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 circuit from a Radio Electronics from February 1966 uses an LDR as a sensor. The transistor is a...
The trigger signal for this circuit is an audio tone. Tones up to close to 100 kHz can be used. The...
This transistor tester configuration can be found on this website with many variations. The...