How do I hitch up two, 9 volt battery to a 3.8 volt LED.

I trying to figure out what size resistor I need and were to get it. (I would prefer to by it at a store. I live near Des Moine, Iowa.)

mathews7 years ago
Assuming you are connecting the batteries in series, the total voltage would be 18v. With 3.8 volt over the LED, this leaves 14.2v over the resistor.

Assuming the LED uses 20mA, or 0.02A.

R = 14.2/0.02 = 710R
S1L3N7 SWAT7 years ago
Why are you powering an LED with 18v?
lemonie7 years ago