A while ago I ordered some LED flickering candles. This particular set comes with build in rechargeable batteries and a charging tray. It comes with a European style 7.5v power supply which is no good to me without a travel adapter. Instead I wanted something simple. No adapters, or buying a new power supply. Opening the charging tray revealed a simple circuit containing a 5V regulator. That's convenient. This makes it well suited for charging via an USB cable.
While checking the circuit I discovered the current limiting resistor for the LED to be very large. 1K ohm, while 210 ohm would have been enough in it's original configuration. I think it was done to reduce the brightness of the LED. With my conversion to 5V I swapped out the resistor for a 100 ohm resistor, which results in 20mAh going through the LED. It's now nice and bright.
Because the batteries don't have any markings on them and there is no overcharge protection anywhere It is not recommended to leave them on charge for a very long time. At least I have now a convenient charging solution.
George Timmermans, Research Toolmaker, Software Engineer and Tinkerer