Did you make sure the GND pins of the power supply, Arduino, and DFPlayer are all connected to each other?
Grounds must be connected. All of them. That includes BOTH ground pins of the DF Player Mini.Note that there is a risk of phantom powering the Arduino through the Tx/Rx pins when it receives no power, but the DF Player does, and the other way around.
Connecting grounds is not just a good idea, it's mandatory for things to work.Phantom powering can always happen when two devices are connected through I/O pins, while one is powered and the other not. Simple solution: power them both from the same supply. Resistors in the I/O lines can help prevent any damage.
You don't need resistors, just connect the grounds and you'll be good to go!If both the Nano and the MP3 player are always powered on together (even with separate supplies), why care about phantom power? Also, why not power the Arduino, the motors, and MP3 player off the same external power supply? Would work perfectly that way.
As mentioned, I want to keep them as 2 different features. So powering both is not what I want.