I recently spotted a new invention.
It is a wireless doorbell.
OK, nothing clever so far in this modern age where remarkable things are the new normal. However, this new doorbell doesn't need batteries. It harvests the energy from your hand to power the transmitter for a moment to send the wireless code. That is cute.
Alas, the other end of the invention does need power. It is a receiver plugged in to a power socket, though you do get a choice of several ring tones.
It seems to me this invention is half baked and lacking.
Let me propose a proper battery free doorbell that is also completely wireless. It harvests energy from your hand and converts it directly to sound. No batteries are needed. The volume is fully adjustable and the range of ring tones is limited only by the user's imagination. This example is undergoing a long term trial at Durham Cathedral.