This circuit activates a relay when the supply voltage drops below the value set in the trimpot. The transistors can be the BC548 and BC558 and the 6 V relay. The circuit was found in a 1999 Poptronics.
This control from a 1989 publication can be assembled with several equivalent components. The SCR,...
In the figure we have a circuit suggested by National Semiconductor, using an operational...
This circuit was obtained from an English magazine from 1976. What this circuit does is start from...