DIY 10W Bluetooth Speaker - No Code Needed – How to use a Bluetooth 4.2 Audio Receiver Board with Inbuilt Amplifier (MH-M38)

I’ve just finished up a new tutorial " DIY 10W Bluetooth Speaker - No Code Needed – How to use a Bluetooth 4.2 Audio Receiver Board with Inbuilt Amplifier (MH-M38). Come hit it up!

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For anyone wanting to complete the extension Tim mentioned in this guide and add the built-in rechargeable battery and battery management module, rather than the easier route of using a USB battery bank I’ve got some tips below.

I’ve tested a battery management module (DFR1026) from stock and the default behaviour is that the unit doesn’t supply power to the output unless there is a short-press on the built-in switch to turn power on. A long-press of 10 seconds disables power to the output.

This means the simplest project build can just use the built-in switch on the power management module and no external switch is required.

If you wanted a button that you can use to turn the device on or off from outside the case you could use a button riser to extend that button to outside the case (This is how buttons work a lot of consumer devices ).
Another option would be to connect a power switch between the battery management module and the Bluetooth receiver, this will allow you to power the device with a simple switch you add yourself but this does mean the battery management module is always operating, so your battery life will drain a lot quicker than using the button on the module to disable power output.

Below is a quick sketch outlining the simplest way to wire the speaker project and add the rechargeable battery.

Awesome solution @Trent5487676 :smiley: I wasn’t aware of this product but it’s really a game changer for battery powered maker projects which are usually really tricky. Its easy to charge a battery, and use it to power a project, but this handles charging and boosting in a really well-integrated and affordable package!

I suppose you could even glue a little ‘push rod’ onto the button that pokes out of the enclosure - so you can easily access it from the outside. I’m imagining using a little 3D printed stem, or the tube from a pen ink, or even a matchstick

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Just a follow up with another configuration option. If you wanted to have the Bluetooth receiver module operate with just a simple toggle switch you can use the amended configuration below. As always in integration there are no perfect solutions only trade-offs. The advantages of this system is a simple intuitive toggle control, the downside is the battery management module is always on and ready to deliver power, so the battery life of the system overall will be lower if the device is left unplugged from a charger.
In the event the battery goes completely flat, you would need to plug it back in via USB to start the charging process, then a short-press on the battery management module to re-enable the output power to the Bluetooth receiver.

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I’ve got another wiring diagram for yet another spin on this project. This version uses a different Bluetooth received with a volume control switch so you don’t have to rely on your Bluetooth device to set your volume. Since that Bluetooth receiver doesn’t have a built in amplifier you can then add one like this after the receiver to get the nicer controls with amplification needed to drive the speakers.

This version looks more interesting. But it will need more work.

Hey @Trent5487676 ! Thank you very much for this solution. I’ve been trying the same as your first drawing but the charge/discharge module switch off after few seconds. Do you have any idea ?
The speakers in my system are 2x 2W.
Thank you very much for you answer. best regards:)

Hi Sebastien,

That charge/discharge module doesn’t include a low current cut-off so it should run until the battery is flat once it’s been turned on.
Can you check the voltage on battery you’ve got connected with a multimeter and see if it’s somewhere around 3.7 Volts?
What is the capacity in milliamp-hours of your battery? If you are using a particularly small battery there may be issues with the instantaneous current draw getting maxed out.

I have m38 circuit, Can you please guide me on connection for using 3.5mm jack with 3 pins at output for passing 10w computer speakers; please note that I dont want to cut wires.

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Is the 3.5mm jack for connecting the module to the speakers (the speakers have a 3.5mm socket) or is it for connecting the speakers to the module (you will add a 3.5mm socket to the module)?

If you have two separate speakers then you will use two sets of mono (two wire) cables. Note that you cannot use powered speakers with the M38.

If the speakers have a single stereo plug or socket (and a connection between the speakers) then you will use a stereo connection.

The module datasheet is here:
BLUETOOTH AUDIO TRANSFER MODULE M18/M28/M38

You will wire one speaker to L+ and L- and the other to R+ and R-. The speakers should indicate which is + and which is -, but if it is not marked then just try to ensure that both speakers are connected the same way around, for instance by using the wire colour.

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I want to connect passive computer speakers without amplifier with stereo cable and 3.5mm pin (male ), female socket to be connected to m38 module,

Need to understand how to get audio ground

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You don’t need an audio ground. Stereo audio is two pairs, conventionally labelled + and -.

If you are using two twin-core cables or one 4-core cable you can connect one side of each pair to the common pin of the plug: just make sure it is same side at the speakers. For instance, connect the - side from each cable together and wire them to the common pin, then wire each of the + sides separately through to the + pins of the speaker, and either one of the - wires to the - pins. If you are using single core shielded, either in a single cable or separately, then use the shields as the common and wire the core to the respective + pins at the speakers.

The socket will be wired to the module to match the plug: that is, one connector from each channel (L-, R-) goes to common (sleeve) and the other side of each channel (L+, R+) goes to the ring and tip respectively. There are some diagrams here.

I initially did the same however as per datasheet of m38, it is not advised to connect two negatives of speaker
So my problem is how to get audio ground i.e. connecting L+, L-, R+, R- TO L,R AND G connection

Hi Avadhut

Most times you can’t do this. The speakers have to be connected with 2 wires each. Labeled L+, L- and R+, R-. They are not necessarily connected to ground

A lot depends on the actual amplifier output circuitry.

And while we are at it what exactly is an M38. I searched it and found a Kodak Camera.
Cheers Bob
Found it in Jeffs post.No you cannot connect the negative side together to give you a "ground. You must use separate pairs. The amplifier will be a class D type and the correct speakers are important as the speaker inductance forms part of the output filter circuit.
Refer quote
The outputs of the two amplifiers may not be connected in parallel or the negative poles may be connected together, otherwise the power amplifier chip may be damaged.
End quote

Looks like an attempt direct translation from Chinese but it is saying the 2 outputs MUST be separate.

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If you don’t want to connect the two L- together, then you can’t use a stereo plug and socket. Use separate pairs and a 2-pin plug and socket for each pair.

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Ohhh m38 is short for mh-m38 bluetooh module

Yes, as I said above I found the link in Jefff’s post.
The M38 version also has the audio power amplifiers and is clearly marked for speaker connection L+, L- and R+, R-.

Without knowing exactly what the output circuitry is you CANNOT join the negative points ad call it ground. The 2 speakers need to be connected separately. As stated above these would be Class D amplifiers.

Are you sure your computer speakers are passive and not powered. Most computers have a 3.5mm stereo socket which can be used for headphones or powered speakers. It seems a bit (read very) unusual to have a pair of passive speakers fitted with a 3.5mm stereo plug. That is tip, ring, sleeve.
Cheers Bob

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Yes its very rare now a days
I have below speakers which are passive and have only stereo cable

https://www.amazon.com/Sony-SRS-P7-Passive-Speaker-Mini-Plug/dp/B00005MOZG

Is there any way i can connect these to mh-m38 Bluetooth module without cutting on wire

No. You would have to replace the stereo plug and cable with two pairs.

It is accurate to the description, but that description was provided before OP had made it clear that they did not want to connect the two negative points through a common wire, as this is not recommended by the manufacturer. Given that OP will not use a common negative connection it is not possible to use a 3-pin stereo plug. OP should use two twin-core cables or one 4-core cable with separate 2-pin connectors and wiring for each speaker. Of course, this will involve cutting wires. Details for the speakers are not available so it is not possible to assess whether or not this is practical.

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