We found this circuit in a 1966 US documentation. It simulates a variable capacitor that can vary capacitance from 1,000 to 1. Transistors are general purpose types.
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...