Input and output devices

Computer Science · Hardware. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An input device lets data enter a computer, for example a keyboard, mouse, microphone, scanner or touch screen. An output device lets the computer show or use its results, for example a monitor, printer, speaker, projector or actuator. Without them a computer could not take in data or give anything back.

A sensor is an input device that measures a physical property of its surroundings, such as temperature or light, and sends the reading to a computer. Sensors allow automatic monitoring and control without a person.

In questions, match the sensor to the property being measured, and know how the common devices work.

Rules to remember

Sensor readings are usually analogue, so an analogue-to-digital converter (ADC) changes them to digital data
A sensor only measures. The microprocessor makes the decision and an actuator carries out the movement
Temperature: heat. Light: brightness. Humidity: water vapour in air. Moisture: water in soil. pH: acidity
Pressure: force or weight. Acoustic: sound. Gas: a gas level. Level: height of liquid. Flow: rate of liquid or gas movement
Infra-red and proximity: something present or near. Accelerometer: movement and tilt. Magnetic field: magnetism
Touch screens: resistive (pressure joins two layers), capacitive (a finger changes the charge), infra-red (a finger breaks light beams)
Printers: laser (charged drum, toner, heat), inkjet (sprays liquid ink), 3D (builds an object layer by layer)

Common mistake

Students write that the sensor decides what to do. A sensor only sends data; the microprocessor compares it with a stored value and makes the decision.

Worked example

A street light must switch on by itself when it gets dark. Describe how the system works.

Brightness is the property to measure, so a light sensor is used.
Its analogue reading is changed to digital by an ADC and sent to the microprocessor.
The microprocessor compares the reading with a stored value.
If the reading is lower than the stored value, it sends a signal to switch the light on. The process repeats continuously.

Answer: A light sensor, with the microprocessor comparing each reading to a stored value.

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