Digital thermometer

Discussion in 'The Projects Forum' started by andrew24, Jun 16, 2009.

  1. andrew24

    Thread Starter Active Member

    Aug 20, 2008
    76
    0
    Hallo, i'm making a digital thermometer using atmega8 and NTC thermistor, connected in the wheatstone bridge circuit.

    http://d3.myfreefilehosting.com/d1/digital_t.JPG

    Can you help me with the schematic, is everything right here?
    Do i need to connect some capacitors between VCC and GND, and AVCC and GND or just on the AREF pin?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You would be much better to use an instrumentation amp to amplify the bridge voltage into a useful range for the A to D converter.

    For that matter, using an LM34 or 35 an an op amp for some gain is simpler, as the temperature output is linearized so you don't need to do software calibration.
     
  3. kurf

    Member

    Jun 15, 2009
    10
    0
    Yes and use more then just 5 Volts differenctial. 10 volts or so. +5V on one end and -5V on the other of your bridge. Depending on how accurate you want the reading it would be helpfull to add another pot so you can tweak out error.

    Do you have any specs set?
     
    Last edited: Jun 16, 2009
  4. andrew24

    Thread Starter Active Member

    Aug 20, 2008
    76
    0
    I wanted to make a project with thermistor, because this way is more interesting :) I agree that instrumential amp should be used.. Or as Kurf said, connect higher voltage to the bridge
    i would first convert analog voltage to digital, then calculate the resistance of thermistor, and finally calculate the temperature using calculation based on the exponential curve. Is the microcontroller capable of using "Ln" function?
     
    Last edited: Jun 17, 2009
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If you can write the algorithm, of course it is.
     
  6. andrew24

    Thread Starter Active Member

    Aug 20, 2008
    76
    0
    Could you tell me what to do with the "reset" pin? do i ned to connect it to the VCC thru a resistor, or just leave it floating? whats the difference?
    I'm not sure if i connected it right in my schematic
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The reset pin connection that you have should be fine. The AVR provides an internal hold-off once the reset pin has reached its inactive state. This allows you to forego the use of a large capacitor to hold off reset using a long RC time constant.

    I would recommend that you provide an ISP6PIN or ISP10PIN header to permit in-circuit programming. The choice of which one is determined by the programmer that you intend to use. That is the real joy and utility of the current generation of AVR and PIC microcontrollers. The luxury of programming the part without removing it from the circuit is a genuine delight.

    hgmjr
     
    Last edited: Jun 18, 2009
  8. andrew24

    Thread Starter Active Member

    Aug 20, 2008
    76
    0
    Hallo, i almost finished my project. I used linearized temperature sensors LM35DZ instead of the thermistors. The result on the LCD are floating too much: for example :
    temp: 24.5,
    after a few miliseconds, temp: 24.2
    after a few miliseconds, temp: 24.7 and so on.. How can i fix that? Is that a LM35DZ problem, or am i sending rezults to the LCD too often?
     
    Last edited: Jul 5, 2009
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