With a single LED on the scale, this voltmeter has low consumption. The circuit was found in the March 1992 issue of Radio Electronics magazine.
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...