Digital thermometer

Discussion in 'Homework Help' started by FBorges22, Apr 5, 2009.

  1. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    Greetings,

    I am trying to create a digital thermomether with the LM35 sensor and a 5V(ref) based ADC.

    Recently, I tried to create a simple signal conditioner with the LM741 opamp in the non-inverting topology.

    However, I can´t get the output voltage right. How I can create a 100°C thermomether with a 12VDC supply with a LM741 signal conditioner?

    Here is the diagram that I made to do this task.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    It would work better if the LM741 had a ground referenced split supply.

    Also the gain looks too high with the value of R19 at 20Ω.

    If the LM35 puts out 10mV per degree you need to specify what output voltage range you need for 0-Tmax. For example - say you want 0-50deg giving 0-5V out. The gain required would be 10X - not ~500X as you seem to have.
     
  3. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    In that case I should replace the R19 to a 2K resistor to get a lower gain.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    At 10 mv/deg, the LM35 will output .01 X 100 volts at 100 C. The op amp gain should be such that the amplified voltage is either full scale into the A to D converter, or scaled so the reading immediately translated to the temperature.

    Our Ebook has a section of figuring op amp gains - http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

    By the way, while there may be a zillion 741's left around, that is not really a good reason to keep using them. Any modern op amp is much better to use in place of it - you get higher input impedance, better frequency response, and can use lower voltages.
     
  5. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    What opamp should I use for this application?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Interesting question as there are so many to choose from. I'm a fan of Burr-Brown, despite their having been subsumed into Texas Instrument. You might look at the OPA237UA, which is designed to work with a single supply and is available in DIP8.

    Doing a parametric search with any supplier and looking for DIP8 (so it can be handled and used with a socket) and single supply will probably turn up hundreds, and of which will work better than the obsolete 741.

    Your biggest problem with the 741 in your application is that it is designed to work with a dual power supply. It is most happy with +/-15 VDC applied. Not a good choice for ground and +12.
     
  7. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    I have a little dificulty in finding the Burr-Brown products in my region. Is there any other popular model that uses a single power-supply?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    See if you can do a parametric search in a catalog of a supplier you can use. The only concern is for single supply operation and in a package you can handle.
     
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