okay - cheasy title but a serious question I've got this: which works fine when I energise the HDD LED + and PWR LED + pins but when I hook it into the PC, power the pc on, both sets of leds (pwr and hdd) come on when there is no HDD activity - I know this is something simple that i've overlooked or whatever but I just cant see the solution. oh ya the LED's are fed with a 12v line. Thanks,
It might be better to try the electronics forum (PM a mod to move it) Are the transistors upside down, shoudlnt the emitter be at the bottom?
HDD you're right to use a PNP, but try without the 330R pull-up, just a base resistor to the -pin. If for some reason the +pin isn't the full 5V it's maybe doing a pull-down enough to turn the tranny on. Easier with optoisolators. I've just read the post again, the 12V supply is the problem. You can't get the PNP bases over 5V, so they're always on. You need the pull-up from the -pin through about 10k to the 12V line, but I don't think that's safe with 5V logic. Either use 5V for the leds, a second NPN transistor to level-shift, or use optoisolators & 1 NPN.
I would have thought that these (BC640's) could handle that voltage but I've never used them with 12v so your probably right Just on the use of optoisolators - why would I still need the NPN? I don't really have a choice with the LED's as they are blue and white and both need over 5v's. Thanks for your help!
so if i where to use an opto and a transistor would this work then: http://www.rapidelectronics.co.uk/r...&CTL_CAT_CODE=&STK_PROD_CODE=M32291&XPAGENO=1 seems a bit more reasonable then buying them indepenently THanks,
The BC640 transistors can handle 12V no problem (80V max) but to turn them off you've got to raise the base voltage to just over (12-0.6)=11.4V which you can't do with a 5V high. The opto-darlington is OK for 50mA (2-3 leds in parallel) but any more the NPN booster is needed. A cheap transistor will handle 200mA or more and will be fully on, the darlington may not be. If you're taking that from the Rapid specs, that's Vf(max) with typos added, normal Vf is around 3.6V. Check the genuine Kingbright specs.
Thanks everyone - after looking at it I did see that it was the 12v. I had switched this to give the LED's more voltage but I just simply took the resitors out all together and am running them off 5v's and its great I had drawn this up when I was planning to use 5v in the first place Cheers again!
It may seem to be working fine but if your driving the LEDS above the recommended fV then they will probably burn out rather quickly.
...And more importantly way above their maximum rated current. Consider the VI characteristic curve for a diode and you'll see why those resistors are needed.
