Newbie question: circuit for harddrive indicator

Folks:

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.

Fob

 

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.

 

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 www.tech-chat.de)

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?

 

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.

 

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

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

Kunal

 

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.

 

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
Notepad.)

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
sake.

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

Fob

 

Have a look at this one:
http://forums.bit-tech.net/showpost.php?p=255359&postcount=8

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.