Trace an automated system's control loop
Follow sensor input through a microprocessor decision to an actuator response.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 6.1–6.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does an actuator do?
What you will learn
- Explain sensor, microprocessor and actuator roles
- Trace input, processing and output
- Connect the response to the sensed condition
Three components work together
A sensor measures a physical condition such as temperature, light or movement and converts it into data. A microprocessor reads that input and applies instructions to decide what action is needed.
Apply and check the rule
An actuator carries out a physical action, such as opening a valve, turning a motor or switching on a heater. The sensor, microprocessor and actuator work together: measure → process → act. A system may repeat the cycle to maintain a target condition.
Pause and explain
Which order describes the normal control loop?
Worked example
A greenhouse sensor detects that soil is dry. Trace how an automated watering system responds.
Show the worked solution
- The moisture sensor sends a reading to the controller.
- The microprocessor compares it with the required threshold.
- If soil is too dry, the actuator opens a valve or starts a pump.
Answer Sensor measures moisture → microprocessor compares with the threshold → pump or valve actuator waters the soil.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Complete an automated room-light loop using a light sensor, microprocessor and lamp actuator.
Give me a hint
State what is measured before choosing an output.
Compare my reasoning
- The light sensor measures brightness.
- The microprocessor compares it with a target level.
- The lamp actuator switches on if the room is too dark.
Sensor measures brightness → processor compares with target → lamp switches on when needed.
Look for these in your work
- Input identified
- Processing decision
- Output identified
A water tank uses a level sensor to control a valve. Describe the response when the level is below the minimum.
Give me a hint
Compare the reading with the stored lower threshold.
Compare my reasoning
- The sensor sends a low-level reading.
- The processor compares the reading with the minimum level.
- The valve actuator opens to allow water into the tank.
The sensor reports a low level; the processor detects it is below the threshold and opens the valve actuator.
Look for these in your work
- Sensor role
- Decision
- Actuator response
A system's sensor is disconnected, but the actuator still receives a command. Explain why the response may be unreliable and suggest one design safeguard.
Give me a hint
The processor needs trustworthy input to make a decision.
Compare my reasoning
- Without a valid reading, the processor cannot know the measured condition.
- A default or stale reading may trigger an incorrect action.
- The system can detect sensor failure and enter a safe state or alert an operator.
The processor lacks current data and may act on an unsafe assumption. Detect invalid or missing readings and use a safe fallback or alert rather than treating them as normal.
Look for these in your work
- Missing input
- Risk explained
- Safeguard
Common mistakes
- Calling the sensor the decision-maker
- Calling an actuator the source of measurement
What can you explain now?
Which component measures a condition?
Compare with the explanation
Sensor
The sensor detects a physical input and sends the measurement to the system.
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Make it stick
What are the roles of a sensor, microprocessor and actuator?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.