Learning Objectives
- Explain the role of sensors as input devices in automated systems.
- Select a suitable sensor for a stated physical quantity or event.
- Describe how sensor data is converted into a form that a microprocessor can process.
- Explain why repeated sensor readings are essential for feedback and reliable control.
Key Terms
- Physical quantity
- A measurable property such as temperature, pressure, light intensity, moisture or distance.
- Analogue data
- Data that can vary continuously across a range of values.
- Digital data
- Data represented using discrete values that a computer can process.
- Analogue-to-digital conversion
- The conversion of an analogue sensor signal into a digital value.
- Threshold
- A boundary value used by a program to decide when an action should occur.
- Sampling
- Taking measurements at intervals so that changing conditions can be monitored.
- Calibration
- Checking or adjusting a sensor so that its readings correspond accurately to known values.
Sensors As The Eyes And Ears Of Automation
An automated system cannot respond to the real world unless it receives information about current conditions. Sensors perform this input role. They detect or measure a physical property and convert it into an electrical signal or data value that can be supplied to the controller.
The sensor must match the condition being measured. A temperature sensor is appropriate for heating control, while a light sensor is appropriate for automatic lighting. Selecting the wrong type of sensor means that the system receives irrelevant data even if the rest of the program works correctly.
In examination questions, candidates are often given a scenario and asked to identify a sensor. The best answer names the sensor and connects it to the required measurement: for example, “a moisture sensor to measure the water content of the soil.”
Common Sensor Choices In Syllabus Scenarios
| Condition Or Event | Suitable Sensor | Possible Automated Use |
|---|---|---|
| Temperature | Temperature sensor | Greenhouse heating, industrial process control, scientific experiment |
| Light intensity | Light sensor | Street lighting, screen brightness, greenhouse shading |
| Moisture | Moisture sensor | Automatic irrigation |
| Pressure or force | Pressure sensor or pressure pad | Industrial safety, vehicle systems, gaming input |
| Distance or nearby object | Proximity or distance sensor | Robot navigation, automatic doors, parking systems |
| Movement | Motion sensor | Security lighting, interactive gaming |
| Wind speed | Wind-speed sensor | Weather monitoring |
| Rainfall | Rain gauge sensor | Weather recording and agricultural decisions |
From Physical Measurement To Digital Data
Many physical conditions change continuously. Temperature can be 21.1, 21.2 or 21.3 degrees, and light intensity can vary across a wide range. A sensor may therefore produce an analogue electrical signal. A computer, however, processes digital data. An analogue-to-digital converter changes the signal into a digital value that the microprocessor can store and compare.
Not every sensor requires the candidate to describe conversion in detail, but understanding the idea helps explain the complete flow. The essential point is that the microprocessor receives a numerical or logical representation of the measured condition rather than directly receiving heat, light or pressure.
Thresholds And Ranges
A threshold is a value at which the program changes its response. An automatic light may switch on when light intensity falls below a threshold. An alarm may sound when pressure rises above a safe limit.
Some systems use a range rather than a single value. A greenhouse may switch heating on below 20 degrees and off above 22 degrees. Using separate limits can prevent rapid repeated switching when the reading fluctuates around one exact value. The syllabus does not require advanced control theory, but candidates should understand that stored rules decide what action follows a sensor reading.
Sampling And Feedback
A single reading gives information only about one moment. Automated control requires repeated readings. The sensor may sample the environment many times per second or at longer intervals depending on the application. A collision-avoidance system requires very frequent data, while a weather station may record a value every few minutes.
The new readings show whether the actuator has had the intended effect. If an irrigation valve opens, later moisture readings should rise. If they do not, the controller may continue watering or report a fault. This is why sensors are part of the feedback loop rather than only the starting point.
Accuracy Reliability And Placement
The value of an automated decision depends on the quality of the input. A damaged, poorly placed or uncalibrated sensor can cause incorrect actions. A greenhouse temperature sensor placed beside a heater may report a value that does not represent the whole greenhouse. A dirty light sensor may incorrectly indicate darkness.
In scenario evaluation, sensor failure is a valid disadvantage or risk because the microprocessor may make a logically correct decision from incorrect data. Systems can reduce the risk through maintenance, suitable placement, calibration, multiple sensors or human supervision.
Worked Examples
Selecting Sensors For A Weather Station
Question: Choose sensors to measure temperature, rainfall and wind speed.
- Match each required physical quantity to a sensor.
- Use a temperature sensor for air temperature.
- Use an electronic rain gauge for rainfall.
- Use a wind-speed sensor for wind speed.
Answer: The system needs separate sensors because each device measures a different physical quantity.
Using A Threshold
Question: A light should switch on when a sensor reading falls below 30 units. The current reading is 18. What should the controller do?
- Read the stored condition: switch on below 30.
- Compare 18 with 30.
- 18 is below the threshold.
Answer: The microprocessor should send a signal to the switching actuator so that the light turns on.
Examination Guidance
- State what the chosen sensor measures; do not only name it.
- Use the scenario wording to choose the physical quantity, not the desired output.
- Explain that sensor readings are input to the microprocessor.
- Where relevant, mention analogue-to-digital conversion without claiming that all sensors are analogue.
- In reliability questions, connect inaccurate sensor data to incorrect automated decisions.
Common Mistakes
- Choosing a sensor that detects the output rather than the required environmental condition.
- Calling a sensor an actuator because both are connected to the controller.
- Saying the sensor makes the decision.
- Assuming one initial reading is enough for a control system.
Knowledge Check
1. What does a sensor do?
2. Which sensor could be used for automatic irrigation?
3. What is a threshold?
4. Why may an analogue-to-digital converter be required?
5. How can a faulty sensor affect an automated system?
