Researcher has developed, at a cost of less than one dollar, a wireless light switch that runs without batteries, can be installed anywhere on a wall and could reduce the cost of wiring a house by …::A U of A engineering researcher has developed a wireless light switch that could reduce the cost of wiring a house by as much as 50 per cent.
I saw a similar device in a friends new build 3-4 years ago. It used the energy from you pressing the switch to transmit to the fitting.
My Xiaomi doorbell button works the same way.