LEDs in Mouse & Resistors

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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

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?



Re: LEDs in Mouse & Resistors

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Try a 1 K Ohm.

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No. Devices will only draw the current they need but they're exposed to
the full voltage.

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Yes in parallel but not in series as there's a voltage drop across each
one so the last one will get less voltage than the first.


Light travels faster than sound. This is why some people appear bright
until you hear them speak.........

Re: LEDs in Mouse & Resistors


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You may have already damanged that LED onto the point where
it'll never be as bright anymore.  LEDs last for many years
driven conservatively, but overdriven that much, they'll go
out like light bulbs or even run faintly in some cases.

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You take the initial voltage (which we assume is the
"Forward Voltage" of 3V, subtract that from the supply
voltage (5V), and divide the current (20mA or 0.02A) using
Ohm's law.  

(5-3) / 0.02A = 100 Ohm

100 Ohm is not too uncommon a value, but 120 Ohm is even
more common and it is safe to round up but likely not
rounding down (don't use 68Ohm).  You'd lose about 15% of
the light output by rounding up, but that's not a lot

Yes you can use 3 LED, but since 3 x 3(V) > 5V, you cannot
put them in series, instead you'll need to put them in
parallel- each with it's own current limiting resistor (120
Ohm) in series because the spec'd forward voltage is only a
guideline, each LED will vary slightly and one would end up
using significantly more current than the others, especially
when you're using a fairly small resistor value.

Each LED does use 20mA (if that's what you choose (as you
should for a maximum) by the resistor value selected), so 3
LEDs would be 60mA.  Keep in mind that while you may not be
exceeding the power of the port yet, that each computer port
pair has finite power capability so if you had other devices
on the same port or port-pair, you'd need to estimate the
total power budget and  how much remains that can be put
towards powering a number of LEDs.

If the base of your mouse is translucent, one ultrabright
LED will produce a lot of light if aimed well.  Personally,
I'd think about painting the inside upper half of the shell
metallic chrome colored, then aim the LED up at it so the
light is reflected in several directions and diffused a bit.
I just think an even light glowing would look better than 3
semi-focused brighter areas but it's not my project so...  I
don't even know what look you're going for.

Finally, many LEDs are spec'd very crudely but if you're
carefully planning with attention to detail, you may find
you can put a bit more current through them.  For example
many of the typical 5 mm are good for 100mW (but check the
spec sheet), so if you had 3V @33mA that might produce the
most light but the further you gove above (roughly) 50mW,
the more you'd need to consider how well the leads are
'sinking away the heat.  Ultimately it is the heat that
kills them sooner, or later.

Here's an LED calculator,

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