Help with PIC16F716 and thermistor

[tomishome]

Hi,

I'm a Physics degree student with no real experience of digital electronics. Part of my project involves making a temperature measuring system which can operate over a range of -10 to 100 degrees celcius. So far I have managed to preform an analogue to digital conversion (via a PIC) on an LM35C thermistor. I then output the digital value via RS232 to a PC.

The problem is the range of values I get out. The voltage on the signal pin of the LM35C ranges between approximately 0.24V at room temp to 0.51 V at approximately 60 degrees C. This seems ok however this only corresponds to a small change in the PICs digital output.

At the moment I have set the PIC output to be a 3 digit number between 0 and 255. This means that I should get 256 divisions in the thermistors range, right? However, the above temperature change only gives a digital change of 15!!!

Does anyone know how I can fix this without Op-amps?
Will lowering the the Vref on the PIC fix this problem?
If I do lower Vref for the A2D will I increase the error on the A2D significantly?


All help gratefully received.

Tom

 

[Jay.slovak]

I'm actually working on temperature sensing too. Lowering Vref will help you resulting in doubling the resolution. Therefore you will get a change of 30. This corespodents to 2.5V Vref and 10 Bit A2D resolution. If you use lower Vref, it should help more, but A2D's resolution may drop to 8 bits. Better solution is to use OP amp, to amplify it so you will get 5V @ 100C. You can try to use 2 bottom bits from the A2D, but it will be not as acure as with using OP amp.

BTW: Using OP amp is not a big deal, it's simple and elegant solution, and you can get full 10Bit resolution.

 

[csaba911]

Hi

I made this one in last year, to monitor my aquarium temperature !
Since my tropical fishes really don't like water below +2C I had no reason to worry for the full scale range.
On the 1st pic you can see, basic config only give you +2C-- +150C
To get the full scale see the 2nd pic !

I designed my shc from the 3rd pic.
Did not have LM385 or other precision ref, only the Lm431 in my parts bin.
@+2C the output is +20mV, @ +102C +1020 mV,
there is 1000mV to use.
10bit =1024, 1000/1024=0.99609375 ~ so 1mV/bit if your refV is 1024mV

If your AD read 256, the C=256/10=25.6, if 258 then C=258/10=25.8
@1024 C=1024/10=102.4~

I hope this help :idea:

Don't ask for the CODE, I wrote it in BASIC, and the PIC molded in epoxy with the tepm sensor and collecting algae in my aquarium :lol:

Use this only as reference, I don't make documentation for myself !
There might be error's or flaw in my design !!
Works for me !! :wink:

STEVE

 

[csaba911]

Here what I made up !

you need to recalculate the res value in the voltage divder to get 1.024V

STEVE

 

[hsingh36]

hi guys
m making a temprature sensor project
using LM35DZ and PIC16F716
THE output is to be displayed using 8 led
it is to measure room temprature
can some one help with the circuit diagram and pic programming
thanks