LM35 Temperature Sensor

The LM35 is a precision analog temperature sensor whose output voltage is linearly proportional to the temperature in degrees Celsius. It outputs 10 mV per °C with no external calibration required, offering ±0.5°C accuracy at room temperature and ±1°C accuracy over the full −55°C to +150°C range. It draws only 60 µA and is available in TO-92, TO-220, and SO-8 packages.

For this interfacing you need the following components:

• Arduino board (Uno, Nano, Mega, etc.)
• LM35 temperature sensor (TO-92 package)
• Breadboard and jumper wires
• USB cable to connect Arduino to your computer
• (Optional) 100 nF capacitor between VCC and GND for noise filtering

Schematic

Connect the LM35 to the Arduino as follows:

LM35 (TO-92)          Arduino
------------          -------
VCC (pin 1, left) --> 5V
VOUT (pin 2, center)   Analog Pin A0
GND (pin 3, right) --> GND

When viewing the flat face of the TO-92 package with pins facing down, pin 1 is the left pin (VCC), pin 2 is the center pin (VOUT), and pin 3 is the right pin (GND).

A 100 nF capacitor between VCC and GND near the sensor reduces noise in the analog reading.

Pin Map

Install necessary Library

No external library is required. The LM35 uses standard analogRead() built into Arduino.

Code with complete explanation

This sketch reads the LM35 output voltage and converts it to temperature in Celsius and Fahrenheit, printing the values to the Serial Monitor.

const int sensorPin = A0;

void setup()
{
  Serial.begin(9600);
  Serial.println("LM35 Temperature Sensor Test");
}

void loop()
{
  int raw = analogRead(sensorPin);

  // Convert raw ADC value to voltage (0–5V mapped to 0–1023)
  float voltage = raw * (5.0 / 1023.0);

  // LM35: 10 mV per °C
  float temperatureC = voltage * 100.0;
  float temperatureF = temperatureC * 9.0 / 5.0 + 32.0;

  Serial.print("Raw: ");
  Serial.print(raw);
  Serial.print("  Voltage: ");
  Serial.print(voltage, 3);
  Serial.print(" V");
  Serial.print("  Temp: ");
  Serial.print(temperatureC, 2);
  Serial.print(" °C  ");
  Serial.print(temperatureF, 2);
  Serial.println(" °F");

  delay(1000);
}

Code breakdown

• analogRead(sensorPin) — reads the analog voltage (0–1023) from the sensor output pin.
• raw * (5.0 / 1023.0) — converts the raw ADC value to a voltage (0–5 V).
• voltage * 100.0 — the LM35 outputs 10 mV per °C, so multiply the voltage by 100 to get °C. At 25°C the output is 0.25 V.
• temperatureC * 9.0 / 5.0 + 32.0 — standard conversion from Celsius to Fahrenheit.

Using the internal 1.1V reference for better resolution

The LM35 only outputs 0–1.5 V over its full range (−55 to +150°C). Using the 5V reference wastes ADC resolution. Change to the internal 1.1V reference for finer granularity:

void setup()
{
  analogReference(INTERNAL); // 1.1V reference (on Uno/Nano)
  // For Mega: analogReference(INTERNAL1V1);
}

void loop()
{
  int raw = analogRead(sensorPin);
  float voltage = raw * (1.1 / 1023.0);
  float temperatureC = voltage * 100.0;
  // ...
}

With the 1.1V reference, the resolution improves from ~0.49°C per step to ~0.11°C per step.

Steps to perform this interfacing

1. Connect the LM35 to the Arduino as shown in the schematic.
2. Optionally place a 100 nF capacitor between VCC and GND near the sensor.
3. Copy the code into the Arduino IDE.
4. Select the correct board and port (Tools > Board and Tools > Port).
5. Upload the sketch to the Arduino.
6. Open the Serial Monitor (Tools > Serial Monitor, set baud rate to 9600).
7. Observe the raw ADC value, voltage, and temperature printed every second.
8. Test by warming the sensor with your finger — the temperature should rise visibly.

Caution

• Only 10 mV/°C: The LM35 outputs 10.0 mV per degree Celsius, giving 0 V at 0°C and 1 V at 100°C. Unlike the TMP36, the LM35 does not have an offset — it cannot read temperatures below 0°C without a negative supply (or a pull-down resistor), as the output would go below 0 V. For sub-zero readings, use the TMP36 (500 mV offset) or the LM35 with a bipolar supply.
• Self-heating: The LM35 draws approximately 60 µA, which causes negligible self-heating in still air (≈0.08°C). In a vacuum or enclosed space without airflow, self-heating can increase by 0.2–0.5°C.
• Analog reference: The default 5V analog reference gives a resolution of about 0.49°C per ADC step. For better resolution, switch to the internal 1.1V reference as shown above. Note: analogReference(INTERNAL) changes the reference for ALL analog pins, not just the one reading the LM35.
• Noise: The LM35 output is susceptible to electrical noise, especially with long sensor wires. Use a 100 nF ceramic capacitor between VCC and GND and a 100 nF capacitor between VOUT and GND near the sensor. Twisted-pair or shielded cable helps for runs longer than 30 cm.
• Pinout: The TO-92 package pinout varies by manufacturer. Some LM35 variants have different pinouts — always verify with your specific datasheet. The common TO-92 arrangement (flat face toward you, pins down) is: left = VCC, center = VOUT, right = GND.