Triggered Oscillator (CB21252E)

This trip switch circuit was found in a Popular Electronics from the 90's. The frequency basically depends on C1 and R1.

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Tunable notch filter (CB7404E)
We found this circuit in an English publication from the 1980s. The circuit uses current...
Telephone Relay (CB15524E)
The circuit shown in the figure can be useful in cases where a common telephone extension is used....
Proximity Switch (CB13489E)
We found this circuit in an American documentation from 1977. The circuit must not be powered by a...