====== Chinese BT FM-transmitter ======

Following is a quick disassembly of a 3€ [[https://aliexpress.com/item/1005008839803108.html|🛒 no-name Chinese BT FM-transmitter]] with Bluetooth 5.4, Aux and SD-Card. Aux-port has plug-in detection an is used as line out. Input is either SD-Card or it is used as Bluetooth audio receiver and its output is either AUX or transmitted over set FM frequency - powered by USB-A plug ([[life_hacks:battery_tips|no batteries, nice!]] :)). \\
Next to the 7 segment display, there is a microphone for hands-free input over bluetooth. \\

If that is too much, there are simpler Bluetooth AUX only adapters for ~2€+ \\
If you need more DIY, there are [[https://aliexpress.com/w/wholesale-fm-transmitter-diy-kit.html|🛒FM transmitter DIY solder kits on aliexpress for ~2-3€]]. \\
If you need a way to set frequency over software for a project, check out [[sony_ericsson:links#mmr-70_fm-transmitter|Sony Ericsson MMR-70 FM transmitter]]. \\

There are [[https://aliexpress.com/item/1005004689652022.html|🛒line-in cassettes (~4.50€)]] (even[[https://aliexpress.com/item/1005009860340243.html|🛒with bt too for ~9.50€]] for your vintage stereo). \\
Also see [[projects:rlink2#gadgets_you_can_get_for_your_car|"Gadgets you can get for your car"]] \\
===== Photos =====

{{gallery>:disassembly:fm-mp3}}
===== Notes =====

The module 'talks': sound when starting, short voices for 'connected' and mode selection (AFAIK it cannot be disabled in a simple way). \\
The clipped USB tin shoe holds the casing together well. \\
Powered by 5V from USB-A, which has data lines going to MCU/SoC. No output on dmesg/lsusb though :/ \\
AUX is 3-pin TRS stereo (no mic). Microphone on PCB is connected over bluetooth. \\
The MCU is //[[https://www.reddit.com/r/AskElectronics/comments/tcqi6f/comment/i0fyeed/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button|AB-series from Bluetrum]]//. 
There is a [[https://gitee.com/bluetrum|gitee repo of bluetrum]]. On first sight, it does not seem to be much of use (?) \\
The closest SoC I could find quickly on the HP is [[http://www.ch-chip.com/Data/ch-chip/upload/file/20200904/AB5305B%20DataSheet.pdf|AB5305B]] \\
Just for reference: There is a [[https://www.cnx-software.com/2021/09/12/getting-started-with-bluetrum-ab32vg1-risc-v-bluetooth-audio-board-using-rt-thread/|getting started tutorial for RISC-V based AB5301A]]. \\
 
On the PCB of the FM transmitter you can add a socket for antennas (or solder directly) (there are pads, just cut the traces behind). \\
Fun fact: protective plastic sheet is still on my 7 segment display (even though it is behind the casing) \\

==== #TODO ====
Find out what file types and sample rates can be used with SD-Card. \\
Measure frequency of AUX output. \\