Metronome using operational (CB22060E)

We found this circuit in an English publication. The transistor can be BC549, and the power supply can be between 5 and 9 V.

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...