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 is from 1977 documentation, but can be assembled with BC548 for Q1, Q2 and Q4, BC558...
Found in American documentation from the 90s, this circuit uses components that are still common...
This high impedance input amplifier is suitable for the use with piezoelectric sources such as...