rite here goes. i have modded my optical mouse by putting some extra blue leds in it just to make it look cool. but they are wired directly into the usb power for the mouse. this works fine but unlike the led on the back of my mouse (the one that is ment to be there) they are always on full. how would i make the other leds dim when this one led does? i have ruled out connecting them directly to the power for that led as i think the current draw might damage the electronics (tell me if im wrong) the only other way i can think of is with a transistors, but transistors are a bit beond my limited know how. can someone tell me how i would do this? Please
i would use resistors, but i dont know a hole lot about that either (WOOT WOOT 75 posts avitar here we come)
So you want the brightness of the new LEDs to match the brightness of the stock LED? Hmm - first thing that comes to mind is to use an op-amp configured as a non-inverting buffer ('voltage follower', see here: http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm). So like you'd have the high side of the stock LED connected to the op-amp input, and the output of the op-amp would be connected to your new LEDs, possibly with a resistor if they were still too bright. There are probabaly easier ways - but I like op-amps.
Transistor for this is pretty straightforward. Since the current is already limited for the first LED, you would hook this straight into the gate input of the transistor. Your hot line goes to the collector, and your LEDs (with resistors of course) are hooked between ground and the emitter. Pretty sure that should work, though Cpemma will probably come and clean up.
Transistor source follower you describe loses about 0.7V (Vbe) from the ingoing, which would severely affect a LED's brightness. michael's op-amp source follower doesn't lose anything (until it gets near the supply voltage rail).
Knew it was coming! Yeah, I've heard about the voltage drop but I've near 0 experience with LEDs, so yeah... Best I be learning about Op Amps.
On average LEDs have a voltage drop of about 2v silicon diodes and transistors are about .6-.8v, everyone just says .7v because most of the time its true. Try taking a multimeter across the LED for the eye when its on high and when its on low setting, also measure the voltage drop across the LEDs you installed, then you can see if its possible to make it work.
I would try a transistor first, really cheap and you should have a few somewhere. TBH, I doubt a few LEDs would damage the circuit.