3 volt string led light adapting to 5v usb charger source
There appears to be no electronics here - just a 3V battery source and a switch. So those white LEDs are all wired in parallel. 3V is applied to every LED.

If you were to substitute a 5V source, the series resistor would have to drop about 2V. Even with 10 mA flowing, you get some light. 80 mA would be quite bright.


10 mA would require a 200 ohm resistor in series
80 mA would require a 25 ohm resistor in series








simulate this circuit &ndash; Schematic created using CircuitLab

Any resistor between 25 ohms and 200 ohms would be appropriate - the smaller value giving more light.