This circuit can be used in measurement bridges. We found it in the September 1995 Electronics Now magazine. The meter can be from 50 uA to 1 mA and the power supply is simple.
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...