In case you wonder why a MP3 player is listed in the hardware section have a look at the ClimArt subproject Tree Aware (Projekt AP 4).
Tutorial: DFPlayer Mini Mp3 Player - DFRobot Wiki
Hint to stabilize power supply, https://forum.digikey.com/t/dfplayer-mini-communication-issue/18159/7:
Also, putting a capacitor or two on the breadboard between Vcc and Gnd might help, as it provides a local energy source for the player during short duration, high current demand periods. The typical value for this is a 0.1uF ceramic cap, and providing an additional larger cap such as 10uF to 22uF low ESR tantalum or aluminum electrolytic cap might give additional support if the high current draw is extended for longer periods, which can cause voltage droop.
In the same posting:
Looking through some of the documentation, it looks like the DFplayer Mini is really designed to be used with around 4.2V,