I purchased a DFplayer mini from Digikey, and have connected it using the default wiring schematic provided by the manufacturer. If I ground the IO pins on the player without any connection to the arduino, I get sounds to play and a little blue light shines on the player indicating a sound is playing.
However, if I connect the DFplayer to the arduino with RX/TX and corresponding pins 10/11 on the arduino per the test code given on the product wiki I do not get any lights on the player and no sounds play. I've tried reversing the RX/TX connections, to no avail, and I have tried formatting the SD card and re-copying files. There's not very many connections to make, with just power/gnd, speaker and the 2 serial connections, so I'm scratching my head as to why this isn't working on either a Nano nor an Uno.
