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 in the figure below is from the 1993 Linear Technology Applications. The...
In the figure we have the way to implement the E (AND) function with two inputs, in the case of...
This circuit can be used both as a power source and as a battery charger. The resistor R1 serves as...