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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.
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
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.
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 |
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.
| 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."
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 |
In an NTC thermistor, heat energy gives electrons more energy to move, increasing the number of free charge carriers. This reduces resistance.
| 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 |
A pressure sensor or force sensitive resistor (FSR) is a component whose resistance changes with applied force.
| Condition | Resistance |
|---|---|
| No force applied | Very high resistance (effectively infinite — open circuit) |
| Force applied | Resistance decreases as force increases |
| Application | How the Sensor Is Used |
|---|---|
| Electronic scales | Pressure sensor measures the force exerted by an object to calculate weight |
| Car seat occupancy detection | Sensor detects whether a passenger is seated (for airbag deployment) |
| Musical instruments | Pressure-sensitive pads on electronic drum kits detect how hard the drummer hits |
| Pedestrian crossings | Pressure pads detect when someone is standing at the crossing |
| Robotic grippers | Pressure sensors allow robots to grip objects without crushing them |
| Medical pressure mats | Detect a patient falling out of bed in hospitals |
A microphone is a transducer that converts sound waves (pressure variations in the air) into an electrical signal.
| Application | How the Microphone Is Used |
|---|---|
| Voice-controlled devices | Smart speakers (Alexa, Google Home) detect voice commands |
| Sound-activated switches | Clap-activated light switches use a microphone to detect sound |
| Security systems | Audio monitoring detects glass breaking or unusual noises |
| Hearing aids | Microphone picks up speech and amplifies it |
| Music recording | Microphones capture vocals and instruments |
| Noise-level monitors | Microphones measure decibel levels in workplaces |
Sensors like LDRs and thermistors are often used in potential divider circuits to produce a varying voltage that can be read by a microcontroller.
A potential divider consists of two resistors connected in series across a power supply. The output voltage is taken from the junction between them.
Formula:
Vout=Vin×R1+R2R2
If one of the resistors is a sensor (e.g. LDR), the output voltage changes as the sensor's resistance changes. This varying voltage is the input signal for the processing stage.
The position of the LDR in the circuit determines whether V_out increases or decreases with light level.
AQA Exam Tip: You may be asked to draw or analyse a potential divider circuit with a sensor. Make sure you understand how the sensor's changing resistance affects the output voltage. Practise both LDR and thermistor versions.
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