Sensors and Actuators
Sensors and Actuators
Syllabus Topic 3.2: Interacting with the Real World
1. The Analogue Problem
Physical properties in the real world (light, heat, pressure) are Analogue—they are continuous and constantly changing. However, the Microprocessor is a Digital device—it only understands discrete $1$s and $0$s.
Analogue-to-Digital Converter
Converts sensor signals for the CPU.
Digital-to-Analogue Converter
Converts CPU commands for Actuators.
An ADC is always needed on the input side, because every sensor produces an analogue reading and the processor cannot accept one. On the output side, a DAC is needed only when the actuator requires a varying analogue signal — a motor whose speed must be adjusted, for example. An actuator that is simply switched on or off, such as a buzzer or a heater, can be driven straight from a digital output.
2. Common Sensors (Input)
A sensor is an input device that continually measures a physical property and sends the readings to a microprocessor. It measures — it does not decide anything.
| Sensor | What it measures | Typical application |
|---|---|---|
| Temperature | How hot or cold something is | Central heating, ovens, greenhouses, car engines |
| Light | Intensity of light falling on it | Automatic street lights, phone screen brightness |
| Pressure | Force applied to a surface | Electronic scales, security mats, tyre monitors |
| Proximity | How close an object is | Car parking sensors, automatic doors |
| Infrared | Infrared radiation, or a broken infrared beam | Burglar alarms, motion detectors, remote controls |
| Humidity | Water vapour present in the air | Greenhouses, museums, air conditioning |
| Moisture | Water content in soil or a material | Automatic irrigation, damp detection in buildings |
| pH | Acidity or alkalinity of a liquid | Soil monitoring, swimming pools, fish tanks |
| Gas | Presence or concentration of a specific gas | Carbon monoxide alarms, air-quality monitoring |
| Flow | Rate at which a liquid or gas is moving | Water meters, medical ventilators, fuel systems |
| Level | Depth or height of a liquid | Fuel tanks, water treatment, washing machines |
| Magnetic field | Presence and strength of a magnetic field | Door and window contacts in alarm systems |
| Acoustic | Sound or vibration | Detecting broken glass, leaks in pipes |
| Accelerometer | Acceleration, and change of tilt or orientation | Rotating a phone screen, airbag deployment, fitness trackers |
3. Actuators (Output)
An Actuator is an output device that carries out a physical action based on commands from the microprocessor.
App: Automated irrigation, Central heating systems.
App: Washing machines, Robot arms, Fans.
App: Microwave "finish" chime, Alarms.
App: Smart thermostats, Automated greenhouses.
App: Irrigation, central heating, fish tank filters.
App: Automatic street lighting, warning indicators.
4. The Role of the Microprocessor
The microprocessor is the decision maker. The sensor measures, the actuator acts, and the microprocessor is the only part that compares anything. It runs a loop that repeats continuously:
- Receives Digital data from the ADC.
- Compares this data against a Pre-set Value stored in memory.
- If the condition is met, it sends a signal to switch the actuator on or off — through a DAC if that actuator needs an analogue signal.
- The process repeats, taking new readings continuously.
That last step is not a detail. A system that checked once would not be a control system; it is the constant re-reading that keeps the property within range.
- The temperature sensor continually measures the air temperature and sends an analogue reading.
- The ADC converts each reading to digital so the microprocessor can accept it.
- The microprocessor compares the reading with the stored pre-set value of 25°C.
- If the temperature is below 25°C, it sends a signal to switch the heater on. If it is above, it switches the heater off and may open a vent using a motor.
- The whole cycle repeats indefinitely, so the temperature is held near the pre-set value.
- Always state that "data from the sensor is converted to digital using an ADC so the microprocessor can understand it." This is a standard mark-scheme requirement.
- Say that the readings are taken continuously and the process repeats. Describing a single check does not describe a control system.
- The sensor does not decide anything and cannot switch a device on. It only measures and sends data. The microprocessor makes every comparison and every decision.
- Name the pre-set value the reading is compared against — questions frequently award a mark for it specifically.