Bluetooth speaker (verified)

Principle: Using [M18 Bluetooth module] as a Bluetooth receiver, the EG8405 power amplifier chip amplifies and outputs the received L and R signals to drive the speaker to produce sound.
[M18 Bluetooth module]
Purchased from TB, the original price is 5-6 yuan. Counting the red envelopes, coupons, etc. usually received on the TB platform, the price is only 3-4 yuan each.
About this module:
 
You can see from the M18 Bluetooth module link that the peripheral circuit is relatively simple, but you need to pay attention to the power supply part. It is best to add a 100mH chip inductor (the 680nH inductor I attached myself). The NPN transistor here can be used for MUTE. S8050, you can also use MMBT4401 transistor.
 
The speaker
TB is purchased for about 4 yuan. If you want left and right channels, buy 2 of them. If you only use it for playing, one is fine. Counting the red envelopes, coupons, etc. you usually receive on the TB platform, the price is not expensive. 4W3Ω speaker link
 
[EG8405 power amplifier chip]
purchased from TB, about 0.5 yuan each, including the red envelopes, coupons, etc. that you usually get on the TB platform. If you buy a few more, it will only cost you a few yuan. EG8405 power amplifier link
About the EG8405 power amplifier:
Both the above two components can be powered by 5V, so there is no need for a step-down module, simplifying the circuit design.
 
The resistors, capacitors, and pin headers accumulated in daily life can be used. The number of components is small and easy to solder. Be careful not to solder falsely, reverse soldering, or short circuit. The soldering iron temperature should not exceed 350°C, otherwise the pad will be easily burned.
R1=51KΩ, you can use 24K and 27K in series. R2=33KΩ, you can use 11K and 22K in series. R3=9.1K, you can use two 18 ohm resistors in parallel. Or it can be soldered together with the four buttons and completely controlled by the mobile phone (this is also the case in actual use).
 
After the soldering is completed, use a multimeter to measure whether there is a path to the 5V and GND of the power supply part. Only after confirming that there is no error, can the power be turned on. The power supply here is 5V. 2A mobile phone charger and USB welding head and cable header to provide power.
 
After powering on, the light of the M18 Bluetooth module starts to flash. At this time, turn on the Bluetooth of the mobile phone and connect ***-M18