Sensors

A sensor is an input device that detects and measures a physical quantity
from the real world and converts it into an electrical signal that a computer
can process.

Sensor = A device that detects a physical change in the environment
and sends a corresponding signal to a computer or microprocessor.

2. How Sensors Work — The Signal Chain

Sensors produce analogue signals (continuously varying).
Computers only understand digital signals (binary 0s and 1s).
So every sensor signal must pass through an ADC.

ADC (Analogue-to-Digital Converter): A component that samples the
analogue signal at regular intervals and converts each sample into a
binary value for the computer to read.

3. Types of Sensors — On the CIE Syllabus

4. Each Sensor in Detail

🌡️ Temperature Sensor

Measures heat or cold in the surrounding environment.

Produces a voltage that varies with temperature.

Commonly uses a thermistor (resistance changes with temperature).

Property	Detail
Measures	Temperature (°C / °F)
Output	Analogue voltage
Converted by	ADC

Used in:

Central heating / thermostat systems

Greenhouses (automated climate control)

Weather stations

Refrigerators and freezers

Industrial ovens and furnaces

💡 Light Sensor (LDR — Light Dependent Resistor)

Measures the intensity of light in the environment.

Resistance decreases as light intensity increases.

The changing resistance produces a varying voltage read by the ADC.

Property	Detail
Measures	Light intensity (lux)
Output	Analogue voltage
Converted by	ADC

Used in:

Automatic street lighting (turns on when dark)

Camera exposure control

Burglar alarms (detecting when a light beam is broken)

Greenhouses (monitoring daylight for plants)

🔊 Sound Sensor (Microphone)

Detects sound waves (variations in air pressure).

A microphone converts sound waves into a varying electrical signal.

Signal is analogue and must be converted by an ADC.

Property	Detail
Measures	Sound level / audio (decibels)
Output	Analogue signal
Converted by	ADC

Used in:

Noise monitoring systems

Baby monitors

Security systems (detecting breaking glass)

Voice-activated control systems

⚖️ Pressure Sensor

Measures force applied over a surface area.

Uses piezoelectric materials or strain gauges that change resistance under pressure.

Property	Detail
Measures	Pressure / force (Pascals)
Output	Analogue voltage
Converted by	ADC

Used in:

Airbag deployment systems (detects sudden impact)

Weather stations (atmospheric pressure / barometric)

Industrial machinery monitoring

Intruder detection mats

Tyre pressure monitoring systems

💧 Humidity Sensor

Measures the amount of moisture (water vapour) in the air.

Capacitance or resistance changes as moisture levels change.

Property	Detail
Measures	Relative humidity (%)
Output	Analogue signal
Converted by	ADC

Used in:

Greenhouses (maintaining ideal growing conditions)

HVAC systems (heating, ventilation, air conditioning)

Weather stations

Museums (protecting artwork from damp)

Server rooms (preventing moisture damage)

🔴 Infrared (IR) Sensor

Detects infrared radiation emitted by warm objects (including humans).

Can detect motion (PIR — Passive Infrared sensor) or measure distance.

Property	Detail
Measures	Infrared radiation / motion
Output	Analogue or digital signal
Converted by	ADC (if analogue)

Used in:

Burglar alarm systems (PIR motion detection)

Automatic doors (detecting approaching person)

Proximity sensors in smartphones (screen off during calls)

TV remote control receivers

Body temperature monitoring

🧲 Magnetic Field Sensor (Hall Effect Sensor)

Detects the presence and strength of a magnetic field.

Voltage output changes in the presence of a magnet.

Property	Detail
Measures	Magnetic field strength
Output	Analogue or digital voltage
Converted by	ADC

Used in:

Detecting whether a door or window is open/closed (security systems)

Speed sensors in vehicles (counting wheel rotations)

Compass applications in smartphones

🧪 pH Sensor

Measures the acidity or alkalinity of a liquid on the pH scale (0–14).

Uses a chemical electrode that produces a voltage proportional to pH.

Property	Detail
Measures	pH level (0–14)
Output	Analogue voltage
Converted by	ADC

Used in:

Monitoring water quality in fish tanks or swimming pools

Greenhouses (monitoring soil or hydroponic solution pH)

Food and drink manufacturing

Chemical processing plants

💨 Gas Sensor

Detects the presence or concentration of specific gases in the air.

Resistance changes when target gas molecules interact with the sensor.

Property	Detail
Measures	Gas concentration (ppm)
Output	Analogue signal
Converted by	ADC

Used in:

Carbon monoxide detectors (home safety)

Smoke alarms

Industrial safety monitoring

Breathalyser devices

Greenhouse CO₂ monitoring

📱 Accelerometer

Measures acceleration, tilt, and vibration — detects changes in motion
or orientation.

