Temperature Sensor: LM35DZ

Why use LM35DZ

I choose this type of temperature sensor because of its following feature:

• Calibrated directly in ° Celsius (Centigrade)
• Linear + 10.0 mV/°C scale factor
• 0.5°C accuracy guaranteeable (at +25°C)
• Rated for full −55° to +150°C range
• Suitable for remote applications
• Low cost due to wafer-level trimming
• Operates from 4 to 30 volts
• Less than 60 μA current drain
• Low self-heating, 0.08°C in still air
• Nonlinearity only ±1⁄4°C typical
• Low impedance output, 0.1 W for 1 mA load

• 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

Basic electrical connection

 

Here is a commonly used circuit.  For connections refer to the picture

• 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.
   

In this basic circuit, we 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. 

Converting analog to Digital 

However in the weather monitoring system, the signal from LM35DZ will need to be converted to digital so that can be read by Altera. In this project, I used ADC0804 to convert the electrical signal to digital.

 Here is the process to get almost accurate data from ADC0804.

• Connect CS pin, V(-ve) pin, AGND and DGND to ground.
• Give a 0 to WR pin, and then a 1. The low-to-high edge will start data conversion. The high-to-low will eventually set the INTR pin to high.
• Check when the INTR pin goes low. If INTR pin is 0, we're sure that the data is converted.
• Give a 0 to RD pin. Data will be present at the data port of ADC. Read it, and then set RD to 1.

ADC Connection