This document describes a series of wireless Bluetooth audio modules (MH-M18, MH-M28, MH-M38) designed for DIY audio projects, offering lossless high-fidelity stereo sound. These modules are low-power solutions that support Bluetooth 4.2 transmission, enabling quick wireless audio streaming. In an open environment, the Bluetooth connection distance can reach up to 20 meters. They are suitable for various Bluetooth audio reception applications and audio DIY modifications.

Function Description

The modules support Bluetooth V4.2 and feature automatic reconnection technology. They are capable of decoding WAV, WMA, FLAC, APE, and MP3 lossless audio formats, providing stereo two-channel output. Once powered on, mobile phones can search for the Bluetooth names "MH-M18," "MH-M28," or "MH-M38" to connect and play music.

A blue indicator light provides visual feedback on the Bluetooth status:

When Bluetooth is not connected, the indicator flashes quickly.
When Bluetooth is connected, the indicator light remains steadily on.
When Bluetooth is playing music, the indicator flashes slowly.

Usage Features

MH-M18 Version: The MH-M18 module features a half-hole interface design, allowing for direct patching or welding onto a board.

PIN Functions:
- KEY (Pin 1): Button control terminal for four button functions. Requires additional resistance.
- MUTE (Pin 2): Mute control terminal. Outputs a high level (3.3V) when muted and a low level during playback.
- VCC (Pin 3): Power supply positive pole (5V). A short-circuit diode is required for 3.7V lithium battery power supply.
- GND (Pin 4): Ground.
- L (Pin 5): Left channel audio output.
- R (Pin 6): Right channel audio output.
MUTE Control Terminal: Used to control a power amplifier board. It outputs a high level (3.3V) when muted and a low level during playback. If the power amplifier board's mute function is active at a high level, it can be directly connected. If the power amplifier board uses low-level control for mute, a reverse circuit is needed.
KEY Button Control: This function requires connecting additional resistors and buttons. It includes a volume power-off retention feature: the default output is 100% volume. Volume can be adjusted (increased or decreased) by pressing the button. If the volume is set below 15% before power-off, it will reset to 15% upon re-powering. If the volume is set above 15%, the original volume setting is retained after re-powering.

M28 Version: The M28 module offers flexible power supply and audio output options.

PIN Functions:
- 5V (Pin 1): 5VDC power input.
- GND (Pin 2): Ground.
- VBAT (Pin 3): Lithium battery input (3.7-4.2V).
- L (Pin 4): Left channel audio output.
- R (Pin 5): Right channel audio output.
- AGND (Pin 6): Audio ground.
Power Supply: Can be powered directly via a Micro USB Android cable (5V), a 5V power supply, or a 3.7V lithium battery. It's important to note that the 5V and 3.7V power supply interfaces are distinct: 5V connects to the 5V and GND pins, while a 3.7V lithium battery connects to the VBAT and GND pins.
Audio Output: An earphone can be directly connected to the 3.5mm audio interface. Alternatively, the L, R, and AGND interfaces can be wired directly to the audio input of a power amplifier board.

M38 Version: The M38 module integrates a dual 5W power amplifier circuit, making it suitable for direct speaker connection.

PIN Functions:
- 5V (Pin 1): 5VDC power input.
- GND (Pin 2): Ground.
- VBAT (Pin 3): Lithium battery input (3.7-4.2V).
- L+/L- (Pin 4): Left channel output (direct speaker connection).
- R+/R- (Pin 5): Right channel output (direct speaker connection).
Power Supply: Similar to the M28, it can be powered via a Micro USB Android cable (5V), a 5V power supply, or a 3.7V lithium battery. The 5V and 3.7V power supply interfaces are separate (5V to 5V/GND, 3.7V lithium battery to VBAT/GND). A 5V2A regulated power supply is recommended for optimal sound quality.
USB Sound Card Support: Supports USB sound card functionality (plug and play). Connecting the module to a computer via a USB data cable allows the computer to play music through the module's amplifier output.
Audio Output: Outputs left and right channel stereo audio signals, capable of driving two 2-8 Ohm, 3-5W speakers, or 4 Ohm, 5W speakers.
Integrated Power Amplifier: This product features its own dual 5W power amplifier circuit. It cannot be connected to another power amplifier. The outputs of the two integrated power amplifiers must not be connected in parallel, nor should their negative poles be connected together, as this will damage the power amplifier chip.

Maintenance Features

General Precautions for all modules:

Power Supply Quality: High power requirements of the Bluetooth audio module can affect audio quality. It is recommended to add an LC filter circuit, especially when using switching power supplies (e.g., mobile phone chargers).
Signal Interference: The Bluetooth antenna is highly susceptible to high-frequency or strong electromagnetic signal interference. It should be placed away from other wireless transmitting equipment and kept at a distance from metal objects on the board.
Current Noise: If current noise is present from the power amplifier board, it is often caused by ground wire interference. It is recommended to solder wires directly to the Bluetooth module's output rather than using plug wires, especially for the ground wire. Wires should be as short and thick as possible. The audio ground of the Bluetooth module must be connected to the audio input ground of the power amplifier. Experimenting with different soldering positions for this ground point can help achieve the lowest current noise. It is recommended to power the Bluetooth module and the power amplifier board separately to isolate their respective grounds.
Handling: As these products are bare board designs, avoid touching the components with bare hands to prevent static electricity from damaging or burning out the chip.
Charging Interface (for modules with charging capability): The charging interface uses a current-limiting charging mode and does not include charging management. For safety, always use a lithium battery with a protective plate from a reputable manufacturer. The company is not responsible for losses caused by improper use.
MH-M18 Specific Power Supply Note: The MH-M18 defaults to 5V power supply. If using a 3.7V lithium battery, the diode needs to be replaced with a 0603 0-ohm resistor, or the two ends of the pad should be directly short-circuited.

Mantech MH-M18 - Page 6

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