Lighting with LEDs Primer


No, I’m not talking Mr. Surfacer 1000 here!  The following assumes you have a basic understanding of wiring, and if you fry yourself through ignorance you:  1.  Have been warned and 2.  Are trying too hard, since this is a battery powered DC circuit.  In order to get a LED to light up, you need to provide a voltage drop somewhere in the circuit.  If you just hook a LED up to a DC (battery) power source, it won't light.  You need a resistor (R) in the circuit (on either side of the LED) to provide current.  LEDs also work only one way—the voltage drop must go from the long lead (+) to short lead (-).  A basic LED circuit would look like this:


Where R is the resistor, and the battery is the +/- thing to the left.

LEDs are generally rated by power or voltage and amps.  Since Power in Watts is related to Volts and Amps, either rating is fine.


Here is the basic formula:


Power in Watts = Volts * Amps


Real World example:


Using a Radio Shack #276-303 Green LED, the ratings are 2.1 Volts @ 2 mA (milli-Amps).  If you have a 3 Volt battery, the resistor needs to drop 0.9 V so the LED can see 2.1 V.  So, how big a resistor do you need?


The basic relation between voltage, current and resistance is:




Where V is again in volts, I in amps, and R in Ohms.

You know the Voltage drop across the resistor:  0.9V; you know the required current:  2mA.  Solving for R:


 Or  or 450 Ohms (2)


This is a nominal value.  I usually find that following this formula will give you a lit bulb, but not a very bright one.  You can crank up the voltage by adding another battery, or decreasing the resistance to increase the current.  Now, using 4 AAA batteries, which should be 6 V, and 440 Ohms resistance, the above LED lit up OK.  Cutting the resistance back to 220 Ohms nearly doubled the brightness (this is according to my eye—not measured).   So, what’s the deal?  I had to use twice the anticipated voltage to get it to light up?  Well, again, the above values are nominal, assuming everything is perfect.  The four batteries I used are not new, so they are probably experiencing some voltage drop-off.  Using 2 brand new Energizer AA batteries (about 3.6 Volts when new), I got the same result—440 Ohms lit but dimmer, 220 Ohms brighter.


So, what to do?  You can’t fit AA or even AAA batteries in a standard Gundam kit where it is easily accessible (say, the torso).  One option that could work is camera batteries—small, high power, high voltage.  If you are using a 6 volt or higher camera battery, remember the LED is only (in this case) supposed to see 2.1-2.8 Volts.  The resistor must provide the rest of the voltage drop, so be ready to crank the resistor values up so equation (2) works out.  If you drop the voltage too low, the LED will get REALLY BRIGHT—Once.  BTW, Radio Shack sells resistors in various values.  For most LED model applications, 100 and 220 Ohm resistors should cover your needs.