Found on a 1964 CQ this circuit is intended for tone frequency change modulation (FSK) telegraphic transmissions. It operates with signals of 2.975 and 2.125 Hz.
This trip switch circuit was found in a Popular Electronics from the 90's. The frequency basically...
This audio-driven relay was published in the July 1977 issue of Radio Electronics magazine. The...
This circuit is from an 80's documentation. Equivalent transistors can be used. The circuit is...