This 1967 Philips circuit recognizes a light signal (visible or infrared) modulated by tone. The circuit makes use of components of the time that are equivalent to the current BCs.
The circuit shown below is from the National Semiconductor's 1994 Linear Applications Handbook. The...
This circuit was suggested by documentation from a 1980’s General Electric. The triacs determine...
This article is from an old publication from 1978. The transistor can be a BC548 and the power...