Soldering 101 | Let’s Put LEDs in Things! | Adafruit Plug in and switch on your soldering iron, it needs a few minutes to fully heat up. While waiting, gather your parts: battery pack, wire, resistors, LEDs and heat-shrink tubing (you’ll also need a lighter, matches or heat gun for the tubing). When I added a resistor to a set of christmas lights where So a string that uses 20 bulbs for a 120V power source will use bulbs designed to operate at 6 volts. And a string that uses 50 bulbs for a 120V source will use bulbs designed to operate at 2. When you are making hundreds of thousands (or millions) of strings you can have custom bulbs made for whatever voltage you wish.