In this chapter, we will explore input and output devices not just by their definition, but also by how they work internally. This is essential knowledge for both theoretical understanding and IGCSE exam preparation.
INPUT DEVICES – How They Work
Input devices are used to send data or signals to a computer system. Let’s look at each device and its internal working process.
1. Keyboard
Function:
Used to enter text, numbers, and commands.
How It Works:
- Each key is part of a matrix (grid) of rows and columns.
- When a key is pressed, it completes a circuit at the intersection of a row and a column.
- The keyboard controller identifies the pressed key by scanning the matrix.
- A scancode is sent to the computer, which maps it to a character via the OS.
2. Mouse
Function:
Controls the on-screen pointer.
How It Works:
- Optical mice use a small LED and a sensor to detect movement.
- The sensor takes rapid images of the surface.
- An onboard DSP (Digital Signal Processor) compares images to detect direction and speed.
- Movements are translated into pointer motion on the screen.
3. Touchscreen (Capacitive Type)
Function:
Combines input (touch detection) and output (display).
How It Works:
- The screen is coated with a capacitive layer that stores electrical charge.
- When touched, a small amount of charge is transferred from the screen to the user’s finger.
- Sensors around the screen detect the change in capacitanc
- A controller calculates the touch location.
Read regarding all touchscreens in detail here. (Please note this is also part of the IGCSE syllabus.)
4. Microphone
Function:
Captures sound input for processing or transmission.
How It Works:
- Sound waves hit a diaphragm (thin membrane) inside the mic.
- The diaphragm vibrates, moving a coil or changing capacitance.
- This movement is converted into an analog electrical signal.
- The sound card or ADC (Analog to Digital Converter) digitizes the signal for the computer.
5. Scanner (Flatbed)
Function:
Converts physical documents/photos into digital images.
How It Works:
- The document is placed on a glass plate.
- A light source (LED or fluorescent) illuminates the document.
- A mirror and lens system reflects the image to a CCD (Charge-Coupled Device) or CIS sensor.
- The image is converted into digital data by an ADC and stored as a file.
6. Bar Code Reader
Function:
Reads data stored in barcode format.
How It Works:
- A laser beam is directed at the barcode.
- Light reflects differently from black (low reflection) and white bars (high reflection).
- A photoelectric sensor captures the reflected light and converts it into electrical signals.
- The pattern is decoded into numbers/letters by the barcode reader’s decoder.
7. Optical Mark Reader (OMR)
Function:
Reads marks made in predefined positions (e.g., exams, surveys).
How It Works:
- A light source scans the paper.
- Marked areas reflect less light.
- Sensors detect these variations and convert them into binary data.
- The software matches positions to known answer templates.
8. Optical Character Recognition (OCR)
Function:
Reads printed or typed text and converts it into editable digital text.
How It Works:
- The document is first scanned like an image.
- OCR software analyzes the shapes of characters.
- It compares patterns to a predefined character set.
- Recognized characters are output as text data.
9. Magnetic Stripe Reader
Function:
Reads information stored magnetically on a card stripe.
How It Works:
- When the card is swiped, a read head detects changes in magnetic fields.
- The stripe contains tiny magnetic particles aligned to represent data.
- Changes are interpreted as binary data and sent to the system.
10. Chip Reader (Smart Card Reader)
Function:
Reads data from embedded microchips in cards.
How It Works:
- Card is inserted into a reader.
- Gold contacts connect to the chip.
- A secure data exchange protocol (like EMV) reads the card’s memory.
- Often used for payments or ID authentication.
11. Biometric Devices (e.g., Fingerprint Scanner)
Function:
Captures and verifies biological traits for authentication.
How It Works (Fingerprint):
- The finger touches a capacitive sensor or optical scanner.
- Ridges and valleys of the fingerprint are mapped.
- The image is converted into a digital template.
- The template is compared to stored records for identification.
OUTPUT DEVICES – How They Work
Output devices allow the computer to present information to the user in various formats—visual, auditory, or physical.
1. Monitor (LCD/LED)
Function:
Displays visual output—text, images, UI.
How It Works:
- Liquid Crystal Display (LCD) uses liquid crystals that twist when electrically charged to control light passage.
- A backlight (LED) shines through pixels.
- Each pixel contains RGB sub-pixels that adjust intensity to form color images.
2. Printer
Function:
Produces a hard copy (physical printout) of documents or images.
How It Works:
Inkjet Printer:
- Tiny nozzles spray ink onto paper.
- Uses thermal or piezoelectric technology to eject ink droplets.
- Good for photo and color printing.
Laser Printer:
- A laser beam draws the image on a drum.
- The drum is electrically charged to attract toner powder.
- The image is transferred to paper and fused using heat.
3. Speaker
Function:
Produces sound output.
How It Works:
- Electrical signals pass to a voice coil inside a magnet.
- The coil vibrates a diaphragm, creating sound waves.
- Converts digital audio to analog signals via DAC (Digital-to-Analog Converter).
4. Headphones
Function:
Private audio output.
How It Works:
- Works just like speakers but on a smaller scale.
- May include noise cancellation, which uses microphones to detect ambient sound and play an inverse wave to cancel it out.
5. Projector
Function:
Projects visual output onto a screen.
How It Works:
- Uses DLP (Digital Light Processing) or LCD technology.
- Light passes through color wheels or LCD panels.
- The image is focused through a lens and projected onto a surface.
6. LED Indicator
Function:
Gives simple visual feedback (e.g., power, charging).
How It Works:
- A current flows through a light-emitting diode (LED).
- The LED emits light based on its material and voltage.
- Often controlled by logic circuits or microcontrollers.
Summary Table: How Input and Output Devices Work
| Device | Category | Working Principle |
|---|---|---|
| Keyboard | Input | Matrix grid detects key presses |
| Mouse | Input | Optical image comparison |
| Touchscreen | Both | Capacitive charge detection |
| Microphone | Input | Diaphragm vibration → electric signal |
| Scanner | Input | Light + sensor → digital image |
| Monitor | Output | RGB pixel control with backlight |
| Printer | Output | Ink/toner placed on paper |
| Speaker | Output | Electromagnetic vibration → sound |
| Barcode Reader | Input | Reflectivity analysis of black/white lines |
Key Takeaways for IGCSE
- Understand how each device works—not just what it does.
- Know the components involved: sensors, lasers, microcontrollers, ADCs/DACs.
- Be able to compare devices based on speed, accuracy, and application.
- Use real-world examples to explain device suitability.
- Practice scenario-based questions where you’re asked to recommend input/output hardware.