Street lights that switch on at dusk, a greenhouse that waters itself, a train with no driver: each one senses the world, decides what to do and acts, over and over, without a person in the loop. Learn how the three parts work together and how to weigh up an automated system for any scenario.
An automated system carries out a task with little or no human input. A sensor measures a physical quantity (such as temperature or light) and sends data to a microprocessor. The microprocessor compares the data with stored values (a set point or range) and decides what to do. If action is needed it sends a signal to an actuator (such as a motor, valve, heater or pump), which changes the environment. The sensor keeps taking readings, so the process repeats continuously.
Fig. 1: sensor, microprocessor and actuator working together in a loop
Section 01
Sense, decide, act.
Every automated system in an exam answer works the same way. Learn the steps in order and use the scenario's own sensor, values and actuator.
Sensor: an input device that measures a physical property, e.g. temperature, light, moisture, pressure, infrared, gas, pH, humidity, proximity, acoustic (sound), flow or level.
Analogue to digital: most sensors produce an analogue signal, so an analogue-to-digital converter (ADC) turns it into digital data the microprocessor can process.
Microprocessor: processes the data. It compares the reading with stored values (e.g. a set point of 25 °C) and decides whether action is needed.
Actuator: an output device that physically moves or changes something, e.g. a motor, pump, valve, heater, fan, light or buzzer. The microprocessor sends it a signal to switch on, off or move.
Continuous: the sensor keeps sending readings and the loop repeats, so the system responds to change without a human.
Worked example Automatic greenhouse
Describe how sensors, a microprocessor and actuators keep a greenhouse at the correct temperature.
A temperature sensor measures the temperature in the greenhouse and sends the data to the microprocessor (via an ADC).
The microprocessor compares the reading with the stored set point, for example 25 °C.
If the temperature is below 25 °C, it sends a signal to an actuator to switch the heater on (and close the windows with a motor).
If it is above 25 °C, it signals the heater to switch off and a motor to open the windows.
The sensor keeps taking readings, so the process repeats continuously.
Answer: sense → compare with a stored value → signal an actuator → repeat
Your turn
An automatic door at a shop uses an infrared sensor. Name the actuator and say what the microprocessor checks.
The actuator is a motor that opens and closes the door. The microprocessor checks whether the sensor reading shows that a person is near the door; if so it signals the motor to open it, and after no one has been detected for a set time it signals it to close.
Examiner trap
"The sensor turns on the heater" earns nothing. Sensors only measure and send data; the microprocessor makes the decision and the actuator does the action.
Section 02
Automation everywhere.
The syllabus names seven areas. In every one you should be able to name a sensor, say what the microprocessor compares, and name an actuator.
Area
Example system
Sensor(s)
Microprocessor decides
Actuator(s)
Industry
Chemical plant tank
temperature, pressure, level
is the pressure above the safe limit?
valves, pumps, heaters, alarm
Transport
Driverless train, adaptive cruise control
proximity (radar/ultrasonic), speed, cameras
is the vehicle ahead too close? is the train at the platform?
An automated street light uses a light sensor. Describe what happens at dusk.
The light sensor sends readings to the microprocessor, which compares each reading with a stored value. When the light level falls below the stored value, it sends a signal to switch the lamp on. Readings continue, and when it is bright enough again the lamp is switched off.
Examiner trap
Name the right sensor for the scenario. A greenhouse that waters plants needs a moisture sensor; a temperature sensor alone cannot tell if the soil is dry.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In an automated system, which part decides whether action is needed?
Quick check+10 XP
Which of these is an actuator?
Quick check+10 XP
Why is an analogue-to-digital converter often needed between a sensor and a microprocessor?
Quick check+10 XP
Which sensor is most suitable for an automatic irrigation system?
Section 03
Worth automating?
Exam questions give a scenario and ask for advantages and disadvantages. Generic points are a start, but marks go to points applied to the scenario.
Advantages
Disadvantages
Faster responses than a human, and it works 24/7 without breaks
Expensive to buy, set up and maintain
More accurate and consistent: the same response every time
A sensor or software fault can cause wrong actions, and it cannot cope with situations it was not programmed for
Safer: people do not have to be in dangerous places (heat, chemicals, heavy traffic)
Can be hacked, which may be dangerous in transport or industry
Saves money over time on wages, energy, water or fuel
Fewer jobs for workers it replaces; staff need retraining
Fewer human errors (tiredness, distraction)
People may rely on it and lose skills; it needs power and regular checks
Worked example Apply it
A farm uses an automated irrigation system. Give one advantage and one disadvantage.
Advantage: water is only pumped when the moisture sensor shows the soil is dry, so less water is wasted and crops get the right amount.
It also works day and night, so the farmer does not need to check every field.
Disadvantage: if the moisture sensor fails, the crops may be flooded or left dry without anyone noticing.
The sensors, pumps and microprocessor are expensive to install across a large farm.
Answer: each point is linked to the farm, not just "it is cheaper"
Examiner trap
"It is more efficient" on its own is too vague to earn a mark. Say what is better, e.g. "it responds to a pressure rise faster than a human operator could".
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 1 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
An automated street light is controlled by a microprocessor. What does the microprocessor do with each reading from the light sensor?
Exam style+20 XP
Which is a disadvantage of an automated system in a chemical factory?
Exam style+20 XP
An adaptive cruise control system keeps a car a safe distance behind the car in front. Which pair of devices is most suitable?
Exam style+20 XP
Why does an automated system keep taking sensor readings after an actuator has been switched on?
Skill unlocked
Automated systems, mastered.
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