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I bought a cheap string of LEDs in the supermarket (the christmas decorationny type) and just out of curiosity I'm trying to figure out how it works, because I can't understand how it doesn't short itself.

It consists of a basic holder for two AA batteries with an on/off switch, and 20 tiny but bright white LEDs on two wires that lead out of the holder. The wires connect to one end of the first LED, and on the other side of the LED two wires lead to the next LED, so each LED has four "legs". The wires on the end of the very last LED simply connect to each other.

The holder
The last LED

The wires themselves are not insulated, (only the LEDs are, they appear to be dipped in some kind of transparent silicone), which What's weird to me because the positive and negative wires are touching all along the length of the strip. Also, you can scrunch up the wires and touch each bit of wire to every other bit of wire, and it still works. In fact, the two wires that come out of the battery holder appear to be touching. How is it not immediately shorting out the batteries, or causing a short when you scrunch it up?

The whole thing lit u

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    \$\begingroup\$ They are insulated, it is called enameled wire \$\endgroup\$ – PlasmaHH Dec 21 '15 at 12:31
  • \$\begingroup\$ I've built a set of 2 strings like this using shiny red enamelled wire and a flip-flop circuit out of discrete components. It was for a tiny christmas tree. The silicone will be strain relief to stop you snapping the legs off at the LED housing or shorting them together. Mine was a series design run off 9V, so each LED only had 2 legs. \$\endgroup\$ – Chris H Dec 21 '15 at 16:39
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The LEDs seems to be mounted in parallel with each other. The four legs are the same two wires running across the transparent blob, which is there to give the mounting some physical strength, I believe.

schematic

simulate this circuit – Schematic created using CircuitLab

The wires have to be insulated. But the insulation must be very thin in this case. Or, maybe, it is enameled copper wire. Otherwise, as you said, the circuit would be shorted out. If you want to take the proof, try to scrape the insulation with a sharp knife or blade and test its continuity.

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    \$\begingroup\$ This would not explain the four terminals and the short circuit at the very last LED. \$\endgroup\$ – Hagah Dec 21 '15 at 12:46
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    \$\begingroup\$ I don't think the end is short-circuited. My guess is that they have a long strip of this LED/wire at the factory and they just cut them at the desired size/led count. Since the wires are sort of parallel, the copper core of each side don't touch each other. The "four legs" are just the same two wires running through the transparent blob thing, which is there just to give some physical strength to the mounting. Running the LEDs in parallel also has the advantage of a step up voltage converter not being necessary, like it would be in case they are wired like in your answer. \$\endgroup\$ – ricardomenzer Dec 21 '15 at 12:52
  • \$\begingroup\$ I made an edit to my answer including your questions, and highlighting my point of view. \$\endgroup\$ – ricardomenzer Dec 21 '15 at 12:57
  • \$\begingroup\$ Interesting. I hadn't heard of enameled copper wire before. Unfortunately my continuity tester got blown so I couldn't simply test it that way. I tried measuring the voltage at various points, and the resistance of some of the LEDs, but I got weird, irregular results. I guess it kind of picked up something because the insulation is so thin? Upon closer inspection it looks like the cores of the copper wires at the end aren't touching at all, which supports your theory that the ends aren't shorted. Thanks for the insights! \$\endgroup\$ – Bas Dec 21 '15 at 14:26
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    \$\begingroup\$ @Bas "the resistance of some of the LEDs" that's because LEDs aren't resistors, and are non-linear over the IxV curve. Almost all digital multimeter have a diode measuring mode. Try that instead. And remember to test the LED off circuit. Depending on the diode, you might have a small glowing in it. \$\endgroup\$ – ricardomenzer Dec 21 '15 at 16:20
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The wires have to be isolated to prevent a short between them. It looks like it's some kind of enamelled copper wire (also known as Magnet wire). Those are wires with a very thin layer of insulation, mostly used in transformers, inductors or electromagnets. See this Link for more information.

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The copper is covered by a very thin, clear insulation. I had to repair one of these and ended up burning off the insulation to reconnect and complete the circuit.

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