I have a 80's Christmas incandescent light string without any kind of electronics. They are static until you replace one normal bulb with one marked bulb.

It seems to be that the marked bulb (that actually also works) has some kind of special filament that burns out but still works, with some different resistance. Then the lights begin to blink.

  1. What is special in that bulb?
  2. If there is a fixed resistance, why does the time between flashes seem to be random?
  3. Why do the lights become fixed and with a mini thump start to blink again?
  4. Is there any way to make them perpetually flash without thumping it?
  • Voltage: 220V
  • Lights bulbs: 100
  • \$\begingroup\$ What do you mean with "marked bulb" ? \$\endgroup\$
    – Klas-Kenny
    Dec 10, 2021 at 13:49
  • \$\begingroup\$ @Klas-Kenny lighst are blue, green, yellow, red and the marked one is normal with a red head. The manual says: replace any bulb with this one in order the lights flash. To simple view, is a normal bulb, just is "marked" with different tint. After colocation seems to be burn out after a seconds and then lights become to flash. But I don't understand how works, and also how it can works "random" \$\endgroup\$ Dec 10, 2021 at 13:51
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    \$\begingroup\$ There is a bimetallic strip inside the bulb - similar to the strip in an old-school thermostat. The two metals (one on each side of the strip) have slightly different thermal expansion constants. As the strip warms from the electric current, it bends and the switch disconnects. It then cools over the next second or so and reconnects the switch. There is a bit of tension on it at room temp and the strip is fairly thick so it has a rather slow response (1 to 2 seconds) open and closed. The first flash takes a several seconds because the strip must heat up before disconnecting the first time. \$\endgroup\$ Dec 10, 2021 at 13:54
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    \$\begingroup\$ "Is there any way to make them perpetually flash" Rather than continuing to string 230 V around your tree protected only by (usually) a single layer of 40-year old insulation I suggest it's time to pension off your old lights and invest in a much safer set of low voltage lights. \$\endgroup\$
    – Graham Nye
    Dec 10, 2021 at 15:54
  • \$\begingroup\$ @GrahamNye I added a 10ma differencial protector and a 5A termical switch off (Sorry idk the translation for disyuntor diferencial and térmica) just for the tree \$\endgroup\$ Dec 10, 2021 at 15:59

2 Answers 2


The red-tipped bulb is a "twinkle bulb" with a bimetallic strip inside which will disconnect the circuit when the bulb heats up. Then the bulb will cool down and the bimetallic strip will close the circuit again. Because all the bulbs are wired in series (fixed ones + twinkle), the whole string will blink off and on.

I suspect you need to fingerkick it just because it's old and due to sparking when making/breaking contact it does not work reliably any more. You need to replace the twinkle bulb but the voltage you need has to match the old one, it's dependent on the number of bulbs in the string (for a total voltage drop of 120 or 230 V depending on where you live).

These seem to also go by the name "Christmas flasher bulbs" but are getting hard to find with the LED revolution steamrolling incandescents.

  • \$\begingroup\$ Thanks, I will update my question with that information, seems to be for someone is "unclear" :D \$\endgroup\$ Dec 10, 2021 at 13:59
  • \$\begingroup\$ This is actually how turn signals on cars used to work, too, just a heavier-duty one of the same type of device. \$\endgroup\$
    – Hearth
    Jul 30, 2022 at 6:45

I don't know how your specific light works, but one way to do this without complex circuitry is using a polyfuse, e.g. a polymeric positive temperature coefficient device (PPTC).

Such a polyfuse trips once its temperature exceeds a certain threshold. Initially, the temperature of the PPTC is low, but once current flows through it, power dissipation will cause the temperature to rise and the PPTC will finally trip. After the fuse has tripped, there's no more current and also no power dissipation, so the PPTC cools down. Once the temperature is below a lower threshold, the PTC becomes condictive and the cycle repeates.

If you find a material that has a tripping point, a high temperature coefficient and can be heated up to 2000-2500°C, then you could make a simple "self-flashing" filament. A simple form of a polyfuse is a bimetal strip used as a switch, like in a relay. It's certainly possible to make it prone to finger kicks as well.

  • \$\begingroup\$ Thanks for your answer, it clarifies me with a lot of more tech information. \$\endgroup\$ Dec 10, 2021 at 14:16
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    \$\begingroup\$ @LeandroBardelli If it was unclear: I added some more explanaition. After rethinking I'm also not sure that it would work just with a PTC element. It would probably settle in a stable temperature without blinking. You actually need a real polyfuse of some kind, I think. \$\endgroup\$
    – Sim Son
    Dec 10, 2021 at 14:46
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    \$\begingroup\$ @LeandroBardelli I think, vaizki's is the better answer for your specific question ;) \$\endgroup\$
    – Sim Son
    Dec 10, 2021 at 14:54
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    \$\begingroup\$ This is not the way Christmas tree light flashers worked. Vaizki's answer is correct. \$\endgroup\$ Dec 10, 2021 at 15:27
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    \$\begingroup\$ @SimSon - I've never heard of PTC elements oscillating. They would normally settle into a stable high resistance state where enough power is dissipated to keep it hot. PTC heater elements exploit this to keep at constant temperature. \$\endgroup\$ Dec 10, 2021 at 17:21

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