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, found in an old magazine of the year 1968, uses a Power germanium transistor. This...
This sequential circuit with an optical coupler was found in a Popular Electronics from the 90s to...
This guitar effect circuit was found in a 2002 documentation. The circuit can be powered by a 9 V...