Typical forum thread... A simple question asked garners responses from the ignorant. Pro tip for those of you who feel compelled to chime in to a forum thread with absolutely no ability to contribute meaningful content: Log Out instead.
Short answer: Yes, you can cut LED xmas light strands to custom length. But it's gonna be challenging. Now for the long answer.
First, the basics...
LED xmas light strands use DC. A widget embedded within the plug-end converts 120vac mains to DC volts, while a specific number of specific resistors limit DC current to the specific number of specific color LEDs (yes, the color of the LED is electrically relevant, believe it or not). For example, your 100-ct LED xmas light strand might contain four LED "segments". Each segment requires a specific current to drive the combined LEDs within the segment to their designed luminosity. This is a simplified reason one cannot simply cut LED xmas light conductors wherever (suggested in a prior post), add a plug and call it done. Judging from your conversation, you seem aware of this already.
Tools:
You're gonna need a DMM and a soldering iron. A benchtop DC power supply would also be mighty handy. You're also going to need some spare plug-ends for each new strand you wish to create. But before you start cutting plug-ends off old lamps, recall that "widget embedded within the plug-end converts 120vac mains to DC volts" thing I mentioned before..? Right. You'll need that exact plug-end with that same embedded widget, else you'll instantly destroy the LEDs in the strand when you plug the thing into the mains. The workaround is not simple. You'll need an external DC power supply. Those things tend to be pricey, they're usually not waterproof, and they must be dialed in somewhat precisely—in both DC volts and current—in order to power ("drive") the mathematically-derived electrical requirements of your strand(s). (Honestly, a DC power supply may be the ideal device for driving all of your strands.) But I digress. Back to your hack...
Unwind the strand:
You must first unwind the LED light strand and separate the constituent conductors (insulated wires). This will be no casual task. The wires are tightly twisted. They will resist your efforts to unwind them and immediately form a Gordian knot. If this thrombosis-inducing frustration doesn't dissuade you from continuing, then you'll soon discover another vexation. Despite appearances, the conductors are not simply twisted the full length of the light strand. A wire may run a few meters, then terminate into a lamp holder or inline resistor (more on these things in a moment) while another wire might originate from a lamp holder mid-strand, travel a few meters, and terminate into another. Yet another wire may run plug to plug. Expect no consistency. You will have to cut wires just to assist detangling the strand in order to make sense of what goes where. Make intuitive cuts, and mark cut ends with colored tape so you can reconnect them later.
Identify segments:
You must determine the location and number of inline resistors servicing the entire strand so you can identify each LED "segment". Every LED xmas light strand contains resistors, one per segment. Resistors are identified as bulbous thingies on the wires or within a lamp socket... perhaps even both 'cause there's never any consistency with these Chinaseum-made things. (Chinese engineers aren't particularly consistent as a rule so expect none, particularly between different brands/styles/configurations of LED xmas light strands.) Confirm a segment by simply yanking any lamp from its holder with the light strand powered ON. Count the LEDs that go out AND mark the lamps on either end within that segment. Repeat this through the entire strand until you've identified each segment and marked segment lamps. You'll eventually cut the two wires between the two adjacent lamps.
Create your first "shortened" segment:
Begin with the segment immediately following the plug-end. Power OFF the strand and cut the wires between the two adjacent lamps you marked earlier. Strip the two wires to copper then tie them together (with solder). Power ON the strand and confirm the shortened strand lights up as expected. Congrats. After an hour or so of unmitigated aggravation you created a segment that is fraction of the whole. This may be the one and only segment you get to work, 'cause making add'l working segments will be difficult.
Where things get tricky:
As long as you ensure 1) each segment has a DC feed with the same voltage and current that 2) immediately goes to an inline resistor, you'll be fine. That means you have to get power from the plug-end to the next segment. You may have noticed the method the manufacturer uses to distribute power to each segment within the strand: via a "pass-thru conductor" either directly from the plug-end or from one of the lamp holders, or maybe some novel method unfamiliar to me (zero consistency). Depending on the configuration of the product you're working with, you might get lucky and one of the three conductors originating from the plug-end feeds the segment adjacent to the one you just made. If the wire is not a power feed, you'll have to tap into the plug-end feeds and run a length of wire all the way to the first LED the segment adjacent to the one you made (I hope this is clear.). You can even use this method to feed power to both segments of your second strand (since the plug-end is powering all the segments in the full strand already!). However, I understand your goal is two independent strands from one. So making that second, independent strand is gonna be real hard without a plug-end with that "widget embedded within that converts 120vac mains to DC volts". Other than that DC power supply thing I mentioned before I have no idea how you can pull this off.
Bottom line:
After hours of frustration and many mistakes, for the effort you'll likely end up with just one working segment of LEDs and only a quarter of the length you desired. Also, you've probably already discovered that you'll never get the conductors to twist back the way they were, so your new strand is a sloppy mess of wires. Also also, because you lack two aforementioned plug-ends you'll have a bunch of LEDs that simply no longer light. That is a LOT of wasted LEDs, amigo—hardly the cost savings that drove you on to this endeavor in the first place. There are workarounds to this, but that's beyond the scope of my already overly-long response. Also beyond the scope of this response, you can most definitely shorten your segment by cutting between LEDs; but doing so changes the cumulative voltage across the remaining lamps (a bad thing) unless you mitigate this electronically.
Wow. After a quick skim of my response and a quick glance at my watch, I kinda sorta understand the terse responses you got. I personally think your idea is meritful though impractical.
Anywho... Happy fiddling!
