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LM3914 - question

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by caslor, Mar 22, 2010.

  1. caslor

    caslor Member

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    Thank you for one more time ;)


    A general question now..

    lets say that we have 10 led in lm3914
    We have select it to use it in dot mode and in step of 0.5 volts
    0v= zero led
    0.5v= 1st led open
    1v = 1st led turns off and the second led goes on ...

    Lets say now that we want to inspect 0,5v 1,5v 4v and 5 volt
    For that we would use the 1st, 3d,8th and 10th led and we would not connect any leds to other pins of lm3914
    But in this way when have 2volts input the led for 1,5v will go off until the 4v come and lm opens the 8th led..

    can we made this in order to have always the led on before the next step we want to inspect??

    i think that it can be done but i am not sure if i have to place a diode to the pins in order not damage the lm3914
     

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  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    You can connect as per your drawing, but you will a slight dimming of the LED's as they step from one pin output to the next,,, try it.
     
  3. caslor

    caslor Member

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    I will ;)

    THANKS !!!!! :) :)
     
  4. dave

    Dave New Member

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  5. caslor

    caslor Member

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    :( :(

    Sorry ! but for one more time i need your help
    I had the last guage that working from 15k to 150k
    it is cylinder temp gauge.. at 15k give 600 F temperature and in 150k gives the 100F temperature

    i try to simulate it in the previous schematics in LTspice but is working in very low voltages volts

    this sender dont work with ground.. takes the +12v and returns it back to the gauge through the resistance ...

    :( :(

    ''oliver'' here :p :p
     
  6. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I will look at your numbers.:)
     
  7. caslor

    caslor Member

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    You are my man Mr Ericqibbs ;)

    without you i still will working to figure out howo i would do all this projects ;)

    The problem with my tries to find a solution to the last gauge settings based on the schematics in Ltspice i had from you, was that all the time , what ever i made.. changing values of the resistances i could achieve only a very short change of volts... (i dont know how to explain this in english... cant remember the words :p )

    from 15k to 150k i had 4,97 to 4,96 range of change
     
  8. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I suspect that the cylinder sensor is a thermistor.??
    If so, the change in resistance versus temperature will not be linear.

    Is it possible that you could check some values at 100F to 600F in 50F or 100F steps.??

    The change in battery voltage [to the sensor] has to be corrected for in the amplifiers.
     
    Last edited: Apr 30, 2010
  9. caslor

    caslor Member

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    I am not sure... but i think that normaly must be thermistor

    yes it isnt

    Based in gauge readings depending from resistances i placed we have :

    600F = 15k
    550F = 17k
    500F = 19k
    450F = 20k
    400F = 23k
    350F = 27k
    300F = 37k
    250F = 40k
    200F = 52k
    150F = 70k
    100F = 150k
     
  10. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Those readings are non linear, it looks like a thermistor.
    I will relook at the design.

    EDIT:
    Look at this plot, I have dropped the 150K/100F in order to expand the plot.

    I think the 300F/37K is incorrect in your table..I make it 33K
     

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    Last edited: Apr 30, 2010
  11. caslor

    caslor Member

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    I get a better trimer more accurate and i have

    resistance K = Fahrenheit

    15=600
    16=550
    18=500
    20=450
    23=400
    25=350
    30=300
    37=250
    47=200
    67-70 = 150
    110=100

    for more accurate readings i will use 2lm3914 here also as the previous temp gauge
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The circuit required to correct the non linearity in the tempr sensor and the effect of a 12V to ~15V battery voltage change will require more than a single LM358.
     
  13. caslor

    caslor Member

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    hi my friend
    thanks for looking it

    what will require in order to work?
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The problems are caused by the 'top' of the thermistor being connected to the +V of the battery and the fact that the thermistor response is non linear.
    The battery voltage could easily vary between approx 12V and 15V.

    Is it possible to access the +12V supply to the 'top' of the thermistor.???
     
  15. caslor

    caslor Member

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    power the thermistor is not a problem..

    we can supply it direct from the circuit with 8v ,9v 10v etc .... or we can do it like the other gauges.. pass from the thermistor the ground !!

    i think ground or + , passing through the thermistor will not be a problem.. it is just a resistance depending from the temperature .. right?
     
    Last edited: May 1, 2010
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Let me know if you can power the thermistor from +8V.???
     
  17. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi caslor,
    Look at this with thermistor powered from the +8V [and electrically isolated from the engine block]

    Set up the LM3914 to suit the Vout.
     

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  18. caslor

    caslor Member

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    Ericgibbs for one more time you save me ;)

    Yes we can power the thermistor from the +8v.

    The thermistor is a washer that goes to the spark and has two wires at the end.
    I think it is electrically isolated from the engine block as we havent any electrical issues until now that it was powerd with +12v

    here it is 2 pictures from the sender and the wire :)
     

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  19. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi caslor,
    This is a simpler version and with improved linearity than the previous circuit.
    Set the LM3914 to suit the output.

    EDIT:
    Deleted, the linearity was worse than predicted.
     
    Last edited: May 3, 2010
  20. Hero999

    Hero999 Banned

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    I haven't read the entire thread, just the last couple of pages so sorry if any of the following has already been suggested.

    Here's a couple of suggestions:

    Could you use the VU bar graph IC to correct the logarithmic curve of the thermistor?

    How about replacing the thermistor with a linear temperature sensor IC?
     
  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi hero,
    The circuit in post #96 works OK, the OP is using that version..
     

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