Re: Light Emiting Diodes

Posted by Frank_NH on Feb 11, 2015; 7:09pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Light-Emiting-Diodes-tp17549p17574.html

I use 5mm clear for on/off indicators and 3mm diffused for clipping (but have used 5mm - the size doesn't make much difference).  You can calculate the resistor value required (to keep the LED from burning out!) knowing the source voltage and LED forward voltage drop and current.  For example, let's assume a forward voltage of 1.6V and typical current around 2 mA  (check the data sheet for your LEDS!).   For a 9V source, and using Ohm law, we have:

R = V/I = (Vsource - Vf)/If = (9 - 1.6)/(2xe-3) = 3.7K ohms

Remember that the indicator LED will drain power from your supply, so using the lowest current while maintaining a reasonable brightness is the way to go (less current = less bright).  Given that fact, and that LEDs vary in forward voltage from 1.6 - 2.0 or more volts, a current limiting resistor (CLR) value of 3.5K - 5K will work fine.  I tend to use 4.7K or 5.1K depending on what I have available, and the light seems bright enough.

Hope this helps.