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.
Didn’t the soviet union have radio bugs that worked like that? No power of their own and hard to detect but if you blast them with RF they can use some of that energy to turn into small very weak signal transmitters. One of the culprits of ‘Havana sickness’ if I remember.
Found the sauce
https://en.m.wikipedia.org/wiki/The_Thing_(listening_device)