In the past, be using the bulb for lighting. The white color ray LED is more and more cheap currently. It can be used for more way.

This small lamp is using 24 pieces LED connecting in parallel and 3 x AA cells. It is useful and convenient.

Its circuit is 3 cells, switch and LED in serial. I add extra USB cable and diode to connect to 3 cells in parallel. The circuit is simple.

The USB port is 5v output. It will be drop  0.7v when current pass through the diode. So 5-0.7=4.3 volt to supply the LED set. When cable be connected to USB port, it will charge the 3 cells and the diode is prevent to current flow back to port. When disconnection, the 3 cells will supply the power to LED set.

Taking your own risk.