In that plastic housing, it looks like they are being used as sounders. They can be used as sensors, sounders, actuators etc. Google is your friend...look up piezoelectricity and go from there
It clicks only one time because it is also a capacitor that charges then holds its charge. It should click again if you reverse the battery wires to it. When it is used as a transducer then it is fed AC that alternates its polarity.
Some piezo beepers have a transistor oscillator inside and a third feedback terminal on the piezo wafer. Then it oscillates at the loudest frequency.
I did see things similar to those in old TV sets, and they were Cadmium Sulphide photo resistors and were used to adjust the brightness of the picture depending on the amount of daylight in the room. They were a bit of a gimmick and fell out of use. Cadmium Sulphide fell out of use too, and photo sensor devices were based on silicon photo diodes.
I did see things similar to those in old TV sets, and they were Cadmium Sulphide photo resistors and were used to adjust the brightness of the picture depending on the amount of daylight in the room. They were a bit of a gimmick and fell out of use. Cadmium Sulphide fell out of use too, and photo sensor devices were based on silicon photo diodes.
I don't think they ever looked anything like that (and I've seen plenty of them over the years - just as I've seen a great many of these sounders in microwaves).
It's a silly ideas which gets revamped every few years.
Many modern TV's still do something similar, you have to disable it during install of the sets - it was a stupid feature back in the 50's, and it's a stupid feature in the 2010's
Incidentally, on modern Sony sets it's called an 'ambient sensor'.