This circuit was found in the 1969 RCA thyristor manual. The circuit has a limiter (clipping) with two 12 V zener diodes in opposition to cut power grid pulses that may influence the trigger.
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...