← IGCSE Computer Science Paper 1

Choosing the most suitable sensor for a given context

The sensor that measures the physical quantity a system needs, e.g. light for street lamps or pressure for a car park. Justify it with 'because'.

Part of: IGCSE Computer Science 0478 — 3.2 Input and Output Devices — Module 50, IGCSE Computer Science 0478 — 6.1 Automated Systems — Module 25

Understanding Choosing the most suitable sensor for a given context in IGCSE Computer Science Paper 1

What it does: Movement or heat from a person is the quantity to measure, so an infra-red sensor is chosen and justified with 'because'. A moisture sensor measures the water content of soil. An acoustic sensor detects sound. Street lighting depends on how bright it is outside, and a light sensor measures that quantity. The fix is a pressure sensor (or proximity sensor) because it detects the weight or nearness of a car in the space.

Common mistakes: When the infra-red sensor detects movement in a burglar alarm system, the alarm actuator must be switched ON, not OFF. The line should be: alarm ← ON. Watering depends on how dry the soil is, so the physical quantity is moisture. A moisture sensor is the suitable sensor; a pH sensor measures acidity, not dryness. It should be: sensor ← moisture.

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Learning
Duo
A street lamp must switch on automatically when it gets dark. Which sensor is most suitable?
Light sensor, because it measures how bright the surroundings arePressure sensor, because it measures how bright the surroundings are
Matches
Use a temperature sensor to detect a car in a parking space, because it measures cars
↓
The sensor does not measure the needed quantity. A car park needs to detect the presence or weight of a car, not temperature, and 'measures cars' is not a real reason.
Applying
Meanings
Term
suitable sensor
Meaning
The sensor that measures the physical quantity a system needs, e.g. light for street lamps or pressure for a car park. Justify it with 'because'.
Practice
Street lights: light sensor because it detects when it gets dark.
Testing
Gaps
Sensor:
Check Answer
Spots
2IF motion = TRUE THEN
3 alarm ← OFF

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Quick Questions

1. A street lamp must switch on automatically when it gets dark. Which sensor is most suitable?
A. Pressure sensor, because it measures how bright the surroundings are
B. Light sensor, because it measures how bright the surroundings are ✓
The quantity to measure is brightness, so a light sensor is the right choice. A pressure sensor measures force, not light.
2. A burglar alarm must detect someone moving in a dark room at night. Which sensor is most suitable?
A. Infra-red sensor, because it detects the heat given off by a moving person ✓
B. pH sensor, because it detects the heat given off by a moving person
An infra-red (motion) sensor works in the dark and detects body heat. A pH sensor only measures acidity or alkalinity.
3. A greenhouse computer must keep the air warm enough for tomato plants. Which sensor should it use?
A. Magnetic field sensor, because the system needs to measure how hot the air is
B. Temperature sensor, because the system needs to measure how hot the air is ✓
Heat is the physical quantity to measure, so a temperature sensor is needed. A magnetic field sensor measures magnetism.
4. A greenhouse must water plants only when the soil is dry. Which sensor is most suitable?
A. Moisture sensor, because it measures how much water is in the soil ✓
B. Acoustic sensor, because it measures how much water is in the soil
A moisture sensor measures the water content of soil. An acoustic sensor detects sound.
5. A water tank must stop filling when it is full. Which sensor is most suitable?
A. Level sensor, because it measures how high the water has risen in the tank ✓
B. Light sensor, because it measures how high the water has risen in the tank
The quantity to measure is the height of the water, so a level sensor is used. A light sensor measures brightness.

Match the Pairs

Street lamps switch on at dusk: light sensor → The system must measure brightness, so a light sensor is the suitable choice
Greenhouse watering: moisture sensor → The system must measure how wet the soil is, so a moisture sensor is the suitable choice
For a burglar alarm that detects someone moving into a room, find the quantity (movement) and choose the sensor that measures it → Use an infra-red sensor because it detects body heat and movement of an intruder
For a water tank that must not overflow, find the quantity (height of the water) and choose the sensor that measures it → Use a level sensor because it measures how high the water is in the tank
Use a temperature sensor to detect a car in a parking space, because it measures cars → The sensor does not measure the needed quantity. A car park needs to detect the presence or weight of a car, not temperature, and 'measures cars' is not a real reason.
Use a sensor in the greenhouse because it is a good sensor → No sensor is named and no physical quantity is identified, so the justification is vague and gets no marks.

Key Term

suitable sensor
The sensor that measures the physical quantity a system needs, e.g. light for street lamps or pressure for a car park. Justify it with 'because'.

Fill in the Blanks

1. Context: burglar alarm Quantity to measure: heat from a person Sensor: infra-red
Hint: Type the sensor that detects body heat given off by an intruder
2. Context: greenhouse soil needs watering Sensor: moisture Because it measures water in the soil
Hint: Type the sensor that measures how wet the soil is
3. Context: street lamps switch on at night Sensor: light Because it measures how bright it is
Hint: Type the sensor that measures brightness
4. Context: water tank must not overflow Sensor: level sensor Suitable because it measures water height
Hint: Type the connecting word that links the choice to its reason
5. Context: car park shows if a bay is taken Quantity to measure: weight of the car Sensor: pressure
Hint: Type the sensor that detects the force of a car pressing on the bay

Spot the Error

Question 1
A sensor ← infra-red
B IF motion = TRUE THEN
C alarm ← OFF
D ENDIF
Line C — When the infra-red sensor detects movement in a burglar alarm system, the alarm actuator must be switched ON, not OFF. The line should be: alarm ← ON.
Question 2
A // Water greenhouse plants
B sensor ← pH
C IF reading < 30 THEN
D sprinkler ← ON
E ENDIF
Line B — Watering depends on how dry the soil is, so the physical quantity is moisture. A moisture sensor is the suitable sensor; a pH sensor measures acidity, not dryness. It should be: sensor ← moisture.
Question 3
A REPEAT
B level ← light sensor
C IF level > 20 THEN
D lamp ← ON
E UNTIL FALSE
Line C — Street lamps should come on when it is dark, which means a LOW light reading. The condition should be level < 20, otherwise the lamps would switch on in daylight.
Question 4
A sensor ← level
B READ height FROM sensor
C IF height >= 90 THEN
D pump ← ON
E ENDIF
Line D — A level sensor shows the water tank is nearly full at 90 or more, so the pump must be switched OFF to stop it overflowing. It should be: pump ← OFF.
Question 5
A FOR each bay
B sensor ← humidity
C IF reading > 0 THEN
D bay ← FULL
E NEXT
Line B — A car in a parking bay presses on the ground with its weight, so the quantity to measure is pressure. A humidity sensor measures moisture in the air and cannot detect a car. It should be: sensor ← pressure.

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