Hello!
I just came across a wireless doorbell design without a battery in the transmitter,
at first i thought, this might be a bit of a simple circuit, but it is quite sophisticated:
https://youtu.be/5cKhyGMZbq8?t=65Sadly i can not just send you one but if you come across one, it seems to be a interesting piece of low current design with energy harvesting.
There is a electromechanical thingie in there wich generates the power, a µC, RF chipset and a DCDC converter.