I have revised the page with the LED resistor calculator by adding a chart below the calculator with forward voltage measurements of selected LEDs. You will see on the chart that the red, green and yellow voltage drops are all similar while the other LEDs have a higher Vf.

I was surprised that the pink’s voltage was so high as I thought it was a modified red, but it turns out that it is apparently more closely related to the blue and white devices.

The UV is also a modified blue, as would be expected. I did not have an infrared pair to test but if I find some I will add them to the chart.

**Update:** I have modified the calculator so that it is more versatile. Enter the current at which the LED is to operate and the calculator will find the resistor value. Alternately, you can leave the LED current field empty and the calculator will report the milliamps being used by the LED. The Voltage Supply and LED forward voltage values must always be entered into the form.

Where’s Bernanke’s Nobel??? ~ ThermionicEmissionsSays: 9:22 pm, December 16th, 2009[…] LED Resistor Calculator […]

RickSays: 8:57 am, January 5th, 2010Hi, what is the formula that you used?

I am also interested in the other formules of the ‘calculate XX’ pages of your great site!

adminSays: 6:07 pm, January 5th, 2010The formula is:

Resistor in ohms = (V-Vf)/Current

V= the voltage supply, and Vf is the LED voltage drop. Current is in Amps.

So, for a 9v powered pedal that we want to use a red LED (1.95 voltage drop) at 10 milliamps, we would have:

Resistor = (9-1.95)/0.01 for a value of 705 ohms

DanSays: 8:49 pm, April 12th, 2010Thanks for this calculator. Does this mean I can order any LED and then look up the specifications to figure out the resistor needed? What specific LED do you suggest? I was looking at the Mouser stock list today and had no idea which LEDs to buy, so I picked this one (kind of randomly!):

http://mouser.com/Search/ProductDetail.aspx?R=WP7113HDvirtualkey60400000virtualkey604-WP7113HD

The operating voltage is 2V, and the operating current is 20mA. So, according to the calculator I need a 350 ohm resistor… Right?

Thanks,

Dan

adminSays: 7:55 am, April 17th, 2010Dan, That LED is fine. You can use the 2v rating in the calculator but the 20ma rating is the maximum that the device can sustain in standard operation… you do not have to use that much current, and if you use lower current, the battery in your effect will last longer. Start with 3ma and see if that is bright enough for you.