Found in American documentation this circuit uses transistors of the time. The circuit can be made with current components. It works as a trigger working with 600 ohm signals.
This filter that rejects determines frequency (notch) was found in a Popular Electronics of 1993....
Found in an Elektor from the 90's, this double door-bell drive circuit switches a relay by...
Using a 555 this circuit was found in old documentation, but it is still valid. | Clique na imagem...