Temperature circuit help

Discussion in 'The Projects Forum' started by mattinho, Mar 25, 2015.

  1. mattinho

    Thread Starter New Member

    Mar 25, 2015
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    I am new to electronics and am still learning so please go easy on me :)

    I would like to build the following but have found myself stumped.

    I would like to make a circuit that can detect both increases and decreases in ambient air temperature. I need 3 LEDs on the circuit, a base led that I can use to set the base reading by use of a pot. The other two would be on either side, one for increases and the other for decreases in the set temperature. If possible I would like to add a buzzer that would sound when either of the outside LEDs light(temperature change). All to be powered by a 9v battery.

    I hope this makes sense. Any help you can offer will be very much appreciated.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    would you like chips/fries with that Sir?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Look up "window comparator" and LM35 thermometer.

    You might want to look at LM3914, which is a fancy window comparator meant for 10 levels of indication, not just 3.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Actually, three comparators. Since they come 4 to an ic package (LM339), this should be compact.

    What are the deltas between ambient and low trip, and ambient and high trip? How many degC or degF? What is the normal ambient range?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Actually, just two are needed for a window. ;)
     
  6. MikeML

    AAC Fanatic!

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    I was figuring he needs to know that he is in the middle of the interval between the high trip and low trip, hence three :p (wasn't born yesterday)
     
  7. wayneh

    Expert

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    Well, then you'd need all 4 on the LM339. Two for the inner window and two for the outer extremes.

    We need some specifications.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the middle one had very little hysteresis, then you could diddle the pot so that its LED was on the verge of flipping back and forth, thereby centering the range on the temperature that exists at that moment.


    Since there is no absolute temperature required, even a glass-bead thermistor could be used...
     
  9. #12

    Expert

    Nov 30, 2010
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    That's why I stayed out of it. Lack of specs quickly degenerates into guessing and second guessing.
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
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    I thought he wanted one LED to light when the temperature was increasing and the other to light when the temperature was decreasing. And I am thrown by the "base reading" LED. You all are magicians for your ability to interpret questions.
     
    #12 likes this.
  11. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    Hello,
    Set the value of mV voltage divider according to your requirements...
    example: there is inverting and non inverting pin + - in any like lm394/lm339
    set one pin to 2.5V if supply is 5V by 10k ties to vcc and 10k to Gnd.

    there are NTC and LM35 for temp. work 10mV at 1*C and 20mV at 2*C!
     
  12. MikeML

    AAC Fanatic!

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    The middle LED and the ability to set it to ambient temperature comes from this sentence: "I need 3 LEDs on the circuit, a base led that I can use to set the base reading by use of a pot."
     
  13. wayneh

    Expert

    Sep 9, 2010
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    What's missing is the behavior of the central LED, whether it should
    1) Stay on even when one of the other LEDs comes on,
    2) Go off immediately as one of the others comes on, or
    3) Go off when temperature leaves the central target zone, but before temperature has exceeded one of the extremes by enough to light one of those LEDs.

    I'd vote for #2 but it's not my project.

    In each case, the temperature behavior needs to be defined - the on and off temperatures for each LED.
     
  14. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    While we are second guessing...

    I would make the middle LED an inverse-connected red-green bi-color one. The middle LED would be driven by an opamp connected so as to drive the LED so that it goes from red to amber to green as the trim pot is diddled to match the ambient temp. The temperature span over which the color change occurs would be much less than the delta temperature from Low LED to High LED.
     
  15. wayneh

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    Sep 9, 2010
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