But I think this is so much better than earlier versions so I deleted them.
Parts list:
Some knowledge of electronics, and you need to be good at soldering. You are responsible if things break! Not me!
Credits to
Thanks to archi for making a PCB
This project shows the way I did it. I'm a hobbyist, not a professional electronics engineer. Proceed at your own risk. The devices use different voltage levels, so there is a chance that you will let the magic smoke out. (Hence the 4066)
Demo https://youtu.be/jGOKGUjlLhI
(R5 and R6 are optional. They are there to measure battery voltage. The zener diode is not needed but protects the arduino if R5 breaks)