Found in a Popular Electronics from the 90's, this circuit is used to indicate the operation of telephones of the time. The circuit is TTL.
Found in a 1967 Electronics World, this circuit can use any modern equivalent FET. Observe the...
This 1967 Philips circuit recognizes a light signal (visible or infrared) modulated by tone. The...
The circuit of the figure serves the same purpose as the previous one (CB5057E), but has a...