Property	Detail
Measures	Acceleration (m/s²), tilt
Output	Analogue signal
Converted by	ADC

Used in:

Smartphone screen rotation (portrait/landscape)

Airbag systems (detects sudden deceleration in a crash)

Fitness trackers (counting steps)

Gaming controllers (motion sensing)

Earthquake detection

5. Sensors in Control Systems

Sensors are a critical part of automated control systems and embedded systems.
They form the input side of the sense → process → act loop.

This loop is called a feedback control loop or closed-loop system.

Worked Example — Greenhouse Temperature Control

6. Advantages of Using Sensors in Automated Systems

Advantage	Explanation
24/7 monitoring	Sensors work continuously without breaks
More accurate	No human error in reading values
Faster response	React in milliseconds — much faster than humans
Safer	Can monitor dangerous environments (toxic gas, extreme heat)
Cost-effective	Reduces need for human monitoring staff
Consistent	Same measurement process every time

7. Summary Table — All Sensors

Sensor	Measures	Key Use
Temperature	Heat / cold	Thermostat, greenhouse
Light (LDR)	Light intensity	Street lighting, cameras
Sound	Audio / noise level	Security alarms, monitors
Pressure	Force / pressure	Airbags, weather stations
Humidity	Moisture in air	Greenhouse, HVAC
Infrared (PIR)	Motion / heat	Burglar alarms, auto doors
Magnetic field	Magnetic presence	Door/window sensors
pH	Acidity / alkalinity	Water quality, food industry
Gas	Gas concentration	CO detector, smoke alarm
Accelerometer	Motion / tilt / vibration	Phones, airbags, fitness

8. Key Vocabulary

Term	Definition
Sensor	A device that detects a physical quantity and converts it to an electrical signal
Analogue	A continuously varying signal with infinite possible values
Digital	A signal represented as discrete binary values (0 or 1)
ADC	Analogue-to-Digital Converter — converts analogue sensor output to digital
Actuator	An output device that causes a physical action in response to a signal
Feedback loop	A system where output is fed back as input to maintain a set condition
PIR	Passive Infrared sensor — detects infrared radiation from warm bodies
LDR	Light Dependent Resistor — resistance changes with light intensity
Thermistor	A resistor whose resistance varies with temperature
Set point	The target value a control system aims to maintain

9. Exam Tips ✅

Always state what the sensor measures and give a specific use —
examiners want context, not just the sensor name.

Remember: sensors produce analogue signals → must use an ADC to convert
to digital before the CPU can process it.

In control system questions, describe the full loop:
sensor → ADC → microprocessor → decision → actuator → effect on environment.

A feedback loop means the system continuously re-reads the sensor and
adjusts — mention this for full marks.

The PIR sensor is the most common infrared sensor in exam questions —
link it to burglar alarms and automatic doors.

Sensors make systems faster, safer, more accurate, and available 24/7
compared to human monitoring — these are standard advantages to learn.

Cambridge (CIE) IGCSE Computer Science — 0478 / 0984

2. How Sensors Work — The Signal Chain#

3. Types of Sensors — On the CIE Syllabus#

4. Each Sensor in Detail#

🌡️ Temperature Sensor#

💡 Light Sensor (LDR — Light Dependent Resistor)#

🔊 Sound Sensor (Microphone)#

⚖️ Pressure Sensor#

💧 Humidity Sensor#

🔴 Infrared (IR) Sensor#

🧲 Magnetic Field Sensor (Hall Effect Sensor)#

🧪 pH Sensor#

💨 Gas Sensor#

📱 Accelerometer#

5. Sensors in Control Systems#

Worked Example — Greenhouse Temperature Control#

6. Advantages of Using Sensors in Automated Systems#

7. Summary Table — All Sensors#

8. Key Vocabulary#

9. Exam Tips ✅#

2. How Sensors Work — The Signal Chain

3. Types of Sensors — On the CIE Syllabus

4. Each Sensor in Detail

🌡️ Temperature Sensor

💡 Light Sensor (LDR — Light Dependent Resistor)

🔊 Sound Sensor (Microphone)

⚖️ Pressure Sensor

💧 Humidity Sensor

🔴 Infrared (IR) Sensor

🧲 Magnetic Field Sensor (Hall Effect Sensor)

🧪 pH Sensor

💨 Gas Sensor

📱 Accelerometer

5. Sensors in Control Systems

Worked Example — Greenhouse Temperature Control

6. Advantages of Using Sensors in Automated Systems

7. Summary Table — All Sensors

8. Key Vocabulary

9. Exam Tips ✅