The big idea: An electronic system senses, decides and does: input, process, output.
The fourth part is the feedback loop: a measurement of the OUTPUT sent back to the process, so the system can correct itself.
The model, then the same four boxes filled in for an oven, a security light and a powered wheelchair.
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| Box | What belongs there | What does NOT |
|---|---|---|
| Input | A device that turns a change in the world into an electrical signal: a thermistor, an LDR, a microphone, a push switch | "Temperature" or "light" — a quantity is not an input |
| 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, a heating element, a display | "Heat" or "noise" — again, a quantity rather than a device |
| Feedback | A sensor measuring the OUTPUT, and a path carrying that measurement back to the process | The user noticing and pressing a button — that is not a feedback loop in the circuit |
The commonest wasted mark in this topic: Writing a quantity where a device belongs.
"Temperature" tells the process box nothing to work on. "A thermistor whose resistance falls as it warms" does, and the rest of the answer follows from it.
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| Product | Input | Process | Output | Feedback |
|---|---|---|---|---|
| Electric oven | Thermistor inside the cavity | Microcontroller comparing with the dial | Heating element | Yes — the thermistor measures the result of the heating |
| Security light | PIR movement sensor | Timer circuit | LED floodlight | None — which is why it goes out while you are still there |
| Powered wheelchair | Joystick and wheel encoders | Motor controller | Two drive motors | Yes — the encoders report how fast each wheel really turns |
The test for a real feedback loop: Ask whether anything in the circuit measures the output.
A timer does not: it counts, whatever happened. A thermistor inside the oven does, which is why one of these three products reaches a set temperature and the other switches off in your face.
How this is tested — describing an electronic system in terms of input, process, output and feedback. It comes up two ways:
Paper 1 — multiple choice
- Identify the input device in a described system.
- Choose which product contains a feedback loop.
Paper 2 — analysing a product
- Describe a named product as an electronic system.
- Explain the purpose of the feedback path in a given product.
The trap: Writing quantities in the input and output boxes. Name the DEVICE — a thermistor, an LED, a relay — and say what signal it produces.
An automatic hand dryer switches on when hands are placed beneath it and off shortly after they are removed. Describe it as an electronic system.
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