We found this bridge circuit in Electronics Now magazine from September 1995. The instrument can be from 50 uA to 1 mA. The trimpot value depends on this instrument.
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...