Temperature is one of the most commonly measured parameter in the world. They are used in your daily household devices from Microwave,fridges, AC to all fields of engineering. Temperature sensor basically measures the heat/cold generated by an object to which it is connected. It then provides a proportional resistance, current or voltage output which is then measured or processed as per our application.
Temperature sensor are basically classified into two types
Non Contact Temperature Sensors: These temperature sensors use convection & radiation to monitor temperature
Contact Temperature Sensors: Contact temperature sensors arethen further sub divided into three type
Resistive Resistance Temperature Detectors (RTD).
Semiconductor based. (LM35, DS1820 etc).
In this article we will be discussing about LM35 Temperature Sensor which is a semiconductor based sensor. LM35 Temperature Sensor is basically a very low cost and easily available Sensor.
What is LM35 Temperature Sensor?
LM35 is an integrated analog temperature sensor whose electrical output is proportional to Degree Centigrade. LM35 Sensor does not require any external calibration or trimming to provide typical accuracies. The LM35’s low output impedance, linear output, and precise inherent calibration make interfacing to readout or control circuitry especially easy.
Features of LM35 Temperature Sensor
- Calibrated directly in Degree Celsius (Centigrade)
- Linear at 10.0 mV/°C scale factor
- 0.5°C accuracy guarantee-able (at a25°C)
- Rated for full -55°C to a 150°C range
- Suitable for remote applications
- Low cost due to wafer-level trimming
- Operates from 4 to 30 volts
- Less than 60 mA current drain
- Low self-heating, 0.08°C instill air
- Non-linearity only 0.25°C typical
- Low impedance output, 0.1Ωfor 1 mA load
Prefix LM stands for linear monolithic
How Does LM35 Sensor Work?
Main advantage of LM35 is that it is linear i.e. 10mv/°C which means for every degree rise in temperature the output of LM35 will rise by 10mv. So if the output of LM35 is 220mv/0.22V the temperature will be 22°C. So if room temperature is 32°C then the output of LM35 will be 320mv i.e. 0.32V.
LM35 Interfacing Circuit
As such no extra components required to interface LM35 to ADC as the output of LM35 is linear with 10mv/degree scale. It can be directly interfaced to any 10 or 12 bit ADC. But if you are using an 8-bit ADC like ADC0808 or ADC0804 an amplifier section will be needed if you require to measure 1°C change.
LM35 can also be directly connected to Arduino. The output of LM35 temperature can also be given to comparator circuit and can be used for over temperature indication or by using a simple relay can be used as a temperature controller.
Alternative to LM35 Temperature Sensor
If you are interested in getting reading in degree Fahrenheit you can use LM34 Temperature sensor. It is same as LM35 except its electrical output is proportional to Degree Fahrenheit. It has same pin configuration and same voltage range as LM35.
You can also check out MCP9700 Temperature Sensor from microchip. MCP9700 can accurately measure temperature from -40C to +150C. The output of the MCP9700 is calibrated to a slope of 10mV/°C and has a DC offset of 500mV. The offset allows reading negative temperatures without the need for a negative supply. Basically the code for LM35 will not work for MCP9700.
If you are using a 8 bit ADC and want to measure 1°C change in temperature then you can also use MCP9701 Temperature Sensor from Microchip. The MCP9701 can accurately measure temperature from -10°C to +125°C. The output of the MCP9701 is calibrated to a slope of 19.53 mV/°C and has a DC offset of 400 mV. The offset allows reading negative temperatures without the need for a negative supply.
LM35 Interfacing Circuits and Projects
- LM35 Interfacing to Arduino: Monitors Air Temperaure using Arduino MEGA and Displays on LCD
- LM35 Interfacing to Raspberry PI: Measures Temperature using RPI3 and MCP3008 ADC.
- LM35 interfacing to PIC Micrcontroller: Measures Temperature using PIC16f876 andDisplays on LCD.
- LM35 Interfacing to MSP430 Launchpad: Measures Temperature and Displays on 16x2 LCD
- LM35 circuit using Op-amps: Senses Temperature and accordingly indicates if it is above or below set level using LED's.
- Arduino Thermometer: Thermometer made using LCD, Arduino & LM35 Temperature sensor
- PIC based Temperature Controlled Fan: A project done using Microchip's PIC16f877a microcontroller which measures the room temperature and as the temperature increases the Fan speed increases.