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 was found in the Linear Technology Applications of 1997. The circuit uses components from...
This is a 5 V power supply from 2002 documentation, suitable for walkmans and diskmans, common at the...
In fact, this 1967 Electronic Design circuit generates two symmetrical ramps whose frequency...