We're familiar with this from restroom visits in hotels and restaurants. 

The room is filled with pleasant music (… in most cases). 

After remodelling a small guest restroom, I wanted to install a device that would automatically play MP3 songs from a playlist when someone entered. 

Of course, this function could be implemented with the help of Alexa, Siri, and consorts. 
BUT 

And I don’t want to deny that the journey is also the destination. 

The dataset contains, amongst others, a detailed description of the device with supporting illustrations (see PDF). 

For this reason, only a brief summary of the main functions of the MP3 player is given here. 

The device and the two speakers are installed - not visible from the outside - in a small vanity unit in the guest restroom. 

Since there is no 230 V mains socket in the cabinet, the MP3 player is powered by two Li-Ion batteries of type 18650.
The batteries are recharged as needed via a charging controller that is powered by an external power supply. 

However, due to the small power consumption in Sleep mode, recharging is only required after 8 to 9 weeks. 
 

The MP3 player consists of the following functional components 

Music Mode
When the PIR proximity sensor detects a person, the supply voltage for the MP3 player is switched on. 

Music playback starts after approx. 3 seconds provided by the embedded DFP player (FN-M16P) with integrated MP3 decoder.
The player can manage thousands of tracks stored on its SD card. 
After level adjustment, the two stereo signals are amplified by constant 24 dB via the PAM8406D Class D amplifier. The amplifier is combined with a LP output filter to minimise radio interference emitted via the loudspeaker cables. 

The volume of the actual track can be changed by a rotary pushbutton which also allows to jump to the next music track (if really necessary). 

The battery voltage is continuously supervised. Undervoltage will be displayed on the OLED but the player remains blocked. 
 

In sleep mode, only the PIR sensor for person detection is supplied with power.
The device control and the audio module are inactive. 

In this operating mode, the quiescent current is only 17 μA, ensuring a long battery life. 
 

Charging Mode 

Charging of the Li-Ion batteries is managed by the ISL6251 controller in combination with a synchronous buck converter. 

The battery voltage and the charging current are continuously monitored. 

Also the temperature of both cells is supervised by means of NTCs supplied by precision current sources REF200. 

The values for over temperature (interruption of charging mode) and re-start temperature can be parametrised by software. 

The measured values of the current & voltage and both cell temperatures are displayed in an alternating sequence of five seconds on the small built-in OLED.

Active / Sleep Sequence 

The active / sleep module as part of the power supply unit has been optimized for low current consumption. 

When a person is detected, the SD602 PIR-sensor executes a preset of the D flip-flop, which permanently turns on MOSFET switch Q3.
The MP3 player is supplied with power, boots up, and begins playing music (new track from the playlist). 

In the event of a shutdown sequence was triggered by the processor (no person detected for 
15 seconds and current track has ended), the output of the D flip-flop is reset via the SLEEP signal. 

MOSFET Q3 turns off, which disconnects the power supply from the battery.
The MP3 player returns to sleep mode. 

The circuit including batteries is located on a four-layer circuit board. 

The device has no housing, as it is installed in the vanity unit.
To protect the components, e.g., when operating the rotary pushbutton or any of the toggle switches, only an acrylic plate was mounted.

More information can be taken from the attached device description.