Help with thermistor project?

[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 degrees 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 256. This means that I should get 256 divisions in the thermistors range. 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?
Are there any probelms in doing this?


All help gratefully received.

Tom

 

[Russlk]

In the first place, the LM35 is not a thermistor! The IC has an output of 10 millivolts per degree Celcius. Since you have .51 volts at 60 deg. C. and .24 volts at room temperature, I calculate that room temperature is 33 deg. C. You must be working in your skivvies, that is pretty warm. You will find some good info in National Semiconductor's application note: AN-460. You need to amplify the signal, why not use an op amp?

 

[evandude]

I agree. amplifying your signal would be very easy. for even more accuracy, you can introduce a negative DC offset so that at your minimum temperature , the voltage is zero... then amplify it so that at the maximum temperature, it is close to the highest value that can be read by the ADC. that will give you maximum resolution and will only require one or two very simple op-amp circuits.

 

[BF9000]

Reference Voltages

Use a 50k potentiometer on the voltage reference pins Vref+ and Vref-. One side of each pot will connect to VCC and the other to GND. The wiper (middle pin on pot) will connect to the reference pins. Use these to adjust your min and max values to get better resolution per bit. You will need an RS232 transceiver to talk to your PC via RS232. www.circuitblocks.com has a nice one that is already mounted in a module that I like to use for prototyping. If you haven't already try download Realterm (free) and using it instead of hyperterminal. This makes for easy debugging.

 

[meguitarist]

what PIC U used?

 

[ericgibbs]

what PIC U used?

hi,
This post is 4 years old.

 

[Dick Cappels]

Perhaps he is still struggling with a method that doesn't use op amps. One hopes he managed to graduate in spite of this.