Parts required:
- String of LED Christmas lights
- Resistors (see chart below)
- Plug in wall transformer from some old appliance
- Plastic milk jug
1. Pull the LED assemblies out of the string of lights. There is usually a little tab which has to be released first (arrow). Then just pull the assembly out.
2. Add a dropping resistor to one lead of the LED. It doesn't matter which one. The value of the resistor is chosen depending on what voltage your plug-in transformer provides. The goal is to get 20mA through the LED. I use a 5 volt supply and I add a 1K resistor, which is an easy choice for any voltage between 3-12 volts. Then add wire long enough to reach your power supply.
Resistor values to provide 20mA
5 volts = 220 ohm
6 volts = 330 ohm
9 volts = 470 ohm
12 volts = 620 ohm
Of course, you can always use larger resistance values, the LED brightness goes down with increasing resistance. I use 1K just because I have a lot of them, and I don't want the structure lights too overpowering.
You should also read the label on your wall transformer and see what the current rating is. This will tell you how many 20 mA LEDs it can handle. 1 Amp = 1000 mA, so if your supply sources 1 Amp, that is the same as 1000 mA, so it will power 50 LEDs at 20 mA each.
Now you have to mark the wires for correct polarity. I don't bother keeping track until I get ready to connect them up. I just touch my two wires to a 6 volt lantern battery and see which way lights the LED. Then I mark the (+) lead with a red marker.
3. Drill a 5/16" hole in your layout at your structure location, and poke the wire down the hole until the LED just sits atop the layout.
4. We do not want to see the LED through the structure windows, and we do not want to see a point source of light, so we add a diffuser. Cut a 1" x 3" piece off a plastic milk jug. Notch the ends and roll it into a cylinder, and place it over the LED. Here we see how the light is diffused by the translucent milk jug plastic.
5. Place your structure over the LED. Try putting some opaque black tape over some of the windows on the inside to suggest the building has different rooms - see the red building in the background below, the windows on the right have been blocked out inside.
