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 figure shows how to use an LM3914 with a variable range moving point. The circuit has an...
This circuit was found in a 90s Popular Electronics. It uses a varicap that admits equivalents. The...
This circuit is from an Electronics Now from 1998 and can be easily assembled with other JFET...