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LM35 Temperature Sensor: The Ultimate Guide

LM35 Temperature sensors are electric devices that detect and measure coolness or hotness before converting them to electrical signals. The types of sensors range from mechanical (thermometers), electrical (thermistor or thermocouple), or Integrated Circuits (MCPXXXX, LMXX, or ADTXXX series). 

In today’s article, we’ll discuss LM35, a temperature sensor IC in the LM-series. In it, we will share knowledge on the IC’s pin configuration, pinout, applications, features, etc. 

What are the lm35 Temperature Sensors?

An LM35 temperature sensor is an integrated circuit that generates an analog output signal equivalent to the instantaneous temperature. 

One advantage of LM35 is that it doesn’t need external calibration to work. Moreover, its outer coating prevents it from self-heating, and can anyone can afford it since it’s cost-effective. 

lm35 Temperature Sensors Pin Configuration

LM35 pinout

TLM35 pinout

LM35 is a three-terminal sensor, and it has the pins below;

Pin1/ Vcc pin / Input pin – It supplies the input voltage ranging from 4V to 30V (average of 5V) to typical applications. 

Pin2/ Out/ Analog pin – It provides the analog output voltage directly proportional to the rise in temperature (in centigrade). 

Pin3/ GND or ground pin – The last pin connects to the circuit’s ground. 

LM35 Specifications and Features

The features and specifications of LM35 are as follows;

  • First, it can measure a temperature range from -55C° to 150°C.
  • Then, it is a small sensor, therefore, recommendable in remote applications. Besides that, because of wafer-level trimming, it’s cheap.
  • It has a current drain that’s less than 60µA. In addition, it should have a low self-heating of 0.08°C in still air. 
  • Its minimum input voltage is -2V whereas, its maximum input voltage is 35V. 
  • The temperature is directly proportional (linear) to its output voltage. In other words, for each 1°C temperature rise, there’s a voltage rise of 0.01V (10mV) – 10-mV/°C scale factor. 
  • It also has an ensured accuracy of ±0.5°C and a low output impedance of 0.1Ω for 1mA load. 
  • Lastly, you can find it in SO8, TO-CAN, TO-220, and TO-92 packages. 
  1. lm35 temperature sensor circuit configuration

You can use LM35 in two circuit configurations that yield different results, as we will discuss below. 

Note; 

Often, the average accuracy for the two configurations is +-1°C even though there’s a temperature reduction between 2°C to 25°C levels. 

The table shows the accuracy level of LM35

ParameterValue
25°C accuracy level±0.5°C
-55°C to 150°C accuracy range±1°C
Temperature slope10-mV/°C

Basic Centigrade Temperature Sensor 

A basic sensor only allows the measure of +ve temperature ranging from 2°C to 150°C. 

Circuit of a basic centigrade temperature sensor 

Circuit of a basic centigrade temperature sensor 

In its circuit, you only need to power LM35 IC and connect its output to an ADC. The circuit can also have an 80k to 100k resistor connected between GND and Vout pin. The connection tightens the Vout pin, therefore, preventing it from floating. 

Full-Range Centigrade Temperature Sensor

A full-range sensor permits using all sensor resources that further measures temperature ranging from -55°C to 150°C. Despite being complex, its results are higher than in basic sensors. 

Circuit of a full-range centigrade temperature sensor

Circuit of a full-range centigrade temperature sensor

As for its circuit, we attach an external resistor that’ll ultimately switch the negative voltage level upwards. We can further use the formula R1 = -Vs/50µA to calculate the value of the external resistor. 

lm35 Temperature Sensors Alternatives

The alternatives of LM35 include DS1620, DS18B20, LM34, and LM94022. 

Furthermore, you can use sensors from the LM35 family such as LM335, LM235, LM135, or LM135A.  Their difference from LM35 is the package types and varying temperature measuring capacities.  

  1. Applications

LM35 sensor has a wide range of applications. Such as;

  • Monitoring battery temperature,
  • Thermal shutdown for components or circuits. 
  • It measures temperatures in any environment and all HVAC applications

Using LM35 Temperature to Calculate the Temperature

The following steps will help you use an LM35 sensor to calculate the temperature.

  • Start by building the circuit. 
  • In the circuit, ensure you connect GND to the ground, then power LM35 VCC with +5 operating voltage (Vs). 
  • Thirdly, connect the VOUT to an ADC input (Analog-to-Digital Converter). After which you proceed by sampling the reading from ADC of the output voltage (VOUT). 
  • Finally, finish by converting the output voltage to temperature.   

Use the formula below to convert voltage to temperature.

Centigrade temperature = Voltage read by ADC / 10 mV (mills volt). 

Alternatively, you can use;

VOUT = -10 mV/°C × T

Whereby; 

VOUT = LM35 output voltage

T = Temperature in centigrade

10mV represents the linear scale factor of LM35. 

Conclusion

Briefly, LM35 is an analog device operating as a temperature sensor and comes in a form factor of a 3-pin TO-92 package. Some of its applications include measuring temperature in HVAC applications. Moreover, it is cheap and reliable. 

For clarifications or queries, kindly reach out to us. 

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Hommer Zhao
Hi, I am Hommer, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me. I really appreciate any help you can provide.

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