Active audio filter (CB21770E)

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

Ultra High Gain FET Stage (CB14146E)
The circuit shown has a voltage gain of 500 times and can use equivalent FETs like the BF245. The...
Light comparator (CB7368E)
Found in a 1968 Novel Electronics Circuit, this circuit compares two light intensities using...
Proximity Relay (CB12835E)
We found this circuit in an old publication from Argentina. The relay must be of the sensitive type,...