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Can I remove the USB plug from USB powered fairy lights?

Nexus-6

Feb 5, 2024
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Hi Guys,
Im making a model that will be internally lit by 2 sets of 120 USB fairy lights.
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Both sets have a USB plug on the end which contains some circuitry to control the pre-programmed modes and a wireless receiver.
I want to wire the lights to a toggle switch on-off-on (SPDT) so I can switch between the 2 sets and have different parts of the model lit.
My question is, can I cut off the USB plug and wire the lights directly into my toggle switch and then connect a standard 2 wire female USB plug to the toggle switch to power the lights.
Nearly all the string lights that have 50 lights, just have a standard USB connector, but the ones that are 100+ all have that extra circuitry in the USB connector, leading me to believe that there may be some switching happening so that only 50% are ever lit at any one time.
Any advice is welcome.
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Harald Kapp

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Nov 17, 2011
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My question is, can I cut off the USB plug and wire the lights directly into my toggle switch and then connect a standard 2 wire female USB plug
Possibly. It depends on the construction of the fairy lights: Do the LEDs have internal current limiting or is current limitig part of the circuit in the original USB plug?

You can check by:
  • cut off the original USB plug. You should be left with 2 wires.
  • add a 100 Ohm series resistor to one of the 2 wires.
  • connect the open end of the resistor to a 5 V voltage source, the open end of the other wire to the other pole of the voltage source.
  • do the LEDs light up? If not, reverse the polarity (swap the connections to the power source).
    {*]once the LEDs are lit, measure thze voltage drop ascross the resistor.
  • add a second 100 Ohm resistor in parallel to the first one. Again measure the voltage drop across teh parallel connection of resistors.
  • show us the results. Depending on the difference in voltage drope we may be able to find out whether there is an internal current limiting in the LEDs or not.
 
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