HW-878 MP3 Prompter

5V MP3 File Playback Module With SD Card Slot And Small Amplifier

This MP3 playback module comes with a SD Card Reader Slot and can play back MP3 songs. A built-in mini amplifier (3W) lets you directly connect the board to a speaker.

The SD Card media is accessible via USB port from a computer.

This board does not require any external components (other than a speaker) and can be operated without a microcontroller (9 tracks selectable). When controlled by a microcontroller, up to 31 tracks are selectable. Via a solder bridge, tracks can be played circular (in a loop).


The board comes with the following connectors:

Connector Description
Speaker one speaker (mono), 4-8 Ohm, 3 Watt (newer board versions: 5W)
Headphone 3.5mm. When a head phone is connected, the speaker turns off.
Power Supply 5V
Trigger Ports 9, can trigger playback of up to 9 song files (31 songs with external microcontroller)
Pause switches between play and pause
Circular Play a solder bridge controls circular (endless) playing of a track
USB Micro-USB for power supply and/or computer access to SD Card media

Micro USB Port

The board comes with a USB port that can supply power. It also allows a computer to access the SD Card like a flash drive. When the computer is accessing the SD Card, it cannot be used to play back music.

Circular Play

The picture below shows the two circular solder pads left of the SD Card Reader: when connected, music tracks are played continuously (circular play):

Trigger Ports

Via the connectors A1 to A9, tracks can be individually addressed.

In manual mode, each connector corresponds to one track (allowing nine tracks to be directly selected).

In microcontroller mode, A1 to A5 are addressed in binary format, allowing 31 tracks to be individually addressed.

To enter microcontroller mode, pin A10 needs to be grounded when the board is powered on.


A status LED flashes when the SD Card is accessed. During music playback, the LED is on (not flashing).


Volume can be adjusted with a potentiometer.

Playing Order

The order in which sound files depends on the physical location of the file on the SD Card media controls the order.

Make sure you freshly format the SD Card media and then save the files to the SD Card in the order in which you want to play them back.

After you are done copying sound files from a computer to the SD Card via USB, disconnect the computer and then power off and on the board. You cannot play audio files while there is a computer connection accessing the SD Card reader.

Manually Controlled

The board can be used stand alone in which case nine trigger buttons can be used to play the first nine tracks saved to the SD Card media. Pull A1 trough A9 to GND for at least 50ms.


If pin A10 is grounded before powering the board, the board switches to coding mode.

In coding mode, pins A1 through A5 control up to 31 songs using binary encoding:

Track A1 A2 A3 A4 A5
1 1 0 0 0 0
2 0 1 0 0 0
3 1 1 0 0 0
4 0 0 1 0 0
5 1 0 1 0 0
6 0 1 1 0 0
7 1 1 1 0 0
8 0 0 0 1 0
31 1 1 1 1 1

To control playback, n control mode pins A7, A8, and A9 are assigned as follows:

  • Next Track: A7 to GND
  • Previous Track: A8 to GND
  • Play or Pause: A9 to GND

File Naming Scheme

The playback order of files stored on SD Card media depends on their physical file location.

To control the file playback order, first format the media, then copy the files in the intended playback order onto the media.

Even though the physical storage location of sound files controls the playback order, the following naming scheme is required:

  • Folder 01: After formatting the SD Card media, create a folder named 01.
  • File Naming: All sound files need to start with a three-digit number (001-999)


The module can automatically start playing a sound file after power-up.

To enable this, add another folder named 99 on the SD Card media, and place the autostart audio file into this folder.

Technical Data

Item Description
Power Supply 5V, 1000mA
Connectors For Sound Playback Trigger Buttons 9
Speaker Impedance 1 Speaker, 4-8 Ohm, 3W (5W newer models)
Bypass Mode High: bypass amplifier
SD Card Reader up to 16GB Cards supported, FAT filesystem
Size 39x39x6mm


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(content created Apr 11, 2024)