This circuit was found in RCA documentation from the 1980s. The circuit uses LDRs of the time and the 50 uA instrument has zero in the center. R1 and R2 may have 2k2 and R3 10k typically.
This Schmitt trigger was found in old documentation. The transistors can be of the general-purpose type like...
This circuit can be used with previous transmitters to decode a signal transmitted via power grid....
This circuit was suggested by General Electric in a 1965 documentation. The circuit can be...