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Thread ID: 28946	2003-01-05 22:19:00	anyone who knows much about electronics...	tango (2697)	Press F1

Post ID	Timestamp	Content	User
111311	2003-01-05 23:58:00	ok cool. do you know how to go about getting it connected to the HDD light output?	tango (2697)
111312	2003-01-06 00:48:00	Not without measuring it with a multimeter, as it will be one of two ways . 1 . The "ground" connection will be switched by the HDD electronics 2 . The "supply" connection will be switched by the HDD electronics . You need a multimeter to determine which . If you use a cheap small signal NPN transistor, connected with the emitter to ground, and the LED -ve connected to the collector . Connect the LED +ve to a 150 ohm resistor, and the other end of the resistor to +12 v . Connect a 2,200 ohm resistor to the base lead of the transistor . The other end of the resistor to the HDD LED . Depending how its driven, one side of the HDD LED will always give blue LED "ON" or "OFF" The other side of the HDD LED may give blue LED "ON" when HDD LED is on, or "OFF" when HDD LED is on . If its in reverse you will have to get a bit more creative and invert the signal, but I don't have any data to tell me which way it is . Use your multimeter to decide . You want the 2,200 ohm resistor to be connected to a signal at the LED on the HDD that goes high (+12v) when the HDD LED is on for the above to work . All risks are yours, not just to the LED but also to the HDD itself .	godfather (25)
111313	2003-01-06 02:21:00	Been there, done that . Yes it does have a Vf of 10 . 5 . So assuming you use a transistor "switch" and run it off 12V, a 47 ohm resistor will allow it to pull its 25mA . It must have a bunch of junctions in series to get a Vf like that --- and the 140 degree visibility . . . normally 20 degrees, but that probably explains the 30 mCd . The high intensity ones gain quite a bit bu having a very narrow beam . So a 10k, a 47R, a BC547 should do you . Cathode of the LED to +12 through 47 ohms, anode to collector of transistor, emitter to ground . 10k resistor between the base of transistor and the disk activity LED . (Usually these supply 5 V through a current limit resistor to the front panel LED . ) My way of driving LEDs from "random" supplies is by putting one of the flashing ones in series . The flashing LEDs have an IC which takes up to 15 V, and incorporates a constant current supply, so you can have a string in series .	Graham L (2)
111314	2003-01-06 05:47:00	Tango, Spend $2.50 on a DSE catalogue if you haven't already got one and turn to page 316. Read the section on opto couplers. If you already have the big LED working, then hook up the input to an opto-coupler to the HD led connection - you can either replace it or hook it in series. Connect your new LED circuit to the output side of the optocoupler. Connect the circuit that you have working n series with the collector-emitter circuit of the optocoupler (make sure you get the polarity right). As others have mentioned, you may need some current limiting in the form of a series resistor if you are running your 10.5 Volt led off the 12 volt rail although there is a 1.5 v forward volt drop across the coupler so it should just work. The 4n25 or 4n28 should do the job okay. Also check out "using LEDs" on page 311. For $2, the DSE catalogue is a really good, simple electronics textbook. Regards, Andy	andy (473)
111315	2003-01-06 06:14:00	Graham, Did you mean 470R? My maths might be wrong: V = 12 I = 25mA, .025A R = 12/.025 = 480 if I got that wrong feel free to make fun :P	tango (2697)
111316	2003-01-06 06:16:00	also, why 10k? I know a transistor needs .6v to operate, but I was just wondering what maths is used to calculate that. Andy, I looked up optocouplers on the dse site, and they have 6 legs. what are they all for?	tango (2697)
111317	2003-01-06 07:54:00	Surely you could just remove the case LED and put in your new one and it will just work ?	-=JM=- (16)
111318	2003-01-06 08:20:00	Tango, transistors are current amplifiers, and whether the base drive was 1k or 10k will not matter, as either will drive the transistor to saturation. A 10k resistor, even driven with 5v will allow 0.5mA to flow. Most transistors of this type have an "amplification factor" of 200 or more, so 200 x 0.5 = up to 100 mA can flow in the collector. As you are limiting it to 25 mA with the series resistor (47R), the transistor is fully saturated anyway, the best way to switch. You do not size base current to meet the expected collector current only in a switching situation, as the power dissapation in the transistor will be excessive. The 47R is based on 12v - 10.5v = 1.5 (as the junction voltage is 10.5 for the LED) NOT the full 12v. 47 ohms is close to what is needed. (1.5/.025=~60 ohms). Graham is probably right that the blue LED is actually several LED's in series, hence the high Vf. Hence the need for a limiting resistor, as there is no internal resistor. E&OE	godfather (25)
111319	2003-01-06 08:45:00	> Surely you could just remove the case LED and put in > your new one and it will just work ? Yes, but some people have to have lights that dance and sing along to their HDD's workings :p	Chilling_Silence (9)
111320	2003-01-06 08:55:00	> Surely you could just remove the case LED and put in > your new one and it will just work ? I tried that, except the voltage coming out of it isn't enough to run the big LED :P	tango (2697)
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