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OCR H446 · Component 1 · 1.1.3

Input and Output
Devices

OCR A Level Computer Science · cszone.co.uk
H446 SpecA Level
Learning Objectives

By the end of this topic you will be able to:

Describe how a range of input devices work and their use cases
Describe how a range of output devices work and their use cases
Explain how sensors convert real-world signals to digital data
Discuss the role of actuators in control systems
Input Devices

Common Input Devices

Manual Input
Keyboard: electrical signals from key presses, converted to ASCII/Unicode character codes
Mouse: optical sensor tracks movement; scroll wheel and buttons generate signals
Touchscreen: capacitive (detects finger conductivity) or resistive (pressure)
Automated Input
Barcode reader: laser reflects off bars; reflected pattern decoded to digits
QR code scanner: camera captures 2D matrix; software decodes pattern
Digital camera: CCD/CMOS sensor converts light intensity to pixel values
Input Devices

Sensors

Sensors convert analogue real-world signals into digital data for processing. An ADC (Analogue-to-Digital Converter) samples the signal at regular intervals and quantises it to binary values.
Temperature
Thermistor changes resistance with temperature. Voltage divider outputs proportional analogue voltage → ADC converts.
Light (LDR)
Light Dependent Resistor: resistance decreases as light intensity increases. Used in cameras, automatic lighting.
Pressure/Sound
Microphone: sound waves vibrate diaphragm → electrical signal. Pressure sensor: piezoelectric material.
Sampling rate and bit depth determine quality of digitised data. Higher sampling rate → more accurate representation (Nyquist theorem: sample at ≥2× the highest frequency).
Output Devices

Common Output Devices

Visual Output
LCD/LED monitor: pixels controlled by liquid crystals or LED arrays; resolution, refresh rate, colour depth
OLED screen: each pixel emits own light; higher contrast, lower power
3D printer: builds layers of material from STL model file; additive manufacturing
Audio and Physical
Speaker: DAC converts digital signal to analogue; electromagnet vibrates diaphragm to produce sound
Inkjet printer: microscopic droplets of ink fired onto paper; resolution in DPI
Laser printer: laser charges drum; toner attracted to charged areas; heat fuses toner to paper
Control Systems

Actuators in Control Systems

An actuator is an output device that causes a physical effect in the real world in response to a digital control signal. They are key components in embedded control systems.
Motor: electric motor controlled by PWM signal — used in robots, fans, automated doors
Relay/solenoid: electrically controlled switch — used in heating systems, automated valves
LED/buzzer: signal lights and audio alerts in embedded systems (e.g. alarm systems)
Control system loop: Sensor → ADC → Microprocessor → DAC → Actuator. Feedback loop compares actual state to target and adjusts output (closed-loop control).
Exam Practice
OCR H446 Style · 4 marks
A greenhouse monitoring system uses temperature sensors to control heating. Describe the steps from sensing temperature to activating the heater.
[4 marks]
1
Temperature sensor (thermistor) produces an analogue voltage proportional to temperature.
1
ADC (Analogue-to-Digital Converter) samples and converts the analogue value to a digital binary value.
1
Microprocessor compares the digital value to a target temperature (threshold) and generates a control signal if too cold.
1
Actuator (relay/motor) receives the control signal and switches the heater on. Feedback continues to monitor until temperature reaches target.
Common Mistakes

Don’t Lose Marks

!
Forgetting that sensors produce analogue signals that must be converted by an ADC before the CPU can process them. Always mention ADC in control system answers.
!
Confusing actuators with sensors — sensors are input devices (they detect), actuators are output devices (they act). In exam answers, clearly label which is which.
!
Saying a capacitive touchscreen works by pressure — it works by detecting the conductivity of a finger (changes electrical field). Resistive screens work by pressure.
1.1.3a Complete
Well done! ✓
Input and Output Devices
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