Obtained in a 1969 Ferranti documentation, this circuit dissipates only 2.2 mW with 4.5 V. The circuit can be designed based on modern equivalent transistors.
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...