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 4-transistor signal tracer or follower was found in Radio Electronics magazine from May 1960....
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
Assembled with SMD components this circuit can be very small. The range is a few tens of meters. We...