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 Rytron transmitter was published in a 1953 book. The circuit can be implemented with a BC558...
Foto-Darlington for applications such as light sensor. In normal applications only the collector and...
This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and...