1M18: MUTE mute control terminal: used to control the power amplifier board. When muting, MUTE outputs a high level of 3.3V, and outputs a low level during playback. KEY button control: This function requires additional connection of resistors and buttons to achieve Volume power off function: The default output is 100% volume. Press the button to control the volume increase and decrease. If the control volume is less than 15%, the volume output will be 15% after poweron; if the control volume is greater than 15%, the original volume will remain unchanged after poweron. 2M28: Power supply: It can be directly powered by the MICRO USB for Android cable 5V, or it can be connected to a 5V power supply or a 3.7V lithium battery. Audio output: The earphone can be directly connected to the 3.5mm audio interface, or it can be wired for the audio input of the power amplifier board. 3M38 Power supply: It can be directly powered by the MICRO USB for Android cable 5V, or it can be connected to a 5V power supply or a 3.7V lithium battery. The maximum working current is 5V2A. It is recommended to use 5V2A regulated power supply for better sound quality. Support USB sound card, free drive (plug and play) Connect the module to the computer with the USB cable, and the computer can play music through the module amplifier output. 4For Bluetooth V4.2 version of for Bluetooth automatic technology, supports WAV / WMA / FLAC / APE / MP3 lossless decoding, stereo two-channel output 5After powering on the module, the phone searches for the for Bluetooth name MHM18 / MHM28 / MHM38, and can play music when connected to for Bluetooth. 6Widely used in various for Bluetooth audio receivers, DIY conversion of various sounds, etc.Item Type:Module Function / model M18 M28 M38 MICRO power supply No Yes Yes Audio headphone Interface No Yes No Built-in amplifier No No Yes(5W+5W) Lithium battery powered Yes Yes Yes Support USB sound card, free drive No No Yes Volume memory/adjustment Yes No No MUTE mute interface Yes No No Button extension