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.
The circuit shown below is from the National Semiconductor's 1994 Linear Applications Handbook. The...
This metronome was found in a 1979 publication, but it can be assembled more easily with the BC548...
This circuit for a maximum current of 1 A is the most common version that uses the 78 and 7905...