Interesting idea and reminds me of the German train security system PZB which acts completely electricity-free at the signal - a mechanical switch connects a ca

Interesting idea and reminds me of the German train security system PZB which acts completely electricity-free at the signal - a mechanical switch connects a capacitor to de-tune a magnetic resonance circuit, which is then detected by a passing train's magloop transmitter. Unfortunately... I see issues: > Those frequencies are above 20 kilohertz, which is the upper limit of sounds humans can hear. Yeah. Humans' range ends there, at least in adulthood. But what about pets? Wildlife? Children and toddlers? Or just people that take care of their hearing by not frying their ears with too much exposure to loudness? It's already an issue with "mosquito teenager repellant" devices.