This circuit was found in an old GE SCR manual. The SCR must be in accordance with the relay and transformer as well.
This circuit, from 1990 documentation, can be assembled with BC558 transistors for Q1 and Q2,...
In this circuit the LED lights up when the voltage drops below the value set in the trimpot. The...
This circuit generates interval signals controlled by a light sensor. The basic time of the sound...