Tuesday, March 20, 2012

Automated Weather Monitoring System : Temparature Sensor Design

Mechanism for measuring atmospheric temperature

Equipment used:
·         LM35 temperature sensor
·         Analog to digital conversion functionality of the microchip

LM35 series produced by National Semiconductors is a precision integrated circuit temperature sensor series which are designed to produce an output voltage that is linearly proportional to the Celsius temperature. The output voltage is 10mV per degree Centigrade.
·         Calibrated directly in Celsius
·         Linear 10mV/`C scale factor
·         Can operate in the range of -55`C – 150`C
·         0.5`C accuracy guaranteeble (@25`C)
·         Less than 60µA current drain
·         Operates from 4V to 35V
·         Low self-heating (0.08`C in still air)
·         Low impedance output (0.1 per 1mA load)
·         Low cost
·         Easily available
·         Suitable for remote applications (low power consumption)
·         The sensor is commonly used for such application

Calculations
Assume the ADC of the PIC gives the values between 0 to n for input voltage of 0 to 5v provided. (eg: if the reading is 0 then input is 0V, if the reading is n the input is 5V). The PICs we are planning to use, PIC16F877a or PIC18 series, have a 10 bit ADC. Thus the max value for the 10 bit ADC will be 1023)
N = 1023
Voltage = (α/n)*5
Voltage in mV = (α/n)*5*1000
Since 10mV = 1`C
Temperature = (α/n)*5*100 = (α/n)*500
Substituting n = 1023
Temperature = (α/1023)*500 = α * 0.48876 `C
We can use this equation to directly calculate the instantaneous temperature of the atmosphere.

Content

Email: