LM35 Temperature Sensor: The Ultimate Guide

What is the LM35 Temperature Sensor?

The LM35 is an analog temperature sensor that provides an output voltage linearly proportional to the temperature in degrees Celsius (°C). It is a low-cost, precise, and easy-to-use sensor that does not require any external calibration or trimming to provide typical accuracies of ±¼°C at room temperature and ±¾°C over a full -55°C to 150°C temperature range.

Key Features of the LM35 Temperature Sensor

How Does the LM35 Temperature Sensor Work?

The LM35 temperature sensor works on the principle of the temperature dependence of a semiconductor’s forward voltage drop. The sensor consists of a precision temperature-dependent voltage source and a series of transistors that amplify and condition the voltage to provide a linear output.

As the temperature changes, the voltage across the semiconductor changes at a known rate. The LM35 has been calibrated to provide an output voltage that increases by 10mV for every 1°C rise in temperature. For example, at 25°C, the sensor will output 250mV (25 × 10mV/°C).

LM35 Temperature Sensor Pinout

The LM35 temperature sensor comes in various packages, including TO-92, TO-220, and SO-8. The most common package is the TO-92, which has three pins:

1. Vcc (Power Supply)
2. Output (Analog Temperature Output)
3. Ground

Interfacing the LM35 with a Microcontroller

To use the LM35 temperature sensor with a microcontroller, follow these steps:

1. Connect the Vcc pin to the microcontroller’s power supply (typically 5V).
2. Connect the Ground pin to the microcontroller’s ground.
3. Connect the Output pin to one of the microcontroller’s analog input pins.

Here’s an example circuit diagram showing the LM35 connected to an Arduino:

Arduino Code for Reading Temperature from LM35

Here’s a simple Arduino sketch that reads the temperature from the LM35 and prints it to the serial monitor:

const int sensorPin = A0; // LM35 connected to analog pin A0

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

void loop() {
  int sensorValue = analogRead(sensorPin); // Read the analog value from LM35
  float temperature = sensorValue * (5.0 / 1023.0) * 100.0; // Convert analog value to temperature in °C

  Serial.print("Temperature: ");
  Serial.print(temperature);
  Serial.println(" °C");

  delay(1000); // Wait for 1 second before the next reading
}

Applications of the LM35 Temperature Sensor

The LM35 temperature sensor finds applications in various fields, including:

1. Industrial Process Control: The LM35 can be used to monitor and control temperatures in industrial processes, such as chemical reactions, food processing, and manufacturing.

2. HVAC Systems: The sensor can be used in heating, ventilation, and air conditioning systems to monitor and regulate room temperatures.

3. Environmental Monitoring: The LM35 can be used in weather stations, greenhouses, and other environmental monitoring applications to measure ambient temperatures.

4. Home Automation: The sensor can be integrated into smart home systems to control appliances, such as fans and heaters, based on room temperature.

5. Medical Equipment: The LM35 can be used in medical devices, such as incubators and temperature-controlled storage units, to ensure precise temperature control.

Advantages and Disadvantages of the LM35 Temperature Sensor

Advantages

1. Linear output, making it easy to interpret temperature readings
2. Wide temperature range, suitable for various applications
3. Low self-heating, ensuring accurate measurements
4. Low cost, making it affordable for budget-constrained projects
5. Easy to use and interface with microcontrollers

Disadvantages

1. Limited accuracy compared to more expensive temperature sensors
2. Susceptible to electrical noise, which may require additional filtering
3. Not suitable for extreme temperature conditions beyond its specified range
4. Requires a stable power supply for accurate measurements

Frequently Asked Questions (FAQ)

1. What is the accuracy of the LM35 temperature sensor?

The LM35 provides typical accuracies of ±¼°C at room temperature and ±¾°C over a full -55°C to 150°C temperature range.

2. Can I use the LM35 with a 3.3V microcontroller?

Yes, the LM35 can work with a 3.3V power supply, but the output voltage range will be limited accordingly. You may need to adjust your code to account for the lower voltage range.

3. How do I convert the analog output of the LM35 to temperature in Celsius?

The LM35 provides an output voltage that increases by 10mV for every 1°C rise in temperature. To convert the analog output to temperature in Celsius, use the following formula:

Temperature (°C) = (Analog Output Voltage (mV) / 10) - 50

4. Can I use the LM35 to measure negative temperatures?

Yes, the LM35 can measure temperatures down to -55°C. However, for negative temperature measurements, you will need to use a negative power supply or a level-shifting circuit.

5. How do I minimize electrical noise when using the LM35?

To minimize electrical noise, keep the wiring between the LM35 and the microcontroller as short as possible. You can also add a small capacitor (e.g., 0.1µF) between the Vcc and Ground pins of the sensor to filter out high-frequency noise.

Conclusion

The LM35 temperature sensor is a versatile, low-cost, and easy-to-use solution for measuring temperatures in various applications. With its linear output, wide temperature range, and low self-heating, the LM35 is an excellent choice for projects that require precise temperature monitoring and control.

By understanding the working principle, interfacing methods, and applications of the LM35, you can effectively integrate this sensor into your projects and create temperature-sensitive systems with ease. Remember to consider the advantages and disadvantages of the LM35 and follow best practices to ensure accurate and reliable temperature measurements.