In this circuit the LED lights up when the voltage drops below the value set in the trimpot. The transistors can be the BC548. The circuit is from a 1999 American publication.
This traditional doubler configuration with the 555 IC can be found in other articles on the...
This traditional circuit with a neon lamp is a flip-flop that draws which of the two lamps will...
This sophisticated circuit is from old Motorola documentation, using common integrated devices...