This circuit was found in Sgnetics documentation from the 80's. The circuit also has loudness control. The power supply must be symmetrical.
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...