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 circuit is from a 1977 timers Cookbook, but can still be easily assembled with the eventual 3130...
This circuit can be used to activate the lamp when it gets dark. We found the circuit in old...
This adjustable source was found in a 1979 documentation, but all components are still common...