LM35 Temperature Sensor

Key Features and Specifications

The LM35 is designed for easy interfacing with analog-to-digital converters (ADCs).

• Calibration: Calibrated directly in Degrees Celsius ($\text{}^\circ\text{C}$).
• Scale Factor: The output has a highly linear scale factor of $+10.0\text{ mV}/\text{}^\circ\text{C}$.
• Accuracy: Typically $\pm 1/4 \text{}^\circ\text{C}$ at room temperature, with an ensured accuracy of $\pm 0.5 \text{}^\circ\text{C}$ at $25\text{}^\circ\text{C}$.
• Temperature Range: Can measure a wide range from $-55\text{}^\circ\text{C}$ to $+150\text{}^\circ\text{C}$ (depending on the specific model, e.g., LM35D is $0\text{}^\circ\text{C}$ to $100\text{}^\circ\text{C}$).
• Operating Voltage: Operates from $4\text{V}$ to $30\text{V}$ (typically $5\text{V}$ in microcontroller circuits).
• Self-Heating: Extremely low current draw (less than $60\ \mu\text{A}$) leads to minimal self-heating (less than $0.1\text{}^\circ\text{C}$ in still air), which is important for accurate ambient temperature readings.

  Pinout and Output Formula

The most common package for hobbyists is the 3-pin TO-92 plastic transistor package.

Pinout (TO-92)

Output-to-Temperature Conversion

The LM35 simplifies temperature calculation because its output voltage is directly proportional to the temperature in Celsius.

• Example 1 (Positive): If $\text{V}_{\text{OUT}}$ is $0.250\text{V}$ ($250\text{ mV}$), the temperature is $250\text{ mV} / 10 = \mathbf{25\text{}^\circ\text{C}}$.
• Example 2 (Maximum): At the maximum $150\text{}^\circ\text{C}$, $\text{V}_{\text{OUT}}$ would be $150\text{}^\circ\text{C} \times 10\text{ mV}/\text{}^\circ\text{C} = 1500\text{ mV}$ ($\mathbf{1.5\text{V}}$).

   

Note on Negative Temperatures: To measure negative temperatures (e.g., down to $-55\text{}^\circ\text{C}$), the sensor requires a dual-supply configuration ($\text{+V}$ and $\text{-V}$) because the output $\text{V}_{\text{OUT}}$ must be able to swing to negative voltages (e.g., $-55\text{}^\circ\text{C}$ requires $-0.55\text{V}$). For positive-only measurements (most common hobbyist scenario), the standard single-supply connection is sufficient.

LM35 vs. Thermistors

The LM35 is often chosen over a traditional thermistor for its simplicity:

The LM35 is the ideal choice for projects prioritizing ease of use, linear output, and direct Celsius scaling.