This circuit from a Radio Electronics from October 1965 shows how to obtain the action of an SCR when triggering a relay using two transistors. The relay is according to the circuit supply voltage.
This circuit from a Popular Electronics shows how to take the audio signal from a telephone line...
The coil of this transmitter has 4 coils of wire 26 or 28 in the form of 1 cm without core and on it...
In the figure we have another configuration of filter that uses the four Norton amplifiers of the...