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Newbie question: circuit for harddrive indicator

Discussion in 'Electronic Basics' started by fob, Jan 5, 2005.

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  1. fob

    fob Guest


    I want to light up an ultra violet reactive 120mm computer case fan
    with UV LED's whenever my harddrive is accessed.

    I figure that I will use 3 UV LEDS to light up the fan, and the 3 LED's
    will be activated by a transistor that is triggered off of the computer
    motherboard's harddrive LED connector.

    Problem is that I don't have any idea how to do this. I figure I will
    need to power it off of a molex connector from the power supply (5v or
    12v) and use a combination of resistors, a transistor and maybe other
    stuff (like caps and/or a voltage regulator). I do know that LED's are
    2.4v devices and are polarized.

    Has anyone seen this type of circuit or know how to make one?
    Thanks for any help you can provide.

  2. Don Taylor

    Don Taylor Guest

    For a first project, when you are a little uncertain about whether
    your project might damage something in the computer, how about this?

    What if you used a light sensor, say a phototransistor, that would be
    illuminated by the light from your harddrive led? The phototransistor
    acts sort of like a switch, when the light is on it the switch is
    closed, no light and the switch opens. That can then be used to turn
    your transistor on and off. And the transistor can turn on and off
    enough power to switch your 3 uv leds on and off. All this could be
    tested without risking wiring it to your computer, so worry can be
    reduced, always a good thing with a first try at something.

    Sound like a reasonable approach? Then wen can move on to the next step.
  3. Another newbie here. I haven't dabbled in optoelectronics yet, so
    I thought I'd try my hand at this circuit...

    Here's what I came up with. Anything wrong with this?

    IDE LED +5V GND GND +12V
    GND + Red Black Black Yellow
    .-------. .---------------------.
    | o o | | o o o o |
    '-------' '---------------------'
    | | | |
    | | | +-------------------------.
    | | | | |
    | V ---> \| | UV |
    | - |---' LEDS |
    | | ---> <| ___ // |
    | | | .--|___|------>|------+
    | | | | 120 Ohm |
    '---' | | ___ // |
    | 120 Ohm |
    | ___ // |
    | 120 Ohm |
    | ___ // |
    240 Ohm New HD LED

    (created by AACircuit v1.28.4 beta 13/12/04

    Well, first uncertainty I don't know is, if the phototransistor
    base (if present) should be grounded if it's not used to force drive
    the transistor or left floating? (The only time I played with one,
    I left the base floating...)

    The old HD LED is probably not enough to drive the phototransistor
    fully, so the resistors should probably be less than what if they
    were hooked to the +5V directly?

    If the old HD LED was recycled to the "new" side, that
    LED+phototransistor could be replaced with an optoisolator. That should
    be a safe way to trasmit the signal (and essentially what that above
    is), but are there any others?
    I would be vary to directly drive a transistor base from mobo IDE LED
    output, since that would mean you'd have to hook the PSU common and
    mobo ground together, right? (Which I'm not sure would be a good idea,
    since they're so far apart from each other... Or even if they should be
    in same ground potential in the first place.)

    Anything else I didn't think of?
  4. Don Taylor

    Don Taylor Guest

    Nicely done.

    I would check to see whether your choice of phototransistor can
    supply enough current to directly drive 4 UV leds or not. That was
    why I suggested using the phototransistor to drive the base of a
    separate transistor, giving more gain and more power, letting you
    adjust the bias on the transistor to make best use of the hard drive
    led intensity levels.
  5. Kunal

    Kunal Guest

    You can use Pin 39 on the IDE connector to detect activity on the hard

    Simply connect it to a transistor driver and put the LEDs as loads.
    Let me know if you need help with the actual circuit.

  6. cpemma

    cpemma Guest

    One pin of the HDD Activity header connects to +5V through a resistor,
    commonly 220R-330R. Other pin goes to pin39 of the IDE cable/drive which is
    an open-collector NPN transistor, switched on activity so that header pin
    goes low and the LED comes on. There are a couple of steering diodes so
    either drive will light the one LED.

    You *can* use a PNP transistor switch and the molex supply to boost the
    current, but it's a lot safer to just plug an opto-isolator's diode in place
    of the LED; check its rating, you may need a small NPN transistor switch as
    well. If you put your 3 UV leds in series (with the appropriate resistor)
    the 12V supply will run them all with minimum wastage.
  7. fob

    fob Guest

    cpemma (et al):

    I hate to do this to you, but could you draw the circuit with actual
    part numbers and values? I know that seems kind of lazy, but I haven't
    had any circuit or breadboard experience and an example would really
    help (although Toni's example was good after I printed it off from

    I think I understand Don's, Toni's, Kunal's and your comments about
    using an optical device to isolate the harddrive activity signal (led
    drive) from a transistor that switches the 5V's current for safety's

    When I say I'm a noobie, I mean I am a nooooooobie!

  8. cpemma

    cpemma Guest

    Have a look at this one:

    For a few leds, most any small NPN transistor will do, a BC107, BC109 or
    2N3904 will take 300mA, a BC547 about 500mA.

    You could also use a 5V supply, changing R1 to a 1k.
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