Actually it needs the CURRENT limited, not the voltage.

access a LED calculator:

LED Lighting Basics

Greg Elmassian web site on large scale trains and garden railroads, cigars, and computers

elmassian.com

Greg

 

we sell LEDs with the resisters on them. they will work on 5-12 volts

 

Thats greg! that looks useful.
I went here:

Conversion Calculator LED Series Resistor

DigiKey's LED series resistor calculator uses supply voltage, forward voltage, and forward current values to help you identify the correct series resistor.

www.digikey.com

So let's say "supply voltage" = 9 (a 9 volt battery)
I believe its a yellow LED, so based on the "typical Vf range" chart I enter "2" in the calculator.
That's 2 out of 3 entries in the calculator. How do I know what the "forward current" is for the third entry in the calculator?

thanks,
Scot

 

Scot,
20ma is max for most leds. So 9v battery,3v forward voltage, 20ma would be full brightness. If that is too bright, then just increase the resister value a couple hundred ohms at a time until the brightness is where you want. Or just substitute a lower ma value like say 15ma in the formula. That will give you a higher resister value to use. The resister value doesn't need to be an exact match. Just don't go any less resistance than the formula says for 20ma.

 

I agree, in the old days 20 ma was the standard, nowadays I go for 10ma, and see how it looks... I got an accucraft K4 wit a surface mount led at the bottom of a nice reflector, it was perfect, brightness, looks, etc... it was running 7 ma...

Greg