Feature:1. M18: 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 power‑on; if the control volume is greater than 15%, the original volume will remain unchanged after power‑on.2. M28: 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.3. M38 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.4. For Bluetooth V4.2 version of for Bluetooth automatic technology, supports WAV / WMA / FLAC / APE / MP3 lossless decoding, stereo two-channel output5. After powering on the module, the phone searches for the for Bluetooth name MH‑M18 / MH‑M28 / MH‑M38, and can play music when connected to for Bluetooth.6. Widely used in various for Bluetooth audio receivers, DIY conversion of various sounds, etc.Specification:Item Type: ModuleFunction / modelM18M28M38MICRO power supplyNoYesYesAudio headphone InterfaceNoYesNoBuilt-in amplifierNoNoYes(5W+5W)Lithium battery poweredYesYesYesSupport USB sound card, free driveNoNoYesVolume memory/adjustmentYesNoNoMUTE mute interfaceYesNoNoButton extensionYesNoNofor Bluetooth versionFor Bluetooch V4.2Support for Bluetooth protocolHFPV1.7, A2DPV1.2, AVRCPV1.5, AVCTPV1.2, AVDTPV1.2Format supportSupport WAV/WMA/FLAC/APE/MP3 un-decoded, stereo two-channel outputOperating Voltage5V/3.7V-4.2VNot connected Broadcast state 5.5MAConnection work state20MADeep sleeping3UATransmission distance20 meters (empty or low interference environment)Operating temperature-40C to +85°CSensitivity-87dbmMH-M18:.NumberPinDescription1KEYButton control terminal (4 button functions, additional resistance required)2MUTEMute control terminal (output high level 3.3V when muting, output low level during playback)3VCCPositive power supply 5V (lithium battery 3.7V power supply requires short circuit diode)4GNDNegative power supply5LLeft channel output6RRight channel outputMH-M28: NumberPinDescription15VPower positive 5V2GNDNegative power supply3VBATLithium battery positive 3.7-4.2V4LLeft channel output5RRight channel output6AGNDAudio groundMH-M38:NumberPinDescription15VPower positive 5V2GNDNegative power supply3VBATLithium battery positive 3.7-4.2V4L+/L-Left channel output (direct speaker)5R+/R-Right channel output (direct speaker)Blue Module:The light blinks when for Bluetooth is not connectedThe indicator light usually lights up when for Bluetooth is connectedFor Bluetooth appears during playback, slow flashingPackage List:1 x Module