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.
A Schmitt trigger inverter can be obtained from each gate of the 4093 with the connection shown in the...
The circuit is from Motorola's Linear Databook from 1990, but the configuration is current because...
Found in a 90s American documentation, this circuit consists of a trigger that generates...