We found this solid-state relay circuit that can function as a shield in a Radio Electronics magazine from December 1989. The optocoupler is a phototransistor type.
This circuit can be found on this website in similar versions. The power supply is given in...
We found this third-order Butterworth asymmetric filter in an old manufacturer documentation. The...
This circuit was found in a Japanese documentation of the 70's. It uses transistors of the era and is...