This simple latching alarm circuit using CMOS gates was found in the May 1990 Radio Electronics magazine. The supply voltage is dependent on the relay.
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...