Lesson 5: Using LM35 Temperature Sensor with Arduino

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Using LM35 with Arduino

List of Content

1. List of Components
2. Connections: LM35 and LED Circuit Schematics
3. How the LM35 Sensor Works
4. The Code and How It Works
5. How to Go Further in Your Temperature-Controlled LED Circuit

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1. Needed Components

• Arduino Board (Arduino Uno or another type)
• LM35 Temperature Sensor
• LED (any color)
• Resistor (330 ohm)
• Breadboard
• Jumper Wires
• Power Supply (USB cable or battery pack)

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2. Connecting Your Circuit

Set Up the Breadboard:

1. LM35 Sensor Connections:

• Connect VCC pin of the LM35 to the 5V pin on the Arduino
• GND pin of the LM35 –> GND pin on the Arduino.
• Vout pin of the LM35 –> analog pin A0 on the Arduino.

1. LED Connections:

• Connect the anode (longer leg) of the LED to digital pin 13 on the Arduino.
• Connect the cathode (shorter leg) of the LED to a 330-ohm resistor, and connect the other end of the resistor to the GND on the Arduino.

Schematics and Pictures

• MAKE SURE TO CONNECT THE LM35 SENSOR’S PINS PROPERLY. Otherwise LM35 will heat and may get damaged as shown below.
• Put the sensor’s legs towards your face and its flat part towards the roof, then follow the below diagran to identify the pins (for TO-92 package)
  
• Circuit Diagram
  

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3. How the LM35 Sensor Works

The LM35 is a precision analog temperature sensor that provides an output voltage linearly proportional to the Celsius temperature. Here’s how it works:

1. The LM35 outputs 10 mV per degree Celsius. For example:

• At 25°C, the output voltage is 250 mV.
• At 30°C, the output voltage is 300 mV.

1. The output voltage is read by the Arduino’s analog input pin (A0).
2. The Arduino converts the analog voltage to a digital value using the analogRead() function.
3. The digital value is converted to temperature using the formula:

   Temperature (°C) = (Analog Value × 5.0) / 1024.0 × 100

LM35 Package Types:

• TO-92: The most common package, looks like a small transistor. Easy to use on a breadboard.
• TO-220: Larger package, often used for higher power applications.
• SOIC: Surface-mount package, used in compact designs.

Where to Get the LM35:

• You can purchase the LM35 from electronics component stores like:
• Online retailers
• Local electronics shops: Check for availability in your area.

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4. The Code and How It Works

Here’s the Arduino code to blink an LED when the temperature exceeds a certain threshold:

// Define pin numbers
const int lm35Pin = A0;  // LM35 connected to analog pin A0
const int ledPin = 13;   // LED connected to digital pin 13

// Define temperature threshold
const float tempThreshold = 30.0; // Blink LED if temperature > 30°C

void setup() {
  // Set pin modes
  pinMode(lm35Pin, INPUT);
  pinMode(ledPin, OUTPUT);

  // Initialize serial communication
  Serial.begin(9600);
}

void loop() {
  // Read the analog value from the LM35
  int analogValue = analogRead(lm35Pin);

  // Convert the analog value to temperature in Celsius
  float temperature = (analogValue * 5.0) / 1024.0 * 100;

  // Print temperature to Serial Monitor
  Serial.print("Temperature: ");
  Serial.print(temperature);
  Serial.println(" °C");

  // Control LED based on temperature
  if (temperature > tempThreshold) {
    digitalWrite(ledPin, HIGH); // Turn on LED
    delay(500);                 // Blink delay
    digitalWrite(ledPin, LOW);  // Turn off LED
    delay(500);                 // Blink delay
  } else {
    digitalWrite(ledPin, LOW);  // Turn off LED
  }

  // Add a small delay for stability
  delay(100);
}

How This Code Works:

1. Variable Declaration:

• lm35Pin is set to A0 for the LM35 sensor.
• ledPin is set to 13 for the LED.
• tempThreshold is set to 30.0 (the temperature threshold in Celsius).

1. Setup:

• The pinMode() function sets the lm35Pin as an input and the ledPin as an output.
• Serial communication is initialized for debugging.

1. Loop:

• The analogRead() function reads the analog value from the LM35 sensor.
• The analog value is converted to temperature using the formula.
• The temperature is printed to the Serial Monitor.
• The if condition checks if the temperature is greater than the threshold (tempThreshold):
  • If true, the LED blinks by turning on and off with a 500 ms delay.
  • If false, the LED remains off.

1. Delay:

• A small delay is added for stability and to avoid rapid readings.

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5. How to Go Further in Your Temperature-Controlled LED Circuit

1. Adjust Temperature Threshold: Change the tempThreshold value in the code to trigger the LED at a different temperature.
2. Add a Buzzer: Include a buzzer to sound an alarm when the temperature exceeds the threshold.
3. Multiple LEDs: Use multiple LEDs to indicate different temperature ranges (e.g., Green for normal, Yellow for warning, Red for high).
4. LCD Display: Add an LCD or OLED display to show the temperature in real-time.
5. Cooling Fan: Integrate a small DC motor or fan to activate when the temperature is too high.
6. Data Logging: Use an SD card module to log temperature data over time.
7. Wireless Communication: Add a Bluetooth or Wi-Fi module to send temperature data to a smartphone or computer.
8. Advanced Logic: Implement hysteresis to prevent rapid toggling of the LED near the threshold.

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This project demonstrates how to use the LM35 temperature sensor to create a temperature-controlled LED system. Experiment with the code and hardware to explore more advanced applications!