs there | What scores nothing |
|---|---|---|
| Input | A device that turns a change into a signal: a thermistor, an LDR, a microphone, a push switch | "Temperature" or "light" — a quantity, not a device |
| Process | The thing that decides: a comparator, a timer, a logic gate, a microcontroller running a program | The product's own name, or "the circuit" |
| Output | A device that acts on the world: a motor, an LED, a buzzer, an element, a display | "Heat" or "noise" — again a quantity |
| Feedback | A sensor measuring the OUTPUT, and a path carrying that measurement back | A timer, which counts whatever happened, and the user noticing |
Three products through the same four boxes — and one of them with no loop at all.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Important: A quantity in the input box. Name the device and the signal it produces. A prefix out by a thousand. Convert to base units first, then check the value is plausible for that component. Calling a timer feedback. Feedback requires something that measures the OUTPUT. Connecting a load straight to a chip pin. An LED with a resistor is fine; a motor, a lamp or a mains load needs a transistor or a relay.
| Group | Members | What each is for |
|---|---|---|
| Passive | Fixed and variable resistor, capacitor, switch, relay | Limit current and set voltages; store and release charge; make or break a circuit; let a small current switch a large, isolated one |
| Active | Diode, LED, transistor | Allow current one way only; convert current into light; let a tiny base current control a large one, as a switch or an amplifier |
| Inputs | Switch, LDR, thermistor, humidity sensor, microphone | Turn a change in the world into a signal — mostly a changing RESISTANCE, which a voltage divider turns into a voltage |
| Outputs | LED, lamp, LCD, braille display, buzzer, speaker, headphones, haptic, motor, relay, printer, plotter | Reach the user by sight, sound or touch, act on the world, or produce a record that outlives the moment |
| Processing | Comparator, op-amp, logic gate, microcontroller | Decide. Analogue signal conditioning amplifies, filters and compares; digital program control converts to a number and decides in software |
Every symbol the guide names, then a complete circuit built from them.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
V = I × R and P = V × I — quote them before calculating. An LED always needs a series resistor; a coil always needs a diode across it. A relay isolates; a transistor does not. An op-amp amplifies the difference between its inputs, and its feedback resistors set the gain. A dot at a wire crossing means a junction; no dot means no connection.
A cycle helmet has a rear light that switches itself on at dusk and flashes brighter when the rider brakes. Describe it as an electronic system.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A student connects a 5 V microcontroller pin directly to a small 5 V DC motor, and the pin stops working. Explain what happened and what the circuit should contain.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A kettle manufacturer adds a microcontroller, a temperature display and a phone app to a product that previously used a bimetallic thermostat. Evaluate this.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Why is "temperature" a wrong answer in an input box?
What is lost when an analogue signal is digitised?
Why does a resistive sensor need a voltage divider?
What is the difference between an open and a closed loop?
Why must a relay coil have a diode across it?
What does an op-amp actually do in a product?
Exam tips
- Put devices in the input and output boxes, and say what the process decides.
- Convert to base units before any calculation, and quote V = I × R or P = V × I first.
- Never claim feedback unless something measures the output.
- Put a transistor or relay between a chip pin and anything bigger than an LED, and a diode across every coil.
- For responsibility questions, give a mechanism and a number, not an adjective.
- For evaluate, separate the features that earn their place from the ones that only add failure modes.