The Basics: Connections
When you buy a new LED (Light Emitting Diode), it has two leads. Typically, one is longer than the other. The longer one is the anode, the other is the cathode. For normal LEDs (the ones that only produce one light color), the anode (longer lead) should be connected to the positive lead of the power supply or circuit, and the cathode should be connected to the negative or ground lead.

If both of the LED's leads are the same length (which sometimes happens if the LED is removed from a circuit), then look for a flat side at the bottom of the LED's case. Look at the LED straight on from the top and you'll see one edge is not round but flat. The lead nearest the flat edge is the cathode, or negative lead.

The Basics: Hook-up
Never connect an LED directly to a power supply - it will self-destruct. Always have at least a resistor in the path so that the current through the LED can be controlled. This resistor is called a "current-limiting" resistor. An LED by itself will happily take whatever current the power supply or circuit has to offer; until blows or breaks.

Doing the Math: Controlling Brightness
Because of the current-limiting resistor, you can control the amount of current the LED is allowed (call it "portion-control"; we want our LED to be on a steady diet so that it doesn't starve and doesn't overeat). The side-effect of controlling the amount of current going through an LED is that the amount of light it emits can be controlled. The less current, the dimmer the light. The more current, the brighter the light. Again, too much of a good thing here, i.e. current, and the LED will blow. Sometimes an application calls for a very bright scenario, and sometimes you want a dimmer LED.

If you have done anything relating to electronics, you will be familiar with Ohm's Law. Ohm's Law states that voltage is equal to current times resistance (V = I * R), where V is voltage (in volts), I is current (in amps), and R is resistance (in ohms). We will use this Law to calculate the size resistor we need for a certain brightness.

When you buy a new LED, the manufacturer will provide some key information about the LED on the package or in electronic format via a PDF file on their web site. Different from a regular diode, an LED can have completely different parameters based on its light color. Different chemical components are used to produce the white, gold, yellow, red, green, blue, etc. colors that are currently available. These have different parameters in normal operation. So, be sure to read the information about the specific LED you are about to use.

In our example, let's assume that the LED "consumes" 3.4 volts from the circuit to run it. Let's also say that the LED's maximum current is 20mA (i.e. 0.020A). Finally, we assume we have a standard 12-volt power supply which we wish to use to run our LED. We are going to assume a simple circuit that turns on the LED when the power supply is plugged in to the wall. To prevent the LED from burning out or aging rapidly, we are only going to provide it with 10mA of current (0.01A). V = I * R R = V / I R = (Vps - Vdrop) / I R = (12.0 - 3.4) / 0.01 R = 8.6 / 0.01 R = 860 Ohms

where Vps is the voltage of the power supply (in my sample that's 12 volts), and Vdrop is the LED's rated voltage drop. So, connecting a 860ohm resistor between the positive lead of the power supply and the positive (anode) lead of the LED will allow 10mA of current to run through the LED.

Locomotive LEDs
Most newer scale locomotives come with the correct color headlight. However, for some older engines, you may need to swap out the existing LED for a new "golden white" LED to better simulate the prototype's headlight color. The newer "bright white" LEDs are also much brighter than the older yellow LEDs. The photo below show three N-scale PRR E8 engines. The engine on the left has a newer bright blue white LED in it. The one in the middle has the new bright golden white LED, and the one on the right has the Kato stock yellow LED.