Temperature help

Discussion in 'The Projects Forum' started by CLOVES101, May 16, 2011.

  1. CLOVES101

    Thread Starter New Member

    May 16, 2011
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    I have a design project where i have to design a sprinkler system that takes in inputs and based on those inputs i have different flow rates and duration of the sprinkler. One of the inputs is a temperature sensor, for the sprinklers to turn on it has to be between 70-90 °F. I set up two comparators so that the "-" sides are tied together and each of the "+" sides are set to each of the two temps. Then I have the two outputs of the comparators going into an XOR gate, so I should only get a high when it's between the two temps. The problem I have is the temperature sensor is the LM35CAZ-ND http://www.national.com/ds/LM/LM35.pdf transitor which increases the voltage by 10mV/°C that means the two voltages are .21 V - .32 V for 70 °F - 90 °F respectively, this is too small of a difference so that it's hard to set the two "+" sides of the comparator (LM339). I'm wondering if either someone knows how i could either amplify this difference or set it up so that i get a greater difference between the two voltages. Any help would be awesome as this is my final project and is due Thursday report and all. Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    Starting with your schematic would work better. It sounds like you have too many parts already, meaning you already have more than enough amplification to do the job. It's just arranged wrong.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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  4. #12

    Expert

    Nov 30, 2010
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    Hot shot!

    Just because you have 10 times the experience here that I do, you can put your mouse on exactly what he needs.
     
    Last edited: May 17, 2011
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  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If you already have comparators working, then just change your Lm35 sensor to a thermistor and a trimpot to adjust it.

    You should easily be able to use the thermistor and get a good working comparator circuit.
     
  6. Ron H

    AAC Fanatic!

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    Google is my friend.:)
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you want a linear reaction in fahrenheit you could look at the LM34.pdf
    This will give you 0.7 Volts @ 70 °F and 0.9 Volts @ 90 °F.

    Bertus
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    I've found that the LM34 is considerably more expensive than the LM35 here in the USA.

    Using an LM34 and an opamp to amplify its' output would be a suitable and inexpensive solution.

    Radio Shack stores generally carry an LM324 quad opamp, which are rated for single-supply operation and can sense inputs down to ground. The three unused opamps would need to be wired as voltage followers (each channels' inverting (-) input wired to its' output, and the noninverting (+) input to ground) so that they won't cause unpredictable oscillations.
     
  9. CLOVES101

    Thread Starter New Member

    May 16, 2011
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    The Opamp worked perfectly thanks all i got to demo it tomorrow
     
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