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 of a few watts of power shows a typical configuration from the 70s using driver and...
Scratch and Rumble are noises that appear when recovering vinyl recordings, both due to scratches on...
This circuit, already explored on this site, can be applied to other comparators in the same...