I'm currently converting an old MP3-player (a Creative Zen Stone) to a standalone music player for my children. One of the things I'd like to do is to add an LED which shows whether the device is powered on or off.
Due to how the MP3-player is built, the only place where I can easily modify the circuit is between the battery and the device itself. Therefore, my idea was to place the LED in parallel to the device and use a transistor to switch it on whenever there's a current flowing through the player.
The battery says that it delivers 3.7V. The green LED needs 20mA at a forward voltage of 2.1V, so given the 3.7V from the battery it needs an 80Ω resistor. I'm currently using a 100Ω one, because that's the closest that I got.
This design somewhat works, in the sense that when I power on the MP3-player the LED lights up. However, it seems the player doesn't get enough voltage now, because it doesn't work properly (which it does without the additional elements attached). The base-emitter saturation voltage of my transistor is around 0.7V (accordingly, I'm measuring that the player only gets 3.0V).
How can I resolve this problem? Is there a different circuit design that I can use or should I start looking for a transistor with a very low base-emitter saturation voltage instead (assuming those exist)?