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 circuit converts a high impedance signal into low impedance. The circuit has unit gain and...
This circuit is from a 1992 documentation, but the components used can still be easily assembled. The...
This circuit was published in a 1979 magazine, but it can still be easily assembled, as the...