is it safe to add an activity LED to an USB charging cable in parallel?

full story: I've recently bought one of these glowing USB charging cables, but it died after a few months of use so now i am thinking to make myself a replacement with a single activity led.

Here it is my plan:

  1. take an USB extension cable and cut a bit of insulation in the middle of it
  2. cut the power and data wires and strip both ends
  3. solder a 330ohm resistor and a led in series with the power and ground wires like this:


simulate this circuit – Schematic created using CircuitLab

The cable will be used only to charge mobile phones, because i think the increased current usage may cause issues to USB controllers.

EDIT: still looking for an alternative design providing some form of current-passing indication using discrete components only.


1 Answer 1


This is 100% safe. Using the 5V rail as a voltage source for a parallel 20mA LED won't affect the supply or the cell phone at all. Your diagram does not show any series connection btw.

This is 100% not an activity light. It will always be solid on, barring an weird supply design. It will be on with no cell phone connected, will be on while charging, and on while not charging.

  • \$\begingroup\$ how can i make the led turn on only when there is charging current passing? \$\endgroup\$
    – eadmaster
    Commented Apr 18, 2017 at 5:33
  • \$\begingroup\$ You need active circuitry for that. A current sensing resistor in series on the 5V or Ground line, and an ic or circuit that measures the voltage across that resistor to turn on the led. It is not a trivial setup. \$\endgroup\$
    – Passerby
    Commented Apr 18, 2017 at 5:42
  • \$\begingroup\$ i've measured that when i connect my smartphone the voltage drops from 4.92V to 4.80V due to the increased load. So if i put a bigger resistor the brightness of the led should change a bit and this may work out as a charging indicator. \$\endgroup\$
    – eadmaster
    Commented Apr 18, 2017 at 7:18

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