Temperature Sensor - The LM35

Temperature Sensor - The LM35

The LM35 is an integrated circuit sensor that can be used to measure temperature with an electrical output proportional to the temperature (in ^oC).

The LM35 - An Integrated Circuit Temperature Sensor

•  Use of LM35s To Measure Temperature
• You can measure temperature more accurately than a using a thermistor.
• The sensor circuitry is sealed and not subject to oxidation, etc.
• The LM35 generates a higher output voltage than thermocouples and may not require that the output voltage be amplified.

•  LM35 Look Like
• Here it is.

• Working of LM35
• It has an output voltage that is proportional to the Celsius temperature.
• The scale factor is .01V/^oC
• The LM35 does not require any external calibration or trimming and maintains an accuracy of  +/-0.4 ^oC at room temperature and +/- 0.8 ^oC over a range of 0 ^oC to +100 ^oC.
• Another important characteristic of the LM35DZ is that it draws only 60 micro amps from its supply and possesses a low self-heating capability. The sensor self-heating causes less than 0.1 ^oC temperature rise in still air.

•  Use An LM35 (Electrical Connections)
• Here is a commonly used circuit.  For connections refer to the picture above.
• In this circuit, parameter values commonly used are:
• V_c = 4 to 30v
• 5v or 12 v are typical values used.
• R_a = V_c /10^-6
• Actually, it can range from 80 KW to 600 KW , but most just use 80 KW.

• Here is a photo of the LM 35 wired on a circuit board.
• The white wire in the photo goes to the power supply.
• Both the resistor and the black wire go to ground.
• The output voltage is measured from the middle pin to ground.l

• What Can You Expect When You Use An LM35
• You will need to use a voltmeter to sense Vout.
• The output voltage is converted to temperature by  a simple conversion factor.
• The sensor has a sensitivity of 10mV / ^oC.
• Use a conversion factor that is the reciprocal, that is 100 ^oC/V.
• The general equation used to convert output voltage to temperature is:
• Temperature ( ^oC) = Vout * (100 ^oC/V)
• So if  Vout  is  1V , then, Temperature = 100 ^oC
• The output voltage varies linearly with temperature.

In order to calculate the Celsius reading from the analog value, we use the following formula to calculate the temperature in Celsius:

Where

Val = is the value send to the computer by the serial port
tempC= is the calculated temperature value (in Celsius)
5 is the reference we are using
1024 is the resolution of the 10 bit internal ADC