This code key uses a PLL and an oscillator that generates two frequencies that must be recognized for the relay to be activated. The circuit is from an 80's Popular Electronics.
This Schmitt trigger was found in old documentation. The transistors can be of the general-purpose type like...
This circuit can be used with previous transmitters to decode a signal transmitted via power grid....
This circuit was suggested by General Electric in a 1965 documentation. The circuit can be...