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.

Delayed SCR trigger (CB19336E)
The circuit shown is from an old European publication. In it we have the way to activate a relay...
Audio Line Amplifier (CB4769E)
We found this circuit in a European publication from the 1990s. Transistors are still common types...
Active three-tone filter (CB20808E)
This circuit was found in an American documentation and can be added to common amplifiers of any...