Hi all, I've got a simple problem I was wondering if I could get some help with, if you don't mind responding please insert all the digression you like (I will be a little verbose, please point out if I have something wrong), as I would like o gain a better understanding of what to do. My knowledge of electronics is quite shameful.
I need to be able to control an LED (perferably two) from my computer, I would also like to control their intensities, but I've come to the impression that it would be hard to do because LED brightness is controlled by current and not voltage (why am I having trouble understanding that?), so something like a DAC hooked to the parallel port won't work (how much current does the parallel port supply anyway?).
Special note: I have a special 300 nm emission LED, which is expensive. The company says it is just like any other LED, but I have to be careful of reverse bias. THe only way this can happen is if I put it in backward, no? It's also got a ground pin, which I can guess goes to ground, but I have difficulty understanding exactly why you need it...
Also here are the specs:
Typical Maximum Condition Forward voltage: 5.5 7.5 IF=20mA
Does this look like a normal LED to you in terms of voltage and current reqs? Should I be able to drive it from the parallel port (+5V vs. +5.5V?) of a PC? Will I need an external powersource (should just a plain old electronics adapter do?)? Is there anything else I need to worry about (potential powerspikes that could blow the LED etc.)?
I was suggested some prestuffed control boards like from iowarrior (it'd be convinient to be able to control these LEDs with USB from my iBook)....but apparently they don't have enough power to drive my LED. And it seems like overkill with a few of these other control boards (I am not religous about software, but my life would be easier if they had linux or OS X software/libs).
So I think that leaves me with a parallel port interface.
Does the following make any sense? (please don't hit me with anything, I hope the ASCII turns out OK):
Resistor (? 5.5V/0.02A = 275ohms? Could I make this a variable resisitor so I could alter the intensity with a knob?) | | | -----pin D7 on parallel port (do I need a \/ | resistor?) | | |
+5.5V -------/\/\/\-----transistor(NPN)------| | ---- \/