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PC LEDs and temp

Discussion in 'LEDs and Optoelectronics' started by silver, Aug 4, 2011.

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  1. silver

    silver

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    Aug 4, 2011
    Hey everyone, new to the forum.
    I have an idea for a project, i want to make 3 different color LEDs turn on/off and 3 200mm fans turn on/off according to the CPU temp.
    The fans speed is controlled via software, so as the CPU heats up the fans go faster, so i want to use transistors to turn off the fans when they reach 40% speed, and also have the LEDs light up as the temp rises.

    i know i can use 100ohm resistors with 5 LEDs and a 12V power source, but i can't figure out what transistors to use and how to connect them best.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
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    Jan 21, 2010
    You have set yourself a pretty complex project there silver.

    How experienced are you (not a lot I guess)
     
  3. silver

    silver

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    Aug 4, 2011
    Yeah not that experienced, took a class about it back in school but nothing seriuos.
     
  4. poor mystic

    poor mystic

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    Apr 8, 2011
    ...so split the work into 2 separate projects.
    Use an LM3904 bargraph generator with whatever temperature detector you end up choosing, and there's your coloured led display.
    How does the fan speed changing circuitry work exactly?
     
  5. silver

    silver

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    Aug 4, 2011
    Don't forget I'm a newb at this, and the fan speed is increased as the cpu temp rises,
     
  6. poor mystic

    poor mystic

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    Apr 8, 2011
    suggestion: google LM3914 and report back

    PS it's OK to ask any question at all
     
    Last edited: Aug 5, 2011
  7. donkey

    donkey

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    Feb 26, 2011
    have you figured out the temperatures you want this to go at? i am going of a very poor memory here and think 43 celcius is the highest a cpu will go without damage (very poor memory so please correct me) and what temp will the fans cut off? also at what temps do you want the leds to light up?
    is there a way to get the temp of the motherboard to drive this ciruit so the heatsink doesn't have to be removed?
     
  8. silver

    silver

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    Aug 4, 2011
    lol CPUs these days go up to 90c or more, and it's not a problem to remove the heatsink.
    but i wanted to use the exhaust fan V to monitor the heat, since it goes faster as the cpu heats up, i my idea was that when the fan is at 40% it turns off(not the cpu fan, the 3 big ones that bring air in and out) and as it goes faster and the V increases LED are lit up,
    say the fan is at 60%, green leds light up, when it's at 80% orange ones, and at 100% red ones
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    The fans are controlled by PWM I think, so you may be able to sample that signal, integrate it (to change it to a voltage proportional to duty cycle) and then use this to drive a LM3914.

    Since the speed is probably proportional to temperature, this would be (indirectly) a measure of temperature.
     
  10. silver

    silver

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    Aug 4, 2011
    any idea how i can change the PWN to voltage?
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    It depends on the amount of current the pwm output can supply.

    If it is high enough, you can place a resistor and a capacitor in series between the PWM output and ground. The voltage across the capacitor will be close to the average voltage of the PWM output.

    The RC time constant will determine how fast the output changes, and how much ripple is on the output.

    If the output cannot drive significant current (and you don't actually need much) then you can place a buffer amplifier between the PWM and the RC circuit.
     
  12. Solidus

    Solidus

    349
    4
    Jun 19, 2011
    Well, I don't know if you're running specialized fans or not - the ones I typically use are all or nothing and usually prompted by the CPU just to turn on at various intervals...if you're using spec fans this may be different.

    You'd have to find a way to maybe mount a thermo sensor to the side of the sink or something....I'm not exactly sure where I'd start if I were you.
     
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