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.
In this circuit the photo-transistor signal is amplified by a Darlington step and then applied to a...
This circuit makes simultaneous control of two up to 150 W incandescent lamps. The circuit is half wave...
This circuit uses an integrated CMOS having the frequency determined by the resistors and...