Active audio filter (CB21770E)

This circuit was found in a 1977 documentation for radio amateurs. Integrated circuits are common operational amplifiers.

556 Multi-Hour Timer (CB19101E)
The 556 consists of a double integrated circuit 555, which can be replaced by them in this...
Capacitive relay (CB23765E)
We found this circuit in a Radio Electronics from January 1967. The SCR is already the C106, which...
Programmable Gain Amplifier (CB13915E)
This circuit is from American documentation from 1970. It can be designed with a BC558 and for the...