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.
We found this interesting configuration in a 1976 technical documentation. Equivalent diodes can...
This circuit can generate signals of different frequencies, switched by a DC voltage. The...
This circuit, from a publication from the 1970s, can be implemented with more modern transistors and 6 or...