IoT – Sensor Interfacing | ECET 2026 ECE Notes

Concept Notes (Deep Explanation + Examples)

🔹 Introduction

In the world of Internet of Things (IoT), sensors are the eyes and ears of the system. They collect data from the environment — such as temperature, humidity, light, or motion — and send it to a microcontroller or processor for processing and decision-making.

Sensor interfacing means connecting a sensor to a controller (like Arduino, ESP32, or Raspberry Pi) so that it can measure real-world physical parameters and convert them into digital information.

🔹 Basic Working Principle

Every sensor has:

1. Input: A physical quantity (like temperature, light, sound).
2. Transducer: Converts physical quantity → electrical signal.
3. Output: Voltage, current, or digital signal.

Example:

• A Temperature Sensor (LM35) senses heat and gives analog output proportional to °C.
• A PIR Sensor detects motion and gives a HIGH or LOW (digital output).

🔹 Types of Sensors (Common in IoT)

🔹 Analog vs Digital Sensors

• Analog Sensors: Output is a continuous voltage (e.g., LM35 gives 0–1V for 0–100°C).
• Digital Sensors: Output is binary (0 or 1) or serial (I2C/SPI/UART).

For example:

• LDR → Analog
• DHT11 → Digital

🔹 How Sensor Interfacing Works

Let’s take an example of LM35 Temperature Sensor with Arduino UNO:

1. Connection:
   • LM35 Vcc → 5V
   • LM35 GND → GND
   • LM35 Output → A0 (Analog Input)
2. Process Flow (Explained Diagrammatically in Words):
   • Step 1: Sensor senses temperature.
   • Step 2: Converts it into voltage (10 mV per °C).
   • Step 3: Arduino reads this voltage via ADC.
   • Step 4: ADC converts analog signal → digital value (0–1023 for 10-bit ADC).
   • Step 5: Microcontroller computes temperature using formula.

🔹 Example Code Logic (Arduino Style Pseudocode)

int sensorPin = A0;
float sensorValue, tempC;

void loop() {
  sensorValue = analogRead(sensorPin);
  tempC = (sensorValue * 5.0 * 100.0) / 1024; // Convert to °C
  Serial.print("Temperature: ");
  Serial.println(tempC);
}

int sensorPin = A0;
float sensorValue, tempC;

void loop() {
  sensorValue = analogRead(sensorPin);
  tempC = (sensorValue * 5.0 * 100.0) / 1024; // Convert to °C
  Serial.print("Temperature: ");
  Serial.println(tempC);
}

This example shows how sensor data → microcontroller → readable form (°C) conversion works.

🔹 Sensor Interfacing Protocols in IoT

Sensors communicate using these protocols:

🔹 Real-World IoT Application Example

Smart Agriculture System:

• Sensors: Soil moisture, humidity, temperature.
• Controller: NodeMCU.
• Data sent via Wi-Fi to cloud (ThingSpeak).
• Automated motor ON when soil is dry.

Circuit (described in words):

• NodeMCU connected to 3 sensors and relay.
• Sensors sense soil and environment.
• NodeMCU uploads data online.
• Relay activates pump automatically.

🔹 ECET Relevance

ECET often asks:

• “Which type of output does LM35 give?”
• “Which protocol uses two wires SDA and SCL?”
• “Which sensor detects motion?”

So, understanding interfacing principles is key for both theory and practical applications.

⚙️ Formulas (Plain LaTeX)

🔟 10 MCQs (GATE-level + ECET Mix)

1. Which sensor gives analog output proportional to temperature?
   A) DHT11 B) LM35 C) PIR D) MQ2
2. The output of a PIR sensor is:
   A) Analog B) Digital C) PWM D) Serial
3. The resolution of a 10-bit ADC with 5V reference is:
   A) 4.88 mV B) 5 mV C) 10 mV D) 1 mV
4. In I2C communication, how many lines are used?
   A) 1 B) 2 C) 3 D) 4
5. The LM35 gives an output of 350 mV. The temperature is:
   A) 25°C B) 30°C C) 35°C D) 40°C
6. The output of an LDR decreases when:
   A) Light increases B) Light decreases C) Temperature increases D) None
7. Which of the following sensors works on ultrasonic principle?
   A) DHT11 B) MQ2 C) HC-SR04 D) LDR
8. The I2C lines are:
   A) TX, RX B) SDA, SCL C) MISO, MOSI D) SCK, CS
9. The unit of ADC resolution is:
   A) Bits B) Voltage per bit C) Bytes D) None
10. For 8-bit ADC, total number of quantization levels are:
    A) 255 B) 256 C) 512 D) 128

✅ Answer Key

Q.No Answer
1 B
2 B
3 A
4 B
5 C
6 A
7 C
8 B
9 B
10 B

🧠 Detailed Explanations

1️⃣ LM35 gives an analog output (10 mV/°C), hence answer B.
2️⃣ PIR sensors give HIGH/LOW (digital) output — B.
3️⃣ Resolution = 5V / 1023 ≈ 4.88 mV — A.
4️⃣ I2C uses SDA & SCL — total 2 wires → B.
5️⃣ 350 mV / 10 = 35°C — C.
6️⃣ When light increases, resistance decreases, so output voltage decreases → A.
7️⃣ HC-SR04 uses ultrasonic waves — C.
8️⃣ I2C → SDA, SCL — B.
9️⃣ ADC resolution is in volts per bit — B.
10️⃣ 8-bit ADC → 2⁸ = 256 levels — B.

🎯 Motivation / Why This Topic Matters (ECET 2026)

IoT sensor interfacing is the foundation of all smart systems — from home automation to industrial IoT.
In ECET, this topic connects analog electronics, communication, and microcontrollers — a triple scoring area.
Mastering it builds your logical understanding and boosts confidence in IoT-based questions.
Keep practicing circuit connections and code logic daily — consistency is the key to top ECET ranks!

📲 CTA (Fixed)

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