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.
Found in an EDN from the 90's, this circuit can be implemented with modern operational amplifiers....
The two power steps below are intended for use with the SAA1027 stepper motor control but can be...
Indicated for discriminators adjustment in FM and VHF receivers, this circuit was found in a 90s...