3.2 Input & Output Devices

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

Converts sensor signals for the CPU.

➔ CPU ➔
DAC
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.
Pressure Sensor: Measures the force applied.
App: Electronic scales, Security mats (burglary detection).
Proximity / Infrared: Detects nearby objects or motion.
App: Automatic doors, Burglar alarms.
Ultrasonic: Measures distance using sound waves.
App: Car parking sensors.
Flow Sensor: Measures the rate of liquid/gas movement.
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.
Motors: Creates rotational movement.
App: Washing machines, Robot arms, Fans.
Beepers / Buzzers: Produces a sound signal.
App: Microwave "finish" chime, Alarms.
Heaters: Converts electrical energy into heat.
App: Smart thermostats, Automated greenhouses.

4. The Role of the Microprocessor

The microprocessor acts as the "decision maker." It follows this logic loop:

  1. Receives Digital data from the ADC.
  2. Compares this data against a Pre-set Value (e.g., Is the temp > 25°C?).
  3. 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?

Automated Systems

Monitoring & Control Systems

Syllabus Topic 3.2.2: Automated Decision Making

1. The Need for Automated Systems

Automated systems use sensors and microprocessors to perform tasks without human intervention. They offer several advantages:

  • Safety: Can operate in dangerous environments (e.g., nuclear plants or chemical vats) where humans cannot go.
  • Consistency: Do not get tired or bored; they perform the same task exactly the same way 24/7.
  • Speed: Can react to changes (like a pressure spike) much faster than a human could.
  • Efficiency: Saves time and money by optimizing resource use (e.g., only heating a room when someone is in it).

2. Monitoring vs. Control

The fundamental difference lies in whether the system affects the environment or just reports on it.

Observation Only

Monitoring Systems

The system records data and alerts a human if values go outside a safe range. It does not take action to fix the problem.

Example: ICU Patient Monitor.

  • Sensors: Heart rate, Temperature, Oxygen.
  • Process: Microprocessor compares data to pre-set limits.
  • Outcome: Sound an alarm or display a warning on screen.
Action & Feedback

Control Systems

The system records data and takes direct action via actuators to maintain a specific state or "Set Point."

Example: Automated Greenhouse.

  • Sensors: Humidity, Light, Temperature.
  • Process: Microprocessor sends signals to actuators if values drift.
  • Outcome: Open windows (motors), Turn on fans, or start irrigation (valves).

3. System Components & Operation

Both systems follow a similar loop, but only Control systems complete the "Feedback Loop" via actuators.

SENSOR ➔ ADC ➔ MICROPROCESSOR ➔ [ALARM or ACTUATOR]
Component Role in the System
Sensor Continuously measures physical properties (Analogue).
ADC Converts analogue sensor data into digital for the processor.
Microprocessor Compares input to "Pre-set" or "Stored" values.
Actuator (Control only) Performs physical work (Valve, Motor, Heater).

4. Detailed Example: Street Lighting

This is a Control System because it changes the state of the light.

  • Sensor: Light Sensor (LDR) measures ambient light.
  • Pre-set Value: e.g., "If light level < 20%".
  • The Loop:
    1. Sensor sends analogue signal to ADC.
    2. Digital value sent to Microprocessor.
    3. Processor compares value to 20%.
    4. If Lower: Processor sends signal to switch on the lamp.
    5. If Higher: Processor sends signal to switch off the lamp.
⚠️ Exam Note: In a Monitoring system, the output is always an alert (Siren, Light, Message). In a Control system, the output is an actuator action (Opening a valve, starting a motor).
End of Monitoring & Control Notes • Ready for Primary and Secondary Storage?