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 Precision Centigrade Temperature Sensor
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.
Advantages/ Features
·         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

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
If the value read from the ADC = α
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.