Re: LEDs in Mouse & Resistors

On Mon, 04 Sep 2006 01:11:24 -0500, Jeff <Jeff@xxxxxxxxxxx> wrote:

Hi,

I want to add some LEDs to my mouse. The output of the mouse is 5v. I
have heard that there is really no such thing as a 5v LED, even if the
LED is labeled as 5v. I'm thinking of using 3v (20ma) LEDs with
resistors. What size resistors should I use? I have already tried a 3v
(20ma) LED without a resistor, but the LED quickly overheats and changes
color.

I have another question. If the output of the mouse is 5v, does a 3v LED
use 3 of the 5v, therefor leaving only 2v of output left? In other
words, let's just pretend there are such things as 5v LEDs...Can I attach
three 5v LEDs to a 5v power source? Or does one 5v LED use all the power
by itself? If it does use all the power, is there a way to get around
this problem?

Thanks,

Jeff

You should get a good entry book in electronics and learn about leds,
ohms law, and voltage and current.

Volts are not what you consider with leds. It is current. If you want,
a led's current is (as you mentioned) about 20 ma. So take the 5
volts, and with ohm's law, compute the resistance to pass 20 MA. Oh,
but there is voltage involved with a led, and we use .7 volts in the
calucuations. So 5V less .7V is 4.3 volts. Now 20 MA... and the
formula gives us about 215 ohms. Nearest standard value is 220 ohms.

Now, the mouse's 5 volt supply. You need to figure out how much
current is *available* to you to use. Remember, the port (either PS/2
or USB) provides some current but not an infinate amount. The mouse
itself uses current, so all the current available at the mouse is not
there for your use (although this is a technical point, there actually
is plenty of current for what you want to do).

Say you have 100 ma of current available. So if each led draws 20 MA,
then you can have five leds for a total current draw of 100 MA.

Oh, and each LED would use its own resistor (so 5 leds, 5 resistors,
each wired seperately to the 5 v source).


.

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