Input Devices

This lesson covers the main input devices required by the OCR H446 specification. For each device, you must understand how it works, what it is used for, and its advantages and disadvantages.

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What Is an Input Device?

An input device is any hardware that allows a user (or the environment) to enter data or instructions into a computer system. The data is converted into a digital form the computer can process.

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Keyboard

How It Works

Type	Mechanism
Membrane keyboard	A pressure pad under each key completes a circuit when pressed. Cheaper, quieter, less tactile
Mechanical keyboard	Each key has an individual switch mechanism. When pressed, the switch closes a circuit. More tactile, more durable
Capacitive keyboard	Pressing a key changes the capacitance of a circuit beneath it. No physical contact needed. Used in some premium keyboards

When a key is pressed:

1. The keyboard controller detects which row and column intersection has been activated (using a key matrix).
2. It generates a scan code (a unique number for each key).
3. The scan code is sent to the CPU via an interrupt.
4. The operating system translates the scan code into a character using a character map (e.g. ASCII or Unicode).

Uses

• Text entry, data input, commands, shortcuts.
• Essential for programming, word processing and general computer use.

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Mouse

How It Works

Type	Mechanism
Optical mouse	An LED illuminates the surface beneath the mouse. A small camera sensor takes thousands of images per second. A DSP compares successive images to detect movement direction and speed
Laser mouse	Similar to optical, but uses a laser instead of an LED. More precise and works on more surfaces
Mechanical (ball) mouse	A ball rolls as the mouse moves. Rollers inside detect the ball's movement in the X and Y axes. Largely obsolete

The mouse sends movement data (delta X, delta Y) and button click events to the computer, typically via USB or Bluetooth.

Uses

• Pointing, clicking, selecting, dragging in graphical user interfaces.
• Gaming, graphic design, CAD.

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Touchscreen

How It Works

Type	Mechanism
Capacitive	The screen has a conductive layer. When a finger (which is electrically conductive) touches the screen, it disturbs the electric field at that point. Sensors at the corners measure the change to calculate the touch position. Supports multi-touch
Resistive	Two thin conductive layers separated by a small gap. When pressed, the layers make contact and complete a circuit. The position is calculated from the voltage at the contact point. Works with any object (finger, stylus, glove)
Infrared	A grid of infrared LEDs and sensors around the edges of the screen. When a finger breaks the infrared beams, the position is calculated from which beams are interrupted

Comparison

Feature	Capacitive	Resistive	Infrared
Multi-touch	Yes	No (usually)	Yes
Accuracy	High	Moderate	High
Works with gloves	No (unless special gloves)	Yes	Yes
Durability	Good	Wears over time (layers degrade)	Good
Cost	Moderate-high	Low	High
Common use	Smartphones, tablets	ATMs, industrial panels	Large displays, kiosks

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Scanner

How a Flatbed Scanner Works

1. The document is placed face-down on a glass plate.
2. A bright light source (LED or fluorescent) illuminates a narrow strip of the document.
3. The light reflects off the document surface.
4. A CCD (Charge-Coupled Device) or CIS (Contact Image Sensor) array captures the reflected light.
5. The sensor converts the light intensity into an electrical signal, which is digitised by an ADC (Analogue-to-Digital Converter).
6. The light/sensor assembly moves incrementally along the document, scanning one strip at a time.
7. Software assembles the strips into a complete digital image.

Resolution

Scanner resolution is measured in dpi (dots per inch). Higher dpi = more detail captured = larger file size.

Uses

• Digitising printed documents, photographs, artwork.
• Combined with OCR (Optical Character Recognition) software to convert scanned text into editable digital text.

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Microphone

How It Works

1. Sound waves cause a diaphragm inside the microphone to vibrate.
2. The vibrations are converted into a varying electrical signal (an analogue waveform).
3. An ADC converts the analogue signal into digital data by sampling the waveform at regular intervals.

Key Concepts