I.7 Input Devices

⬆️ Back to: P-I.I. Introduction to Computing

🧭

Page Outline — click a heading to jump.

🎯

Exam pattern (2016–2026): Not asked. Keep it for MCQs only.

---

1. Input Devices at a Glance

2. Grouping by Data Captured

• Text: keyboard.
• Pointing: mouse, trackball, joystick, touchpad, touchscreen.
• Image/optical: scanner, digital camera, barcode reader.
• Audio/video: microphone, webcam.
• Handwriting/drawing: light pen, graphics tablet.
• Exam tip: group devices this way in long answers instead of a random list.

3. Device-by-Device Notes

3.1 Keyboard

• How: key press closes a circuit → controller sends a scan code.
• Types: membrane, mechanical, virtual.

3.2 Mouse

• How: LED/laser and optical sensor track surface movement.
• Types: ball (obsolete), optical, laser.

3.3 Microphone

• How: diaphragm vibrates → electrical signal → sound card digitises it.
• Types: dynamic (rugged, no power), condenser (sensitive, needs power).

3.4 Webcam and digital camera

• How: lens focuses light on a CMOS/CCD sensor → pixel data.
• Webcam: live stream over USB; camera: stored images.

3.5 Joystick

• How: sensors measure tilt and direction → axis values.
• Use: flight simulators, games, industrial controls.

3.6 Barcode reader

• How: light reflects off bars → sensor decodes pattern.
• Types: laser, CCD, 2D (also reads QR codes).
• Optical, not magnetic.

3.7 Trackball

• Stationary; only the ball moves.
• Pro: little desk space, less wrist strain.
• Con: slower for big movements.

3.8 Light pen

• How: sensor detects the CRT scanning beam at the touch point.
• Status: largely obsolete with LCD screens.

3.9 Graphics tablet

• How: sensor grid reads stylus position, pressure and tilt.
• Pressure levels: commonly 2,048–8,192 on professional units.
• Use: digital art (Wacom), CAD, signature pads.

4. Scanners

4.1 How a scanner works

• Light hits the page → sensor measures reflected light, line by line.
• CCD sensor: mirrors and lens; sharper, better depth.
• CIS sensor: sits close to the page; thinner, cheaper, low power.

4.2 Types

• Flatbed: page still, sensor moves.
  • Best for books, fragile papers, photos; slow for many pages.
• Sheet-fed / ADF: sensor still, page moves through rollers.
  • Best for large stacks of loose paper; often duplex; no bound books.
• Handheld: user drags it; portable, uneven quality.
• Drum: rotating drum + photomultiplier tube; top quality, expensive.
• Film/slide: light passes through film.

4.3 Flatbed vs ADF

4.4 DPI and colour depth

• DPI: samples per inch in each direction.
  • Text/OCR ≈ 300 DPI; photos ≈ 600–1200+ DPI.
• File size grows with the square of DPI.
• Worked example — A4 page (8.27 × 11.69 in), 24-bit colour (3 bytes/pixel):
  • At 300 DPI: $(8.27 \times 300) \times (11.69 \times 300) = 2481 \times 3507 \approx 8.70 \text{ million pixels}$
  • Size: $8.70\text{M} \times 3 \text{ bytes} \approx 26.1 \text{ MB}$
  • At 600 DPI: $4962 \times 7014 \approx 34.8 \text{ million pixels} \rightarrow \approx 104.4 \text{ MB}$
  • Doubling DPI → $2^2 = 4$ times the size.
• Colour depth: 1-bit = black/white; 8-bit = 256 greys; 24-bit ≈ 16.7 million colours.
• Optical vs interpolated DPI: only optical DPI adds real detail.

4.5 OCR and related tech

• OCR: software that turns a scanned image of text into editable, searchable text.
  • Needed because a scan is only a picture (a grid of dots).
  • Without it: no search, no copy-edit, no screen reader.
• ICR: handwriting recognition.
• OMR: bubble answer sheets (FPSC/PPSC papers).
• MICR: magnetic ink on bank cheques.
• CSS angle: Punjab's LRMIS land-record digitisation used this scan-then-recognise pipeline.

---

5. ⚠️ MCQ Traps

• Trackball is stationary; the mouse moves as a whole.
• Barcode reader is optical, not magnetic.
• OCR is software, not scanner hardware.
• Flatbed: sensor moves. ADF: paper moves.

6. ✅ Self-Test Questions (with Answers)

Q1. List three pointing/directional input devices.

• Mouse: optical sensor → relative cursor movement.
• Trackball: stationary; ball rotation moves cursor (CAD, tight spaces).
• Joystick: tilt and direction → axis values (simulators, games).
• Also acceptable: touchpad, touchscreen (absolute coordinates), light pen.

Q2. Mouse vs trackball?

• 🧠 A trackball is an upside-down mouse.

Q3. Two devices for handwriting or freehand drawing?

• Graphics tablet: stylus on sensor grid; pressure-sensitive; Wacom, signature pads.
• Light pen: detects CRT beam timing; now obsolete.
• CSS angle: signature pads at NADRA centres and in branchless banking.

Q4. Flatbed vs ADF scanner?

• Mechanism: flatbed moves the sensor; ADF moves the paper.
• Use: flatbed for books/photos; ADF for bulk loose sheets.
• Use the table in 4.3.

Q5. What does DPI measure, and how does it affect quality and size?

• Measures: samples per inch.
• Quality: higher DPI = finer detail.
• Size: grows with DPI squared (300 → 600 DPI = 4× size).
• Show the A4 working from 4.4.

Q6. What is OCR and why is it needed after scanning?

• OCR: software that recognises characters in an image.
• Why: a scan is only pixels; OCR makes it searchable, editable and readable by screen readers.
• Trap: OCR is software, not hardware.