I found a spair of Razer headphones, model Electra V2 (USB), which were damaged. Not wanting to throw them away, I decided to turn them into Bluetooth.
Although at first I tried another board, as you will see in the video below, I did not like this one the way it worked and so I got another one.
Inside is a small Bluetooth board (the M18 from here), a battery from an old cell phone, a microphone amplifier from old PC speakers, a lithium battery charger and a Dc-Dc Step Up Power Module Booster.
Total cost was around 10 €.
What I can say is that I made these headphones for fun and finally I use them every day. Let me say that I have used and other headphones, such as the Sony WH-1000XM3, but I didn't like the fact that I had everytime to connect them to my TV manually.
This process was too boring for me, as it had to be repeated every time I wanted to see something on TV with the headphones and so I still insisted on using my favorite piece of headphones, the MDR-IF330R, again by Sony, which you just wear them on your head and they work. So simple, so fast and so... lazy-friendly.
With the Electra V2 Bluetooth, however, things are just as easy as with the MDR-IF330R. I just turn them on by turning the switch and they connect directly to the TV (maybe the switch sucks in appearance, but it does its job perfectly).
As soon as the headphones are turned on, my TV instantly shows a pop-up message telling me that a device (the headphones) wants to connect. I select "Yes" and now the sound is transmitted through the headphones. Immediately, without much work, like I want my systems to work.
From my tests, the headphones can last up to 8 hours. When the battery drops below ~20%, the sound starts to "break" and this may be solved if I give a little more voltage through the DC-DC Booster.
For their full charge, it takes ~30', while unlike most models out there, they can work while charging. See in the video below (in Greek, sorry) of the initial process of converting the headphones. I did not have the camera available when I finished the conversion, though.