This audio-driven relay was published in the July 1977 issue of Radio Electronics magazine. The circuit uses CMOS components, and the relay must be a sensitive type.
Found in a Popular Electronics from the 90's, this circuit is used to recognize the frequency of a...
This is the version of the previous circuit for vehicles with positive ground. The circuit is from...
This circuit was obtained from an English publication in 1979 but can be easily assembled with...