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Comparator circuit for temp indicator

Discussion in 'Electronic Basics' started by dwbauer, Mar 26, 2006.

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  1. dwbauer

    dwbauer Guest

    I have two circuits at http://bellsouthpwp.net/k/e/kelmark2180/therm.html to
    show what I am thinking. I am trying to make a lead acid charging
    temperature monitor. The circuits are from existing url's that I found by
    searching Google. My question is repeated on the page as follows:
    I want to set up a temperature monitor for charging lead acid batteries. I
    think the sensor can be imbedded into the negative battery post. Distance
    from the sensor to a LED indicator would be around 15 feet. I don't think
    there is a need to have exact temperatures. Likely a range within 10 degrees
    will suffice.

    I would like to have the output from the temperature sensor light up a red
    LED when the temperature exceeds 130° Fahrenheit in place of the voltmeter
    in the first circuit (1). Since the first circuit is supposed to scale the
    output and provide 10 mV/degree, could a proper comparator circuit be made
    to indicate when the voltage exceeds 1300 mV or 1.3V? (Circuit B)

    I have no clue how to calculate what resistor values would be needed in the
    comparator circuit (2). I am assuming there will need to be a trimmer to
    vary when the LED would turn on, maybe at V - REFERENCE? Would I need to
    supply the comparator voltage from a different source and what would that
    be?

    Any help is certainly appreciated; Don
     
  2. ....
    Look into the lm34 temp sensor ic's, they look like a transistor. Its
    output is a voltage that is linear to temp change.
     
  3. Andrew Holme

    Andrew Holme Guest

    One input of the comparator can be connected to where the voltmeter is in
    the other circuit; and the other input can be connected to the wiper of a
    trimmer. The trimmer can be connected between the power supply rails (like
    RV), or you could connect the top end of the trimmer to the +5V regulated
    output from the LM7805. The trimmer could be anything from 10K to 100K;
    it's not critical. You might also connect a large resistor (>1M) between
    the comparator output and the (+) input to provide positive feedback for
    hysteresis.
     
  4. dwbauer

    dwbauer Guest

    Thank You so very much for the reply. I think I can follow your instructions
    and give it a try.
    Thanks Again; Don
     
  5. John Fields

    John Fields Guest

    Try this: (view in Courier)


    +12V>--+---[78L05]--+-------+----------[750R]--+
    | | | | |
    [0.33µF] | [0.1µF] | |
    | | | | |
    +------+-----+ | |
    | | [HLMP4700]
    | | |
    | | |
    | +------+-------+ |
    | | | | |
    | | | +---|--[1M]--+-[750R]--+
    | [3740R] | | | |
    | | | | | |
    | +------|---+--|+\ |
    | | | | >------+
    | | [LM34]---|-/ U1A
    | | | |
    | [1000R] | +-------+
    | | | | |
    | | | +--|+\ |
    | | | | | >-+
    GND>---+-----+------+-------+--|-/ U1B
    LM393


    Use the 12V from the battery to run it, wire it up on a little piece
    of perfboard and keep everything pretty close together. Instead of
    running a 15 foot set of leads to the sensor and building everything
    around the LED, wire everything around the sensor and run the leads
    to the LED.

    I'd use a twisted shielded pair for the LED. with the shield
    grounded.
     
  6. John Fields

    John Fields Guest

    ---
    Better yet:

    +12V>--+---[78L05]--+-------+--------------------/ /--+
    | | | | |A
    [0.33µF] | [0.1µF] | [HLMP4700]
    | | | | |
    +------+-----+ | +------+---/ /---+
    | | | |+
    | | [1K5] [1µF]
    | | | |
    | +------+-------+ | GND
    | | | | |
    | | | +---|--[1M]--+
    | [3740R] | | | |
    | | | | | |
    | +------|---+--|+\ |
    | | | | >------+
    | | [LM34]---|-/ U1A
    | | | |
    | [1000R] | +-------+
    | | | | |
    | | | +--|+\ |
    | | | | | >-+
    GND>---+-----+------+-------+--|-/ U1B
    LM393
     
  7. AIUI the OP wants it to trip at 130F. Wouldn't your divider be a bit
    low (I calculate ~1.055V) or does leakage thru the LED pull it up?
    Couldn't the OP use a 5K linear pot in place of the 3.74K resistor to
    fine tune the trip point?
     
  8. John Fields

    John Fields Guest

    ---
    No, you're right. It should be 1300 ohms. I did the math properly
    but transcribed the 1300 onto the post as 1000. Good catch, thanks!
    :)
    Yes. Matter of fact, he could replace both resistors with a 5k pot.
    That might even be preferable if he wanted to make the trip point
    adjustable:

    +12V>--+---[78L05]--+-------+--------------------/ /--+
    | | | | |A
    [0.33µF] | [0.1µF] | [HLMP4700]
    | | | | |
    +------+-----+ | +------+---/ /---+
    | | | |+
    | | [1K5] [1µF]
    | | | |
    | +------+-------+ | GND
    | | | | |
    | | | +---|--[1M]--+
    | | | | | |
    | | | | | |
    | [5kR]<---|---+--|+\ |
    | | | | >------+
    | | [LM34]---|-/ U1A
    | | | |
    | | | +-------+
    | | | | |
    | | | +--|+\ |
    | | | | | >-+
    GND>---+-----+------+-------+--|-/ U1B
    LM393

    It should be sealed, though, since it's probably going to live
    pretty close to H2SO4 vapor. :-(
     
  9. No biggy. :)
    I was playing around and with 5% resistors, the trip point could vary
    between ~120F - ~139F. Sounds like some kind of adjustment would be a
    good idea. 1% resistors would narrow that down to something that's
    probably negligible (127F - 131F).
    Not a good place to live. Is there any worse place (on Earth) for
    electronics than in an automotive environment?
     
  10. John Fields

    John Fields Guest


    The 78L05 has an output voltage which can vary from between 4.75V to
    5.25V, so with an output voltage of 4.75V and worst case
    resistances for the 3740 ohm and 1300 ohm 1% resistors for a low
    reference out we have:


    4.75V
    |
    [3774R]
    |
    +----1.208V ~ 121F
    |
    [1287R]
    |
    GND

    At the high end we have:

    5.25V
    |
    [3737R]
    |
    +----1.365V ~ 137F
    |
    [1313R]
    |
    GND

    Which with the OP's request for a 10% machine would be fine.

    Unless he actually meant +/- 5%, in which case the pot would be
    needed to adjust out the variation in the 78L05's output.
     
  11. "John Fields
    Oops, I didn't even think about the voltage variation.
    Ah, I've heard it's nasty in there.
     
  12. dwbauer

    dwbauer Guest

    Thanks for all the expertise.
    Actually, this will be used to monitor charging on a large 12 volt 1200 amp
    hour battery bank with 150 amp alternator and a 120V 70 amp charger when on
    shore power. Realizing that we don't get all those amps in reality but it's
    a start.
    I'll get busy and order parts.
    Thanks again for everyone's help and the time for circuit drawing; Don

     
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