Christmas lights come in all shapes, sizes and colors, but for some buyers, there are really only two categories: blinking and nonblinking. If your Christmas lights are not blinking and you would prefer to have them flash, that's a relatively easy fix. There are also more elaborate approaches for serious hobbyists.
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How Christmas Lights Are Wired
Before you start, it's important to know that your Christmas tree lights can be wired in a couple of different ways. One method is to have them hooked up in series, which means the whole string of lights makes one circuit. This allows the wiring to be simpler, but if one bulb goes out, you'll lose the whole string.
An alternative is to wire them in parallel, meaning each light is on its own little secondary circuit, so if one light blows, the rest continue to work just fine. This is less frustrating for you, but the wiring is more complex and costs a bit more to manufacture. Modern lights often combine both approaches, wiring short substrings in series and then joining those together in parallel. If one light goes out, a short segment of the string will stop working, but the rest will continue.
Some high-quality incandescent lights may have a built-in system of fuses or shunts, so when one bulb blows, a secondary route is created for the electricity. With LED lights, the blown light itself provides a path for electricity, so in both of those cases, the other bulbs will keep working.
Most sets of lights come equipped with at least a few spare bulbs so you can keep the holiday spirit going even if you have a bulb go out on you. Most of these are ordinary bulbs, but you may also see a bulb that's marked with a red or silver tip. That's the blink bulb – the one that makes all of the other lights on the same circuit blink. If your lights are not currently blinking, it's because you need to add one of these special bulbs to the string.
This is why it's important to know how your lights are wired. If the whole string is in series or parallel, placing one blink bulb on the string will make them all turn on and off. If the string is subdivided into multiple circuits, you'll need to have one of those special blink-controlling bulbs for each substring.
The bulb-replacement process is the same in any case. You'll just have to do it a few more times.
Test your lights before you begin and make sure they're all working. Next, unplug the string of lights before working on it. Choose a light bulb near the beginning of the string and remove it. Some bulbs simply unscrew, but the newer push- or bayonet-type bulbs push and snap into place.
To remove those safely, get your fingernails between the bulb's base and the receptacle on the string of lights and pull out carefully. Otherwise, you may end up pulling out just the bulb itself, and the base will be left stuck in your string.
Insert the marked blink bulb and plug in your lights. After a few minutes, the blink will begin working. If the whole string lights up, you're done. If not, unplug the string and replace one of the nonblinking bulbs with another blink bulb. Repeat until the whole string flashes.
Other Effects With Holiday Lights
Note that this technique will only make your lights blink on and off at a predetermined tempo. If you want to be able to make them blink faster or slower or sync with music, you'll need to have a special controller to do that.
Controllers and lights are available as kits, but if you're handy with electronics building, your own can be a fun DIY project. You can make a small string of lights blink with just a 555 timer and a few other components, but more elaborate and ambitious projects require more parts and a lot more electrical knowledge. If you opt to go the DIY route, be sure you understand the necessary precautions to avoid creating a fire risk or harming yourself.