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.
This negative voltage source was found in a Popular Electronics from the 90's. The transformer has 120V...
In this circuit the signal from the phototransistor is amplified by a Darlington stage and then...
This circuit is from an 80's documentation. Equivalent transistors can be used. The circuit is...