AQA 7517 · A-Level Computer Science · ~12 min read
Input Devices
Input devices allow data to enter the computer system. AQA requires understanding of the technology behind them, not just their names.
Keyboard: matrix of switches; pressing a key sends a unique scan code to the CPU
Mouse: optical mouse uses LED/laser reflecting off surface; IR sensor detects movement; encoder calculates x/y displacement
Scanner/Barcode reader: CCD (Charge-Coupled Device) sensors detect reflected light; converts image to digital signal
Microphone: sound waves → diaphragm vibration → electrical signal → ADC → digital audio
Digital camera: light → CCD/CMOS sensor → charge per pixel → digital image data
Touchscreen: capacitive (detects electrical conductivity of finger), resistive (pressure-based layers)
Output Devices
LCD Monitor: liquid crystal panel with backlight; pixels blocked/allowed by liquid crystal orientation controlled by electrical charge
LED Monitor: like LCD but uses LED backlight instead of fluorescent — more energy efficient
OLED: each pixel is its own light source — deeper blacks, better contrast
Inkjet printer: tiny ink droplets fired onto paper from cartridge nozzles; uses thermal or piezoelectric to fire drops
Laser printer: laser beam charges drum electrostatically; toner sticks to charged areas; heat fuses toner to paper
Speaker: DAC converts digital → analogue; amplified electrical signal drives magnet → cone vibration → sound waves
Secondary Storage Technologies
Type
Technology
Access
Speed
Durability
HDD
Magnetic platters; read/write head
Sequential + random
Slower
Fragile — moving parts
SSD
NAND flash memory cells
Random (direct)
Very fast
Durable — no moving parts
Optical (CD/DVD/Blu-ray)
Laser reads pits/lands on disc
Sequential
Slowest
Fragile if scratched
USB Flash drive
NAND flash (like SSD)
Random
Fast
Durable — no moving parts
HDD vs SSD — Detail
HDD: platters spin at 5400–7200 RPM; actuator arm positions read/write head over correct track/sector; data stored magnetically. Access time includes seek time (arm movement) + rotational latency.
SSD: data stored in NAND flash cells as trapped electrons (charge = 1, no charge = 0); no moving parts; access time near-instant; more expensive per GB; limited write cycles.
Exam tip: For each device, know the underlying technology, not just its name. Questions often ask "explain how a laser printer works" or "compare HDD and SSD". Be precise: HDD = magnetic + moving parts + seek time; SSD = NAND flash + no moving parts + faster but limited write cycles. Inkjet vs laser: thermal droplets vs electrostatic drum are common exam angles.
▶
Click through the slides at your own pace. Use arrow keys or click to advance.
Click slide or press arrow keys to navigate
Worksheet — 4.7.3 External Hardware
8 questions · instantly marked · AQA 7517 standard
Q1Explain how an optical mouse detects movement across a surface.[3]
✅ Mark scheme
Mark scheme
An LED or laser shines light onto the surface [1]; the reflected light is captured by a sensor (CMOS/CCD) [1]; an image processor compares sequential frames and calculates the x/y displacement of the mouse [1].
Q2Describe the process by which a laser printer produces a printed page.[4]
✅ Mark scheme
Mark scheme
A laser beam selectively charges the surface of a drum electrostatically [1]; toner (fine powder) is attracted to the charged areas of the drum [1]; paper passes over the drum and the toner is transferred onto it [1]; heat (fuser unit) permanently bonds the toner to the paper [1].
Q3Give two advantages of an SSD over an HDD for use in a laptop.[4]
✅ Mark scheme
Mark scheme
Any two with explanation: Faster access time — no mechanical seek time as no moving parts [1+1]; More durable — no moving parts so resistant to shocks and drops [1+1]; Lighter and smaller — solid state components [1+1]; Quieter operation — no spinning platters [1+1]; Lower power consumption — no motor to spin platters [1+1].
Q4Explain how data is stored on a hard disk drive (HDD).[3]
✅ Mark scheme
Mark scheme
Data is stored magnetically on spinning platters [1]; a read/write head on an actuator arm moves to the correct track and sector [1]; the head magnetises tiny areas of the platter surface to represent 0s and 1s [1].
Q5How does a capacitive touchscreen detect touch?[2]
✅ Mark scheme
Mark scheme
A capacitive touchscreen has a grid of conductors storing an electrical charge [1]; the human finger (being a conductor) distorts the electrical field at the point of contact, which is detected to identify the touch location [1].
Q6Explain how optical storage media (e.g. CD-ROM) stores and reads data.[3]
✅ Mark scheme
Mark scheme
Data is represented as pits and lands (bumps and flat areas) pressed into the disc surface [1]; a laser is shone onto the spinning disc [1]; differences in reflection between pits (land edges cause phase change) and lands are detected by a sensor and interpreted as binary 0s and 1s [1].
Q7How does a microphone convert sound into a digital signal?[3]
✅ Mark scheme
Mark scheme
Sound waves cause a diaphragm inside the microphone to vibrate [1]; this vibration is converted to an analogue electrical signal [1]; an ADC (Analogue-to-Digital Converter) samples the signal and converts it to digital binary data [1].
Q8State one disadvantage of SSDs compared to HDDs.[2]
✅ Mark scheme
Mark scheme
SSDs are more expensive per GB than HDDs [1]; additionally, NAND flash cells have a limited number of write cycles before they wear out [1].
Topic Quiz
Question 1 of 15
You scored
out of 15
Card 1 of 8
Click to reveal definition
🎉
All cards reviewed!
Term
Definition
🎯
Mini Test — External Hardware
10 questions · 10 minutes
⏱ 10:00
Section A — Multiple Choice [5 marks]
Q1An SSD stores data using:
Q2A laser printer uses which process to print?
Q3Capacitive touchscreens detect touch by sensing:
Q4Which storage type has moving mechanical parts?
Q5A microphone converts sound waves to digital using:
Section B — Short Answer [5 marks]
Q6State one advantage and one disadvantage of HDDs over SSDs.
Mark schemeAdvantage: cheaper per GB [1]. Disadvantage: slower (seek time due to moving parts) / more fragile / louder [1].
Q7How does an optical mouse detect movement?
Mark schemeLED/laser reflects off surface [1]; sensor compares consecutive images to calculate x/y displacement [1].
Q8What is 'seek time' in an HDD?
Mark schemeThe time taken for the read/write head to move (seek) to the correct track on the platter [1].
Q9What do pits and lands represent on an optical disc?
Mark schemePits and lands are physical features on the disc surface [1]; changes between pit and land (transitions) represent binary data detected by laser reflection differences [1].
Q10Why do SSDs have limited write cycles?
Mark schemeNAND flash cells store data as trapped electrons [1]; each write/erase cycle slightly degrades the insulating layer, eventually preventing the cell from holding a charge reliably [1].