Learning Objectives
- Explain how a microprocessor applies programmed rules to sensor data.
- Explain how actuators convert control signals into physical actions.
- Distinguish processing decisions from the physical output of a system.
- Construct complete descriptions of automated responses in unfamiliar contexts.
Key Terms
- Programmed rule
- A stored instruction or condition used to decide what output should follow an input.
- Control signal
- A signal sent by a controller to an output device or actuator.
- Actuator
- A device that converts an electrical control signal into a physical effect.
- Motor
- An actuator that creates rotational or linear movement.
- Valve
- A mechanism that controls the flow of a liquid or gas.
- Relay
- An electrically operated switch that allows a low-power control signal to switch another circuit.
- Output state
- The current condition of an output, such as on, off, open, closed, moving or stopped.
The Microprocessor As Controller
The microprocessor receives data from one or more sensors and executes a stored program. The program may compare a reading with a threshold, check several conditions together, calculate an average or decide whether an output should remain in its current state.
A microprocessor does not understand the purpose of the system in the human sense. It follows instructions. If the program states that a fan should be switched on when temperature exceeds a limit, the processor performs the comparison and sends the specified signal when the condition is true.
The program can run repeatedly, allowing the system to react to new readings. It can also store data, produce warnings or send information to a display, but the syllabus focus is on its control role between sensors and actuators.
Decision Rules
Simple rules may contain one condition: IF temperature is above 30 THEN switch on the fan. More realistic systems may combine conditions: IF soil moisture is low AND rain is not detected THEN open the irrigation valve. The exact programming syntax is not required in this topic, but the logical relationship between input conditions and output actions should be clear.
A controller may also maintain an output until another condition is reached. For example, it can keep a pump running until a water-level sensor reports that the tank is full. The later sensor reading causes the next decision.
Actuators Produce Physical Change
An actuator receives a control signal and changes something in the physical environment. A motor can rotate a fan, drive a conveyor or move a robotic joint. A valve can allow water to flow. A relay or electronic switching circuit can turn a heater, lamp or alarm on and off.
The actuator is the link from digital control to real-world action. The microprocessor may produce only a small electrical signal, so an interface or driver circuit may be needed to operate a higher-power device. For syllabus answers, the important point is that the controller sends a signal and the actuator carries out the physical response.
Matching Actuators To Required Outputs
| Required Physical Action | Possible Actuator |
|---|---|
| Move a conveyor or robotic arm | Motor |
| Open or close water flow | Solenoid valve or motorised valve |
| Switch heating or lighting | Relay or electronic switching circuit |
| Create sound | Buzzer or speaker controlled as an output |
| Change vehicle direction | Motor or steering actuator |
| Release an item in a game or industrial process | Motorised mechanism |
Input And Output Must Not Be Confused
In many questions, the same physical system contains both sensors and actuators. An automatic door may use a proximity sensor as input and a motor as the actuator. The sensor detects a person; the motor moves the door. Writing that the proximity sensor opens the door skips the processing and output stages.
A complete description is: the proximity sensor detects a person and sends data to the microprocessor; the microprocessor applies the programmed rule; it sends a control signal to the motor; the motor opens the door. A later reading can cause the door to close when the area is clear.
Safe And Predictable Control
Automated outputs can affect machinery, vehicles and people. The program must therefore define safe behaviour when readings are missing or outside expected limits. A system may stop a motor, close a valve or alert a human if a fault is detected.
Candidates do not need to design complex safety systems, but they should recognise that a microprocessor can make decisions only from its program and input data. Incorrect programming, failed sensors or failed actuators can lead to incorrect physical actions.
Worked Examples
Automatic Tank Filling
Question: Describe how a tank can be filled automatically without overflowing.
- A level sensor measures the water level.
- The microprocessor compares the reading with the full level.
- If the level is below the target, it sends a signal to open a valve or run a pump.
- When the sensor reports the full level, the program sends a signal to close the valve or stop the pump.
Answer: The sensor, microprocessor and actuator create a feedback loop that fills the tank and stops at the required level.
Separating The Roles
Question: In an automatic greenhouse, which component decides to open a vent and which component physically opens it?
- The sensor supplies temperature data.
- The microprocessor applies the rule and makes the decision.
- A motor or other actuator moves the vent.
Answer: The microprocessor decides; the actuator physically opens the vent.
Examination Guidance
- Use verbs that match each component: sensors measure, processors compare or decide, actuators move or switch.
- Describe the control signal from the microprocessor to the actuator.
- Name a suitable actuator rather than writing only “output device.”
- When several conditions are given, explain how the program combines them.
- For safety or reliability, trace how a fault could lead to an incorrect physical action.
Common Mistakes
- Claiming that the microprocessor itself moves a mechanical part.
- Naming an actuator that cannot produce the required action.
- Ignoring the programmed comparison or rule.
- Describing only the first output and not how later readings can stop or change it.
Knowledge Check
1. What does the microprocessor do with sensor data?
2. What is an actuator?
3. Which actuator could open an irrigation pipe?
4. Why may a relay be used?
5. Who makes the decision in an automated door system?
