Originally posted by cruiser78
I'm wanting to put a blue LED into my Logitech Mouseman Wireless Optical and can only seem to find 5v LEDs and the max voltage going through the LEDs is around 1.7 or so... somewhere I read about adding a resistor or something into the circuit so that the 5v LED would work properly on a 3v system but I just can't seem to find that page again (It was on a page about customizing the micro RC cars and adding lights to them using a 3v button cell). If I can't find a blue LED that is bright enough I might try some other color... or infrared (it'd be unique but boring... I've heard that it works good cause the sensor picks up infrared signals best or something like that)
The voltage drop resistor needed for an LED is actually quite simple. An LED has 2 major parameters:
Vf = the forward voltage drop across the LED when forward biased (turned on). Any voltage above this value needs to be dropped across a resistor.
Imax = the max current the LED will handle before it starts to cause damage.
Given these values, you can compute the resistor needed by the following:
R=(Vcc-Vf)/Imax
Where "R" is the resistor value in ohms and "Vcc" is the supply voltage. For safety, I usually use a value slightly less than the Imax value in my calculations. The computed resistor value will most likely be a "non-standard" value. You'll have to find one that is close or use a couple resistors in parallel, but if you need to round the value, make sure you round the resistor value
up. Rounding down will increase the current through the resistor.
Unfortunately, in a device as small as the mouse, you may have a bit of trouble locating the existing resistor to replace it with the new value. It may even be a surface mount device (really small and easy to overheat with a soldering iron).
You may have a tough time getting a blue LED to work in a cordless mouse that used 2xAA batteries. This makes the supply no more than 3v total and a blue LED needs at least 2.5v or more to run. The end resistor value would be very small indeed.
Incidentally, if you want to use LEDs in other projects (like case mods, etc) and want to use the 12v supply, you also have to take the power rating of the resistor into consideration otherwise it may get too hot since it has to drop so much voltage. This is calcuated by:
P=I^2*R
Where "P" is the power in watts, "I" is the current (which is the same as the Imax value), and "R" is the resitor value in ohms.
Ok, enough Electronics class. Have fun.