This circuit was found in the May 1990 Radio Electronics magazine. The timings depend on the capacitors and the supply voltage depends on the relay used.
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...