A sensor is still, technically, an input device — but instead of a human pressing keys or clicking a mouse, a sensor measures a physical property of its environment: light, sound, temperature, pressure, moisture, and so on. It then converts that physical measurement into digital data the computer can process.
If a keyboard is like a person telling the computer something, a sensor is like the computer having its own eyes, ears, and skin that constantly "feel" the world around it without anyone needing to say a word.
Monitoring systems vs Control systems
This is one of the most commonly tested distinctions in this chapter, so let's be really precise about it:
Monitoring system: tracks the state of a system — it gathers data and may issue warning messages, but it does not take direct action itself.
Control system: actually controls the system based on sensor input — it takes automatic action in response to what the sensor reports.
Here's the classic worked example from the chapter, walked through step by step:
- A temperature sensor in a fish tank checks the water temperature.
- If the temperature drops below the acceptable level, a control system automatically starts up a heater (this is an actuator — an output device — being triggered).
- The system keeps checking the water temperature readings (these are now the system's outputs being fed back in).
- Once the temperature is back within the acceptable range, the system switches the heater off.
Feedback loop: a process where outputs are recycled and used as inputs, creating a continuous cycle. This is exactly what happens in the fish tank example above — the temperature reading (an output of the sensing process) becomes the next input that decides whether the heater stays on or off.
Quick Test For Yourself
Ask: "Does this system just
report/warn — or does it actually
do something automatically?" If it just reports → monitoring. If it takes automatic action → control.
The 14 sensors you need to know
| Sensor | What it measures | Typical use |
|---|
| Acoustic | Sound levels | Industrial machinery noise, noise pollution monitoring, security systems detecting suspicious sounds |
| Accelerometer | Acceleration rate, tilt, vibration | Detecting sudden vehicle movement (safety features), detecting device orientation in phones |
| Flow | Rate of gas, liquid, or powder flow | Detecting changes in flow through pipes in a water system |
| Gas | Presence of a gas (e.g. carbon monoxide) | Detecting gas levels in confined spaces, detecting gas leaks |
| Humidity | Levels of water vapour | Monitoring humidity in greenhouses |
| Infra-red | Motion or a heat source | Security systems detecting intruders breaking a beam; emergency services detecting people via heat radiation |
| Level | Liquid levels | Petrol in a car tank, water in a tank, detecting a leak via a drop in level |
| Light | Light levels | Automatically switching on street lights or headlights when it gets dark |
| Magnetic Field | Presence and strength | Anti-lock braking systems, monitoring rotating machinery like turbines |
| Moisture | Presence and levels of moisture | Soil moisture monitoring, dampness in buildings |
| pH | Acidity or alkalinity | Optimum soil growing conditions, chemical process monitoring |
| Pressure | Gas, liquid, or physical pressure | Tyre pressure, pipe pressure during manufacturing |
| Proximity | Distance | Position of objects in robotics, preventing collisions (e.g. reversing sensors in cars) |
| Temperature | Temperature (°C, °F, or K) | Maintaining swimming pool temperature, controlling chemical process temperature |
Common Mistake
Students often confuse
Infra-red and
Proximity sensors since both can detect "something nearby." Remember: infra-red typically detects motion or heat (breaking a beam / detecting body heat), while proximity specifically measures
distance to an object — this is the one used for parking/reversing sensors, not infra-red.
Worked Example (from the source material)
What type of sensor would be used to help drivers when reversing? Is this a monitoring system or a control system?
Practice Question
A smart greenhouse automatically opens its roof vents when humidity gets too high, then closes them again once humidity drops back to normal. (a) Which sensor is being used? (b) Is this a monitoring or control system? (c) Explain why this is an example of a feedback loop.