Long overdue demand from the family : be able to play CDs in the living room. Doing that with the Blu-ray player is cumbersome and needs the TV to be on. I don't have room for proper speakers in the living room so I settled for a shared, portable half sized sound bar from Sony a few years ago and that plays streamed music like Spotify but... sometimes a CD is wanted. 

New goal set, I just couldn't find a stereo that has a BT **transmitter** ie that can send anything played on it via BT to a receiver (a portable speaker, a BT headset / headphones etc). I did purchase long ago a BT audio transmitter / receiver to add that to my old (but full HD) TV from 2009 and which refuses to die, so I could watch a show with my BT / BLE noise cancelling headphones. I didn't like the fact it couldn't be installed permanently, it is battery powered, pairing was a bit tedious but it worked. It had background noise too when hooked to the TV usb for permanent supply, so I had to use it on battery. Time to retrofit it...

it was one of those things.
www.amazon.fr/R%C3%A9cepteur-Bluetooth-Adaptateur-Haut-parleurs-Ecouteurs/dp/B0B6ZZV8GT/

The problem is that the pairing, power on/off is operated with a single switch (momentary) long presses short presses, clear the pairing list, pair, etc... tedious. While I can pair manually once and hope it sticks, I wanted the system to at least power on the module (long press, until the blue led starts flashing) when I turn the stereo on.

So, I looked for micro stereos with a CD player (eventually the FM radio and/or more) for cheap on our french craiglist "the good corner" (Le bon coin) and found this Philips model, with Bluetooth **receiver**, virtually the same model I got my younger one, for 6.99 € shipped (!), with the cabinet speakers. It was not the desired color, so I bought a second one with black and silver color scheme, for 12€ (local pickup at the seller's apartment) and I started hacking, and coding a small automation of the BT module switch, using an arduino, which detects power, boots, turns the module on, stalks the LEDs (blue and red) on its inputs for a el-cheapo heuristic to guess in what state the module is (paring, turning on, off etc) and reflect the 2 leds state on a WS2812 (pixel) 5mm LED on the front panel

The idea was simply to have the BT module switch automated with a long press until it turns on, after the stereo is powered. I located a switched +5V voltage source on the front panel, but it was noisy and dirty (digital supply, feeding the 7 segments led display) so instead, I grabbed a clean 7V supply on the source selector / audio mixer chip, straight on the decoupling capacitor and found the output of the mixer using its datasheet. Finally, I vampired the L+R signals (with proportional volume control from the front panel knob) right after the DC cutting capacitors and fed those into the BT module.

I also added a master switch that goes to the arduino and which allows for the user to override the BT switch, manually, from the front panel : the switch is also used to clear the pairing list with a 8 seconds press, this sort of thing, and I surely didn't want to have to open the box to do that. Finally, the LEDs states are mirrored to the front panel with that pixel that shows the state of the BT link to the speaker. A simple 3D printed housing in ASA is installed at the back of the stereo, I had space to hide the whole thing inside but the metal casing would have prevented a good radio waves transmission. Since there's also a BT module in the front panel, I separated the 2 to prevent destructive wave interaction between the 2 if they got to operate at the same time.
As a matter of fact, I can still use the BT input of the stereo to send music from my phone, to be then forward with the added BT module, to the speaker. Which is silly and serves no purpose vs. a direct BT link, but it works :-)