This circuit was obtained from ancient American documentation based on the application of neon lamps. The SCR is according to the controlled load. It really is a power control.
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...