Input and Output Devices
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.
ADC
Analogue-to-Digital Converter
Analogue-to-Digital Converter
Converts sensor signals for the CPU.
➔ CPU ➔
DAC
Digital-to-Analogue Converter
Digital-to-Analogue Converter
Converts CPU commands for Actuators.
2. Common Sensors (Input)
A sensor is an input device that measures a physical property and sends data to a microprocessor.
Light Sensor: Measures ambient light levels.
App: Street lights, Smartphone screen brightness.
App: Street lights, Smartphone screen brightness.
Pressure Sensor: Measures the force applied.
App: Electronic scales, Security mats (burglary detection).
App: Electronic scales, Security mats (burglary detection).
Proximity / Infrared: Detects nearby objects or motion.
App: Automatic doors, Burglar alarms.
App: Automatic doors, Burglar alarms.
Ultrasonic: Measures distance using sound waves.
App: Car parking sensors.
App: Car parking sensors.
Flow Sensor: Measures the rate of liquid/gas movement.
App: Water meters, medical ventilators.
App: Water meters, medical ventilators.
3. Actuators (Output)
An Actuator is an output device that carries out a physical action based on commands from the microprocessor.
Valves: Controls the flow of liquids or gases by opening/closing.
App: Automated irrigation, Central heating systems.
App: Automated irrigation, Central heating systems.
Motors: Creates rotational movement.
App: Washing machines, Robot arms, Fans.
App: Washing machines, Robot arms, Fans.
Beepers / Buzzers: Produces a sound signal.
App: Microwave "finish" chime, Alarms.
App: Microwave "finish" chime, Alarms.
Heaters: Converts electrical energy into heat.
App: Smart thermostats, Automated greenhouses.
App: Smart thermostats, Automated greenhouses.
4. The Role of the Microprocessor
The microprocessor acts as the "decision maker." It follows this logic loop:
- Receives Digital data from the ADC.
- Compares this data against a Pre-set Value (e.g., Is the temp > 25°C?).
- If the condition is met, it sends a Digital signal to the DAC to trigger the Actuator.
⚠️ Exam Tip: When answering questions about sensors, always mention 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.
End of Sensors & Actuators Notes • Ready for Primary and Secondary Storage?