Sensors and actuators are the physical interface between a control system and the real world. Sensors convert physical quantities into electrical signals that the controller can read; actuators convert control signals into physical actions. Selecting the right sensor and actuator for a given application is a fundamental engineering design decision.
KEY TAKEAWAY: The sensor must be able to measure the controlled variable accurately over the required range; the actuator must be able to produce sufficient force, speed, or output to achieve the desired system response. Both must be compatible with the controller’s electrical specifications.
A sensor is a transducer that converts a physical input (temperature, light, position, speed, pressure) into an electrical signal (voltage, current, resistance, or digital pulse) that the controller can process.
| Criterion | Consideration |
|---|---|
| Measured quantity | Matches the controlled variable (temperature, position, speed, etc.) |
| Output type | Analogue (continuously variable voltage) or digital (HIGH/LOW) |
| Range | Covers the full range of expected values |
| Sensitivity | Sufficient resolution to detect required changes |
| Accuracy | Error is within acceptable limits for the application |
| Response time | Fast enough to track changes in the controlled variable |
| Operating conditions | Suitable for temperature, humidity, vibration in the environment |
| Interface | Compatible with microcontroller input type (analogue, digital, PWM, I2C/SPI) |
| Sensor | Physical quantity | Output type | Typical application |
|---|---|---|---|
| Thermistor (NTC) | Temperature | Analogue (resistance → voltage via divider) | Temperature control, over-temp protection |
| Thermocouple | Temperature (high range) | Analogue (small voltage) | Industrial furnaces, engine management |
| LDR | Light intensity | Analogue (resistance) | Automatic lighting, light-triggered control |
| Ultrasonic (HC-SR04) | Distance | Digital (pulse width) | Object detection, level measurement |
| IR sensor | Proximity / object presence | Digital HIGH/LOW | Obstacle detection, line-following robot |
| Photointerrupter | Speed, position (encoder) | Digital pulse train | Motor speed feedback, position counting |
| Hall effect sensor | Magnetic field, speed | Digital or analogue | Motor speed sensing, position detection |
| Limit switch (microswitch) | Mechanical contact | Digital HIGH/LOW | End-of-travel detection, door position |
| Strain gauge / load cell | Force, weight | Analogue (resistance bridge) | Weighing systems, force measurement |
| Potentiometer | Angular or linear position | Analogue voltage | Manual control input, position feedback |
| Microphone | Sound pressure | Analogue | Sound-activated control, voice detection |
EXAM TIP: For each sensor in a described control system, state: (1) the physical quantity it measures, (2) its output type (analogue or digital), and (3) how its output connects to the controller.
An actuator converts an electrical control signal into a physical output — motion, force, light, sound, or heat.
| Criterion | Consideration |
|---|---|
| Output type | Rotational, linear, light, heat, sound |
| Force/torque | Sufficient for the mechanical load |
| Speed/range | Achieves required output in required time |
| Control type | ON/OFF, analogue proportional, or PWM speed control |
| Power supply | Voltage and current requirements must be met |
| Interface | Can be driven directly from controller or requires a driver circuit |
| Actuator | Output | Control method | Typical application |
|---|---|---|---|
| DC motor | Continuous rotation | PWM (speed), H-bridge (direction) | Conveyor, fan, pump |
| Servo motor | Angular position (0–180°) | PWM pulse width | Robot joint, camera gimbal |
| Stepper motor | Precise angular steps | Step/direction pulses | 3D printer, CNC positioning |
| Solenoid | Linear push/pull | ON/OFF | Valve, door latch, stamping |
| Relay | Switching higher-power circuit | ON/OFF | Mains voltage switching, motor start |
| LED / lamp | Light | ON/OFF or PWM (brightness) | Indicator, status display |
| Buzzer / speaker | Sound | ON/OFF or frequency | Alarm, notification |
| Heating element | Heat | ON/OFF or PWM | Temperature control |
APPLICATION: A microcontroller’s output pin typically sources/sinks only 20–40 mA. Motors, solenoids, and relays require far more. Always use a transistor switch or dedicated motor driver IC (e.g. L298N) between the microcontroller and the load.
The central design question is: what physical variable needs to be controlled, and what sensor/actuator pair enables that control?
| Controlled variable | Sensor | Actuator |
|---|---|---|
| Temperature | Thermistor, thermocouple | Heating element, fan, Peltier module |
| Motor speed | Hall effect encoder, photointerrupter | DC motor (via PWM driver) |
| Position / angle | Potentiometer, optical encoder | Servo, stepper, DC motor + limit switches |
| Liquid level | Ultrasonic, float switch | Pump, valve (solenoid) |
| Light level | LDR, photodiode | LED array, motorised blind |
| Object presence | IR sensor, ultrasonic, limit switch | Conveyor stop, alarm buzzer |
Worked example — Automatic greenhouse ventilation:
VCAA FOCUS: A common extended response question presents a system description and asks you to: (a) identify the sensor and actuator, (b) describe their operating principles, (c) explain why they were chosen, and (d) describe the control logic. Practise writing complete, structured answers to this type of question.
Raw sensor outputs often require conditioning before the controller can use them:
| Conditioning step | Purpose | Example |
|---|---|---|
| Voltage divider | Convert resistance change to voltage | Thermistor + fixed resistor |
| Amplification | Boost small signal to ADC range | Thermocouple amplifier module |
| Filtering | Remove electrical noise | RC low-pass filter on analogue input |
| Digital conversion | Analogue → digital | Built-in ADC in microcontroller |
| Pulse counting | Frequency → speed | Timer/counter peripheral counting encoder pulses |
STUDY HINT: When drawing a sensor interface circuit, always include the signal conditioning stage. A thermistor alone is not connected directly to a microcontroller — it is one leg of a voltage divider that produces a usable voltage proportional to temperature.
