We found this circuit in a documentation for radio amateurs from the 70s. The transistors can be the BC548. With it, small speakers of 4 or 8 ohms can be used as microphones.
The oscillator in figure 1 can be used in alarm, call or warning systems, producing a sound of...
In this circuit, the LEDs light up in sequence until the last one, when the end of the timing is...
This simple circuit, which appears elsewhere on this site, was found in a transistor manual from...