LUMINA is a USB current sensing device with LED driver outputs. It detects the current flowing between its terminals with a current sensing resistor, and amplifies this to turn on and off LEDs. It also features a multi-function LED output that is solid when the current is below the threshold, and pulses when it's above.
The idea is it sits inline with a USB charging setup, and a LED pulses while your device is charging. After charging is done, the current level drops below the threshold and it stops. The level at which it triggers is adjustable by a trim pot.