Electronic Systems: Inputs (Sensors)

This lesson covers the input stage of electronic systems, focusing on sensors that detect changes in the environment. Electronic systems form part of AQA GCSE Design and Technology (8552), Section 3.1.4.

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The Systems Approach

Electronic systems can be broken down into three stages:

Stage	Function	Examples
Input	Detects a change in the environment (a stimulus)	Sensors, switches, microphones
Process	Makes decisions based on the input signal	Microcontrollers, comparators, timers
Output	Produces a response (action)	LEDs, motors, buzzers, speakers

This Input → Process → Output model is fundamental to understanding all electronic products.

The diagram below shows the electronic systems model with example components at each stage:

graph LR
    A["**INPUT**\nLDR, Thermistor,\nSwitch, Microphone"] -->|Electrical signal| B["**PROCESS**\nMicrocontroller,\nTimer, Comparator"]
    B -->|Control signal| C["**OUTPUT**\nLED, Motor,\nBuzzer, Speaker"]
    C -->|Feedback| A

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What Is a Sensor?

A sensor (also called a transducer) is an electronic component that converts a physical quantity (light, temperature, pressure, sound) into an electrical signal that can be processed by a circuit.

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Light Dependent Resistor (LDR)

An LDR is a component whose resistance changes with light level.

Condition	Resistance	Current Flow
Bright light	Low resistance (e.g. 100 Ω)	High current
Darkness	High resistance (e.g. 1 MΩ)	Low current

How an LDR Works

The LDR is made from a semiconductor material (typically cadmium sulphide). When light photons hit the material, they release electrons, allowing current to flow more easily — hence the resistance decreases.

Applications of LDRs

Application	How the LDR Is Used
Automatic street lights	LDR detects darkness; the circuit switches on the street lamp
Camera exposure meters	LDR measures light level to set the correct shutter speed
Solar garden lights	LDR detects nightfall and activates the LED
Burglar alarm beams	LDR detects when a light beam is broken by an intruder
Greenhouse controllers	LDR monitors light levels for plant growth optimisation

AQA Exam Tip: Remember: LDR resistance decreases in light and increases in darkness. A common error is to say the opposite. Think: "Light = Lots of current = Low resistance."

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Thermistor

A thermistor is a component whose resistance changes with temperature. The most common type used at GCSE is the NTC (Negative Temperature Coefficient) thermistor.

Condition	Resistance (NTC)	Current Flow
Hot	Low resistance	High current
Cold	High resistance	Low current

How a Thermistor Works

In an NTC thermistor, heat energy gives electrons more energy to move, increasing the number of free charge carriers. This reduces resistance.

Applications of Thermistors

Application	How the Thermistor Is Used
Central heating thermostat	Thermistor monitors room temperature; the system switches the boiler on or off
Fire alarm	Thermistor detects rapid temperature rise, triggering the alarm
Car engine temperature gauge	Thermistor in the engine block sends temperature data to the dashboard
Baby bottle warmer	Thermistor monitors water temperature to prevent overheating
Fridge/freezer control	Thermistor maintains the set temperature by switching the compressor on and off
Incubator	Thermistor ensures a constant temperature for premature babies

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Pressure Sensor (Force Sensitive Resistor — FSR)

A pressure sensor or force sensitive resistor (FSR) is a component whose resistance changes with applied force.