LM3194 and LM35 Temp sensor guage

Discussion in 'The Projects Forum' started by MustBeMattyXD, Dec 7, 2011.

  1. MustBeMattyXD

    Thread Starter New Member

    Dec 7, 2011
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    Hello Guys,
    So bassically im new to this forum and want help on a curcuit design for a project im doing for my college.
    im trying to design a curcuit that detects change in tempreture from a heatsink and depending on the change in tempreture the bar display either goes up or down. now people have been taking about using a resistive temp sensor for the LM3194 but i will be using the LM35 which has increments of 10mV - 1°c. now i was just wondering how to connect this up. the LEDs will have to be full brightness and it has to shine through a smoked plastic sheild, to put it straight it will be part of a PS3 Fan control system. the sensor will be used on a separate design aswell to controll fan speed using PWM but this is a separate curcuit. ive looked on the forums about people making sensors for the cars and i found a diagram but im only wanting to use about 10 LEDs, the design is below.
    any help would be greatly appreciated
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    You need to specify the high and low temperatures (voltages), that will give you 0 LEDs or 10 LEDs. You should be able then to use a single LM3914 but you MIGHT need an op-amp in between to amplify and/or offset the LM35 output.
     
  3. MustBeMattyXD

    Thread Starter New Member

    Dec 7, 2011
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    0
    Well normal room temp is about 25°c and highest temp ive measured is about 90°c, so its about 250mV (0.25V) to 900mV (0.9V), so that probably means im going to have to use an op-amp, just wondering now how id set it up for the LM3914 ?
     
  4. MustBeMattyXD

    Thread Starter New Member

    Dec 7, 2011
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    okay so ive worked out ill need an op amp which has a gain of x5 but now i have no idea which one to use ?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, you can adjust the values of the resistors for the internal reference and LED intensity..

    There is an LM3915 calculator that you can download from Rod Elliot's download page:
    http://sound.westhost.com/download.htm
    Look for "LM3815 - Nifty little calculator to determine the values for the LED meter IC"
    It's 4th from the bottom under "Executables".

    For working out the low side of the ladder, just remember that the internal ladder is 10k. So, if RHI is 900mV and you need RLOW @ 250mV, (900mV-250mV) / 10k = 65uA; 250mV/65uA = 3846 Ohms you need from RLOW to GND.
    You can get close (3841 Ohms) with a 4.3k resistor in parallel with a 36k resistor.
    You could also use a 3.6k resistor in series with a 240 Ohm resistor to get 3840 Ohms.

    [eta]
    I don't know what you are doing with the 470 Ohm resistors across RHI and RLO.
    There is a limit on the resolution of the LM3914; I don't remember what it is offhand.
    I see you have three LM3914's - are you hoping for a 30 LED display?
    I am just about out of time this AM.
     
  6. MustBeMattyXD

    Thread Starter New Member

    Dec 7, 2011
    19
    0
    OKay so ive been doing a bit of research and work and ive got this far , here is the image attached and following what SgtWookie said ive attache the resistors to the RLOW. hoping this would work, might end up just getting the parts and trying it :)
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you have 470 Ohm resistors across RHI and RLO, you have blown the calculation I've done out of the water.

    If you are actually using three of the 3914's in series to get a 30 LED display, you need to substitute 30k for the 10k and then re-do the calculation. I can tell you right now that you will wind up with 3846/3.
     
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