Touch Switch (CB6449E)

This resistive sensor was found in a 1969 US documentation. The circuit uses transistors of the time, but current equivalents can be employed.

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Notch Filter (CB12169E)
This filter that rejects determines frequency (notch) was found in a Popular Electronics of 1993....
Double door bell (CB7559E)
Found in an Elektor from the 90's, this double door-bell drive circuit switches a relay by...
Sine to square converter (CB7329E)
Using a 555 this circuit was found in old documentation, but it is still valid. | Clique na imagem...