Found in an old Popular Electronics this circuit is still current. The SCR can be of the 106 series and the relay according to the voltage used in the supply. The voltage can be 12 V.
This circuit recognizes a tone determined by the tuned input circuit (R3 / R4 / C2). The circuit...
This CMNOS crystal-based circuit generates a 2 Hz signal. It was found in the October 1994 issue of...
This is one of the possible configurations using the LM139, 239 or 339 voltage comparators. The...