Sensors & Actuators: Types, Working, Interfacing & Applications

1. Introduction to Sensors and Actuators

Sensors and actuators are the eyes and hands of embedded systems, enabling interaction with the physical world.

• Sensors detect physical quantities (light, temperature, motion) and convert them into electrical signals.
• Actuators convert electrical signals into physical actions (movement, sound, heat).

Why are they essential?

Embedded systems rely on sensors to monitor environments (e.g., temperature in a fridge) and actuators to control devices (e.g., turning on a fan). Without them, systems would be "blind and deaf."

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2. Classification of Sensors

Sensors can be categorized based on:

1. Input Quantity Measured (Physical vs. Chemical)
2. Output Signal Type (Analog vs. Digital)
3. Sensing Mechanism (Active vs. Passive)

Comparison Table: Sensor Types

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3. How Sensors Work: A Deep Dive

A. Analog Sensors

• Output: Continuous voltage/current proportional to input.
• Example: Thermistor (temperature sensor)
  • Working: Resistance changes with temperature (NTC: resistance decreases with heat; PTC: increases).
  • Equation: $$R_T=R_0\times e^{\beta \left(\frac{1}{T}-\frac{1}{T_0}\right)}$$ Where:
    • $R_T$ = Resistance at temperature $T$,
    • $R_0$ = Resistance at reference temperature $T_0$,
    • $\beta$ = Material constant.

Worked Example: Fridge Temperature Control A fridge uses an NTC thermistor to monitor temperature. At 25°C, $R_0=10k\Omega$, $\beta =3950$. If the fridge cools to 5°C, calculate the new resistance. Solution: $$R_T=10k\times e^{3950\left(\frac{1}{278}-\frac{1}{298}\right)}\approx 27.6k\Omega$$ The microcontroller reads this change and adjusts the compressor.

B. Digital Sensors

• Output: Binary data (e.g., SPI/I2C protocol).
• Example: Ultrasonic Sensor (HC-SR04)
  • Working: Emits sound waves and measures echo time to calculate distance.
  • Formula: $$\text{Distance}=\frac{\text{Speed of Sound}\times \text{Time}}{2}$$ (Divide by 2 because the sound travels to the object and back.)

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4. Actuators: Converting Signals to Action

Actuators execute commands from the microcontroller. Types include:

1. Mechanical (Servo motors, Stepper motors)
2. Electrical (Relays, Solenoids)
3. Thermal (Heaters, Coolers)
4. Pneumatic/Hydraulic (Air pressure, fluid flow)

Comparison Table: Actuator Types

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5. Interfacing Sensors and Actuators with Microcontrollers

A. Analog Sensor Interfacing (ADC)

• Analog sensors (e.g., thermistor, potentiometer) require an Analog-to-Digital Converter (ADC).
• Steps:
  1. Connect sensor output to microcontroller’s ADC pin.
  2. Configure ADC settings (resolution, reference voltage).
  3. Read digital value using ADC_Read() (e.g., in AVR or Arduino).

Example Code (Arduino):

int sensorPin = A0;  // Analog pin connected to thermistor
int sensorValue = 0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  sensorValue = analogRead(sensorPin);  // Reads 0-1023
  float voltage = sensorValue * (5.0 / 1023.0);  // Convert to voltage
  Serial.println(voltage);
  delay(1000);
}

B. Digital Sensor Interfacing (SPI/I2C/UART)

• SPI (Serial Peripheral Interface): Fast, full-duplex (e.g., MPU6050 accelerometer).
• I2C (Inter-Integrated Circuit): Two-wire (SDA, SCL), multiple devices (e.g., DHT11).
• UART: Asynchronous, simple (e.g., GPS modules).

Mermaid Diagram: SPI Communication

C. Actuator Control

• Servo Motors: Controlled via Pulse Width Modulation (PWM).
  • Example: Move servo to 90° (1.5ms pulse).

  #include <Servo.h>
  Servo myServo;
  void setup() { myServo.attach(9); }
  void loop() { myServo.write(90); delay(1000); }
  

• Relays: Controlled via GPIO pins (high/low signal).

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6. Real-World Applications in Nepal

A. eSewa & Khalti (Digital Payments)

• Sensors Used: NFC/RFID (for card swipes), Fingerprint sensors (security).
• Actuators Used: Vibration motors (confirmation), LCD screens (transaction details).

B. Pathao (Ride-Hailing)

• Sensors:
  • GPS (location tracking),
  • Accelerometer (detecting sharp turns for driver safety).
• Actuators:
  • Electric motors (bike/scooter propulsion),
  • Horn (alert system).

C. NTC (Electricity Distribution)

• Sensors:
  • Current sensors (overload detection),
  • Temperature sensors (transformer cooling).
• Actuators:
  • Circuit breakers (automatic shutdown),
  • Relays (switching high-voltage lines).

Worked Example: Traffic Light Control System A traffic light system uses:

• Sensors: Infrared sensors (detect vehicles),
• Actuators: Relays (switch traffic lights),
• Logic: If sensor detects a car → Activate green light for 30s, then switch to red.

Mermaid Diagram: Traffic Light Logic

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7. Common Challenges & Solutions

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8. Exam Tip

What to Focus On:

1. Definitions: Clearly distinguish between analog vs. digital sensors and active vs. passive sensors.
2. Calculations:
   • Thermistor resistance at a given temperature.
   • Ultrasonic sensor distance calculation.
3. Interfacing:
   • How to connect a sensor to ADC/SPI/I2C.
   • PWM for servo motors.
4. Applications:
   • Relate sensors/actuators to real systems (e.g., Khalti’s fingerprint sensor, Pathao’s GPS).
5. Diagrams:
   • Always label sensor/actuator connections (VCC, GND, data pins).
   • Draw timing diagrams for SPI/I2C communication.

Common Mistakes to Avoid:

• Forgetting to divide by 2 in ultrasonic distance formula.
• Misconfiguring ADC reference voltage (e.g., using 3.3V instead of 5V).
• Not pulling up/down I2C lines (SDA/SCL).

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9. Summary Checklist

Before the exam, ensure you can: ✅ Explain the difference between a sensor and an actuator. ✅ Calculate resistance/temperature for a thermistor. ✅ Describe how an ultrasonic sensor measures distance. ✅ Interface a servo motor using PWM. ✅ List 3 sensors and 3 actuators used in eSewa/Khalti/Pathao. ✅ Draw a timing diagram for SPI communication.