Electronic Systems: Outputs (Buzzer, LED, Motor & Lamp)

This lesson covers the output stage of electronic systems — the components that produce a physical response (light, sound, movement) based on signals from the processing stage. This completes the study of electronic systems for AQA GCSE Design and Technology (8552), Section 3.1.4.

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Recap: Input → Process → Output

Stage	Function	Key Components
Input	Detect environmental changes	LDR, thermistor, pressure sensor, microphone, switch
Process	Make decisions	Microcontroller, 555 timer, comparator
Output	Produce a response	LED, buzzer, speaker, motor, lamp, solenoid

The output stage converts electrical energy into another form of energy — light, sound, kinetic (movement) or heat.

The diagram below shows how output devices fit into the systems model and the energy conversion that each one performs:

graph LR
    P["**PROCESS**\nMicrocontroller /\n555 Timer"] --> L["LED\n(electrical to light)"]
    P --> B["Buzzer / Speaker\n(electrical to sound)"]
    P --> M["Motor\n(electrical to kinetic)"]
    P --> S["Solenoid\n(electrical to linear motion)"]
    P --> R["Relay\n(switches mains load)"]

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Light Emitting Diode (LED)

An LED is a semiconductor component that emits light when a current flows through it. LEDs are the most common output device in modern electronics.

Key Properties of LEDs

Property	Detail
Forward voltage	Typically 1.8–3.3 V depending on colour (red ~2 V, blue/white ~3.3 V)
Current	Typically 20 mA (0.02 A) for standard LEDs
Polarity	Must be connected the correct way round — anode (+) and cathode (−)
Efficiency	Very high — converts most energy to light, not heat
Lifespan	25,000–50,000 hours
Response time	Switches on/off almost instantly (nanoseconds)

Calculating a Current-Limiting Resistor

An LED must always be used with a current-limiting resistor to prevent it from burning out.

Formula:

$R = \frac{V_{supply} - V_{LED}}{I_{LED}}$R=ILED​Vsupply​−VLED​​

Example: A red LED (V_LED = 2.0 V, I_LED = 20 mA) is connected to a 5 V supply.

$R = \frac{5 - 2}{0.02} = \frac{3}{0.02} = 150 \ \Omega$R=0.025−2​=0.023​=150 Ω

A 150 Ω resistor (or the next standard value up, 180 Ω) should be used.

Types of LEDs

Type	Description	Application
Standard LED	Single-colour indicator (red, green, amber, blue, white)	Power indicators, status lights
RGB LED	Contains red, green and blue elements; can produce any colour by mixing	Mood lighting, displays, decorative products
High-power LED	Much brighter; requires heat sink	Torch/flashlight, room lighting, car headlights
LED strip	Flexible strip with many LEDs	Under-cabinet lighting, decorative accents, signage
7-segment display	Seven LEDs arranged to display digits 0–9	Digital clocks, microwave timers, scoreboards

AQA Exam Tip: Always mention the need for a current-limiting resistor when describing an LED circuit. Forgetting this is a common error that loses marks, especially in circuit-drawing questions.

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Buzzer and Speaker

Buzzer

A buzzer is a simple output device that produces a single-frequency tone when a voltage is applied. There are two main types:

Type	How It Works	Notes
Piezoelectric buzzer	A piezoelectric crystal vibrates when a voltage is applied	Requires an AC signal or built-in oscillator; very thin
Electromagnetic buzzer	An electromagnet vibrates a metal diaphragm	Operates from DC; built-in driver circuit in active buzzers

Speaker

A speaker (loudspeaker) is a more versatile output device that can produce a range of frequencies and volumes, allowing it to play music, speech and sound effects.

How a Speaker Works

1. An audio signal (varying current) flows through a coil of wire (voice coil).
2. The coil sits inside a permanent magnet.
3. The varying current creates a varying magnetic field, causing the coil to move back and forth.
4. The coil is attached to a cone (diaphragm), which vibrates the air, producing sound waves.

Comparing Buzzer and Speaker

Feature	Buzzer	Speaker
Sound range	Single tone	Full range of frequencies
Complexity	Very simple to connect	Requires an amplifier for larger speakers
Cost	Very cheap (~£0.10)	Varies (£0.50 to £100+)
Typical use	Alarms, alerts, timers	Music playback, voice output, announcements
Size	Very small and compact	Varies widely

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Motor

A motor converts electrical energy into kinetic energy (movement). Motors are essential output devices for products that involve rotation or linear motion.

Types of Motors

Type	Description	Application
DC motor	Runs on direct current; speed controlled by voltage or PWM	Fans, toys, small robots, CD drives
Stepper motor	Rotates in precise increments (steps), typically 1.8° per step	3D printers, CNC machines, robots (precise positioning)
Servo motor	Rotates to a specific angle (0–180°) and holds position	Robotic arms, RC aircraft, door locks
Linear actuator	Converts rotary motion into push/pull linear motion	Adjustable desks, car window mechanisms

Motor Control Concepts

Concept	Explanation
Speed control (PWM)	Pulse Width Modulation rapidly switches the motor on and off; a wider pulse = faster speed
Direction control (H-bridge)	An H-bridge circuit allows current to flow in either direction through the motor, reversing its rotation
Motor driver IC	A dedicated chip (e.g. L293D) that handles the high current a motor requires, controlled by a microcontroller
Gearing	A gearbox reduces motor speed but increases torque (turning force)

AQA Exam Tip: If asked about controlling a motor with a microcontroller, mention PWM for speed control and an H-bridge (or motor driver IC) for direction control. Explain that the microcontroller cannot supply enough current directly, so a driver circuit is needed.

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Lamp (Filament and Halogen)

A lamp converts electrical energy into light and heat. Traditional filament lamps are being replaced by LEDs due to energy efficiency, but they are still referenced in the AQA specification.

Filament Lamp

Property	Detail
How it works	Electric current heats a thin tungsten filament to ~2,500 °C, producing light
Efficiency	Very low — ~5% of energy is light; 95% is waste heat
Lifespan	~1,000 hours
Advantages	Cheap to buy; produces a warm, natural light; works with dimmer switches
Disadvantages	Very inefficient; short lifespan; hot to touch (fire risk)

Comparing Light Output Devices

Feature	Filament Lamp	LED
Efficiency	~5% (light), ~95% (heat)	~40–50% (light)
Lifespan	~1,000 hours	~25,000–50,000 hours
Response time	Slow (heats up gradually)	Instant
Cost	Low purchase price, high running cost	Higher purchase price, very low running cost
Heat output	Very hot	Cool to warm
Voltage	Mains (230 V) or low voltage (e.g. 6 V, 12 V)	Typically 2–3.3 V per LED

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Solenoid

A solenoid is an electromagnetic device that converts electrical energy into linear (push/pull) motion. It consists of a coil of wire around a movable iron core (plunger).