The LM35 temperature sensor

The LM35 ICs are a series of precision temperature measuring sensors. LM35s are very popular among members of the Arduino community. Their popularity is a direct result of the sensors being easy to setup and easy to interface with the Arduino development environment. Common members of LM35 IC series include the LM35C and the LM35D.

For most Arduino projects, you will likely use the LM35 with a transistor package that has three leads. However, some versions, such as the LM35D can also be found in 8 lead surface mount packages as well.

The LM35 is documented to have an accuracy of ± ¼°C at room temperature.

LM35 ICs are calibrated directly in °Celsius. This means that, the output signal from the IC can be directly translated to a temperature reading in degrees Celsius °C.

LM35 operating range.

The LM35 can operate over a full temperature range of -55°C to +150°C . It can also operate at a basic temperature range of +2°C to +150°C as shown in the datasheet extract.

Temperature output from the LM35.

The LM35 produces an analog signal that we use to determine the temperature reading. The output pin of the LM35 is raised by 10mV for every 1°C increase in temperature. For example, at a temperature of 0°C the output pin would read 0mV. On the other hand, at a temperature of 25°C, the output pin would read 250mV i.e (25 x 10mV).

Power requirements

It operates over a voltage range of 4V to 30V and it draws only a small current of 60uA.

Measuring the temperature of a surface

The LM35 can be glued or cemented to a surface. In such an arrangement, its temperature will be within 0.01°C of the surface temperature. However, this assumes that the temperature of the surrounding air is approximately the same as the surface temperature.

A common pitfall when measuring the temperature of a surface using LM35 and how to avoid it

Where the temperature of the surrounding air temperature is much higher or lower than the temperature of the surface in question. Then the LM35 will be at an intermediate temperature between the temperature of the surface and the temperature of the air.

Also, with LM35s with plastic packaging. Where the copper leads are the main thermal path that carry heat into the device. If the ambient air temperature is much different from that of the surface being measured. Then it is likely that the device’s temperature will be closer to the air temperature than the temperature of the surface of interest.

To minimise this problem, we have to ensure that wiring of the LM35 is at the same temperature as that of surface being measured as it leaves the device. One way of achieving this is by insulating the LM35 and its wiring from the ambient air.

Below is a list of the LM35 features as provided in the data sheet.

LM35 Features

• Calibrated directly in ˚Celsius (Centigrade)u Calibrated directly in ˚Celsius (Centigrade)
• Linear + 10.0 mV/˚C scale factor
• 0.5˚C accuracy guaranteeable (at +25˚C)
• Rated for full −55˚ to +150˚C range
• Suitable for remote applications
• Low cost due to wafer-level trimming
• Operates from 4 to 30 volts
• Less than 60 µA current drain
• Low self-heating, 0.08˚C in still air
• Nonlinearity only ± 1⁄4˚C typical
• Low impedance output, 0.1 Ω for 1 mA load